Sneaky 401cc stroker- the easy way and more.

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lucas98wolvy

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Here’s a list of required parts to make a 401 stroker
From a 99-12 big bear 400 single range 5 speed
- Crankshaft
- Crankshaft Balance shaft
- Cylinder Jug
- Timing chain
- Front timing chain guide (longer than 350 guide)
- 1.5mm overbore Namura wolverine 350 piston



Now after much measuring, swapping, and re-swapping this is what I have gathered as far as differences and similarities.
Both motors use an 83mm bore. Both us a piston with a compression height of 1.116”. Both use identical sleeves in the cylinder jug. The cylinder heads are identical. The main cases of a SR 5Spd. Big Bear 400 4x4, and our 350 Wolverine are identical, and pretty much every part can be swapped between the two. Want to turn your 400 into a 350… you can, but why would you.
Now for some differences, the Crankshaft has a 71.5mm stroke vs a 64.5, the connecting rod is 5mm longer than a 350’s, the cylinder sleeve, while the same length overall is in a jug that is 8.5mm longer (it has 1 extra cooling fin). In other word’s the Big Bear 400 has a raised deck height, 8.5mm higher than a 350, which makes sense, 5mm longer rod + ½ of stroke difference (5mm+3.5mm=8.5mm). The timing chain also has 2 more links in it to compensate.
It makes sense that Yamaha wanted to preserve the rod ratio and keep rod angularity low, so they increased rod length, which keeps the piston more stable in the bore and keeps friction low when they upped the stoke. However, this also makes the rotating assembly heavier, which becomes a problem when you go to a super big bore, anything over 85mm, but there are ways around this as well.
Back on track now, Split your cases, shoe horn the bigger crank, rod, and balancer in, then slap on the big bear jug with a 1.5mm over factory style 9.2-1 piston, which with the extra cc’s makes the actual compression 10.6-1 good for pump gas. You can use a JE piston, or Wiseco 10.5-1, as well but the compression will end up in the 11.6-1 range. I have heard of guys still running pump gas at this range, but you will need to hyper polish the piston top and the combustion chamber to a mirror like shine to even attempt this by keeping detonation causing hotspots down. Rule of thumb here is if your cranking psi is over 180 psi, you’re in race gas territory, but I have seen engines well over 200psi with modern high-squish chambers and the aforementioned tricks run w/o a hint of detonation, so… that’s your comfort zone to deal with. The last thing to do is re-drill the hole in the top motor mount (3/8'' drill bit) about 10mm higher.
Now about the aforementioned rotating assemble weight issues, when you go to a big bore kit (85mm+), with the stroker crank, and heavier con-rod, you risk breakage due to the spindly factory rod not being quite strong enough (not quite enough cross-sectional area to handle the sudden accel & deccel chores of racing). The way to circumvent this is to get an aftermarket rod, which for our purposes mean custom built from Carillo Peformance for about $300 (ouch, but bulletproof).
Also watch your piston selection in general, certain performance piston kits are heavier than the stock piston. Wiseco is guilty here, irony is the weight is mostly in the piston pin in their kits. The end result is that a 10.25-1, 83mm Wiseco is almost 40grams heavier than a stocker. While the JE 85mm 12-1 piston is 30 grams lighter overall with a giant dome adding significant weight to the piston.
Really the best way to do this build, or any bigger bore builds would be to have a custom made piston, 4032 alloy for tighter clearances, better temperature stability, less noise, and much more longevity than a 2618 ALLOY, spec’d with partial skirts, and a small dish that is a reflection of the cambers shape to stay around a comfy 10.5-1. This actually has 2 large benefits, very light weight, and the ability to make a high squish chamber by keeping piston to head clearance around .040.
Ok, so I went a little deeper into this than originally planned, but I want to give you as much information before you go down that road of 446+ strokers, being there are so few machine shops across the country that will even bore our case anyway, and that the 401cc is so easy to build, and make stone dead reliable with readily available factory replacement parts that don’t require special services to build, but the best part is, the big bear jug isn't labeled with its displacement, so no one would ever know ;)
 
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This is great info thank you for posting. Are you currently running this setup?
 
Good post Lucas. I have gone through all this in the past myself when i did mine. I looked at all the options and ended up just finding a whole BB motor to use. I have 28" tires and needed the gearing for the tires anyways. I am only 1mm over too so that gives me room if I need to do one more bore down the road. I run the Wiseco piston in mine and with close to 12:1 compression its very hard on starters, the starter clutch and the starter gears.
Your very accurate about piston weight too. I have already had to do a rod bearing.
 
Good post Lucas. I have gone through all this in the past myself when i did mine. I looked at all the options and ended up just finding a whole BB motor to use. I have 28" tires and needed the gearing for the tires anyways. I am only 1mm over too so that gives me room if I need to do one more bore down the road. I run the Wiseco piston in mine and with close to 12:1 compression its very hard on starters, the starter clutch and the starter gears.
Your very accurate about piston weight too. I have already had to do a rod bearing.

Gearing aside how do you like this setup?
 
I have one spare motor, the one shown, that is now fully built. I will swap it in this spring, wish I could say I've had a chance to run it. It will be in for the hair scramble...:). Other things I am working on are an adjustable base timing via machining and fab to stator cover. Which will allow me to run a more radical cam without the mandatory compression bump to keep it civil. I have done the big bear main shaft and cluster swap, but kept the wolvy's secondary reduction. So top speed w/25" will be around 62-3 ish. I am also using a big bear 02-06 front differential w/electric dis-connect. Which is being installed as we speak, I cannibalized a big bear harness to make a stand-alone, for it as well as the radiator and cooling fan from it. So look for more threads and updates.
 
I have one spare motor, the one shown, that is now fully built. I will swap it in this spring, wish I could say I've had a chance to run it. It will be in for the hair scramble...:). Other things I am working on are an adjustable base timing via machining and fab to stator cover. Which will allow me to run a more radical cam without the mandatory compression bump to keep it civil. I have done the big bear main shaft and cluster swap, but kept the wolvy's secondary reduction. So top speed w/25" will be around 62-3 ish. I am also using a big bear 02-06 front differential w/electric dis-connect. Which is being installed as we speak, I cannibalized a big bear harness to make a stand-alone, for it as well as the radiator and cooling fan from it. So look for more threads and updates.

That's awesome! I am hoping to swap in the BB front diff this spring. Just have to pull the trigger on ebay. I was looking at swapping in a 400 BB motor but have my doubts if all of the work would be worth it or if I should just put together a stroker like you did. Not looking to turn monster tires (25's max most likely) just looking for a little more umphh lol.
 
Excellent post, thanks for taking the time to share!

I have a 2004 Wolverine that is getting pretty tired, it's time for a rebuild. If money were no object I would send my motor to FST and have them install a 446 kit along with several other mods...but I live in Canada and with the weak dollar and added expense of shipping that's really out of the question.

I have been researching this topic for a while now and I keep coming back to this post, this is the way I want to go with my build. I have a few questions, I hope you don't mind taking the time to discuss.

Now after much measuring, swapping, and re-swapping this is what I have gathered as far as differences and similarities.

Both motors use an 83mm bore. Both us a piston with a compression height of 1.116”. Both use identical sleeves in the cylinder jug. The cylinder heads are identical. The main cases of a SR 5Spd. Big Bear 400 4x4, and our 350 Wolverine are identical, and pretty much every part can be swapped between the two. Want to turn your 400 into a 350… you can, but why would you.

I have been comparing part numbers between the Wolverine, Big Bear and Warrior to see what parts are the same. The cases have different part numbers between the Wolverine (4KB-15150-04) and Big Bear (4KB-15100-04). I don't doubt that you are correct about them being interchangeable, but did you happen to find anything different between the two?

The piston also has a different part number. The heads are identical and the bore is the same but the bikes have different compression ratio's. I'm guessing the domes must be different on the pistons, does that sound right to you?

I was surprised to see that the cam's are identical, I thought the Wolvy and the Warrior would have a more aggressive cam.

Now for some differences, the Crankshaft has a 71.5mm stroke vs a 64.5, the connecting rod is 5mm longer than a 350’s, the cylinder sleeve, while the same length overall is in a jug that is 8.5mm longer (it has 1 extra cooling fin). In other word’s the Big Bear 400 has a raised deck height, 8.5mm higher than a 350, which makes sense, 5mm longer rod + ½ of stroke difference (5mm+3.5mm=8.5mm). The timing chain also has 2 more links in it to compensate. It makes sense that Yamaha wanted to preserve the rod ratio and keep rod angularity low, so they increased rod length, which keeps the piston more stable in the bore and keeps friction low when they upped the stoke. However, this also makes the rotating assembly heavier, which becomes a problem when you go to a super big bore, anything over 85mm, but there are ways around this as well.

My biggest concern with this setup is the Big Bear connecting rod. As you have identified, and I seem to recall Mickey Dunlap saying the same thing, the rod is the weak link. Does it only become an issue when you go over 85mm? I would like to go with a JE 85mm piston (more on that later), any issues with this, aside from the high compression?

Back on track now, Split your cases, shoe horn the bigger crank, rod, and balancer in, then slap on the big bear jug with a 1.5mm over factory style 9.2-1 piston, which with the extra cc’s makes the actual compression 10.6-1 good for pump gas. You can use a JE piston, or Wiseco 10.5-1, as well but the compression will end up in the 11.6-1 range.

I have some concerns with the Namura piston, they have a lot of negative reviews. I would be happier with a JE piston but the compression ratio would be too high since they only have a 10.5:1 option available. I have been trying to figure out a way to use the JE piston and as I see it have the following options:

1. Custom connecting rod from CP-Carrillo. A shorter connecting rod should lower the compression ratio and the CP rod will be much stronger. The downside is price of course and I wouldn't know where to begin in spec'ing out a custom design and what sort of impact that would have.

2. Custom piston from JE. I have already been in contact with JE about what they can do and am waiting to hear back from them. I remember Mickey Dunlap saying that they have a minimum order of 4 for their custom pistons though, not sure if that is still the case. I have also enquired if they could modify their 85mm 10.5:1 piston rather than going full custom in case that makes a difference.

3. Custom head work. Increasing the combustion chamber volume should lower the compression ratio and I could get the head ported at the same time. I have no idea what the costs would be.

4. Use a thicker head gasket. It occured to me that it may be as simple as using a thicker head gasket. I found a compression ratio calculator online (http://www.csgnetwork.com/compcalc.html) and starting playing with the numbers.

The calculator has the following inputs:

- Cylinder Bore Size
- Piston Stroke Length
- Head Gasket Bore Diameter
- Compressed Head Gasket Thickness
- Combustion Chamber Volume In CCs
- Piston Dome Volume In CCs
- Piston Deck Clearance

Bore and stroke are easy, but the other details are harder to find. I couldn't find what the stock gasket thickness was, but the compressed thickness for a Cometic EST gasket is 0.027" (0.6858mm). I found on another forum that the volume for a stock Warrior head is 36cc and the deck height is somewhere around 0.030" (0.762mm). Using this information the calculated compression ratio is 8.9:1, stock ratio for the Wolverine is 9.2:1. I played with the Piston Dome Volume value until the calculated compression ratio was close to 9.2:1 and a value of 1.25 got me pretty close so now I have some base values:

- Cylinder Bore Size: 83
- Piston Stroke Length: 64.5
- Head Gasket Bore Diameter: 83
- Compressed Head Gasket Thickness: 0.6858
- Combustion Chamber Volume In CCs: 36
- Piston Dome Volume In CCs: 1.25
- Piston Deck Clearance: 0.762

If I change the bore to 85mm and the piston dome volume to 5.7cc it calculates a compression ratio of 10.5:1. This should be a close representation of the JE 85mm 10.5:1 piston in the Wolverine. If I change the stroke to match the Big Bear (71.5mm) it bumps the compression up to 11.5:1 which is close to your estimation. From there I tweaked the head gasket thickness to get the compression ratio back down to where I want it.

I found that by doubling up on the gasket thickness (1.4478mm) it dropped the compression ratio to 10.5:1. Cometic offer custom gaskets available in the following thickness: .027", .030", .036", .040", .045", .051", .060", .066", .070", .075", .080”, .086”, .092”, .098”, .120". Something in the .051-0.60" range should work quite nicely.

Am I missing something, could it really be that simple?

I'm not sure what the stock piston weighs, but if the JE 85mm 12:0:1 piston is 30 grams lighter than stock, the JE 85mm 10.5:1 piston should be 40 grams lighter since it is 10 grams lighter than the 12.0:1 piston. I think this would be perfect.

I'd love to get some feedback on this!

Thanks for taking the time!
 
I went the same route as you in finding part numbers comparing differences, than I took the next step and bought the parts off eBay:)
Now, I measured every hole dimension, swapped side covers, transmissions, crankshafts, cylinder jugs, everything interchanges. Checked u-joint sizes, spline counts, sensors, all are the same. It really is that simple.

I used the namura piston as an example, but even an OEM Yamaha piston and ring set is only 90 bucks for rings, pin and piston. You can actually use a 2000 Kodiak 400 piston from the liquid cooled it's 10.5-1 and it's the 1.5 mm over needed to hit 401. Yamaha has a habit of making many things interchangeable, lol.

As far as a custom rod and piston I would go thru CP/Carillo they are sister companies, and mode flexible than JE. You can get a custom piston x2 with undercrown milling for $660.00, a custom made con rod for about 300. The CP piston is an ultra modern design and extremely light, like their 89mm 450 pistons are even lighter than the JE pistons in our bikes lol. The Carrillo rod is as light as factory but on another level of strength. Plus you can spec a 4032 forging that runs quieter and last longer than a 2618, unless your running nos or boost lol.

Factory BB 400 rod is plenty strong but, when you slap on the 88/89mm heavy over bore old school style pistons that stresses the factory rod and bearing to much, that's when problems occur. The 85mm JE is a good choice because it reduces stress on the rod, even the factory piston is a good choice. Problem with weisco is heavy @$$ piston pin they use, it actually weights more by almost an ounce.

Head gasket is a no-no on compression, you are also increasing squish distance and increasing chances of detonation all while losing power. Best bet is to have the head ported and increase chamber size, or have a machinist cut the dome down and repolished the crown, which was easy for me because I have access to a lathe.

I think that covers everything you asked, if not let me know.
 
It occurred to me last night that the thicker head gasket might effect the squish area but that's not a topic I'm very familiar with. I wasn't even sure if these engines utilized the squish area and if they did how a thicker gasket would effect things. A little reading on the YFM350.com forums and I see that they do and the gaskets would have a big effect, so as you say, thicker gaskets are a no-no :(

I was thinking that the higher compression pistons must alter the effect of the squish area as well, I have been looking for a picture of the JE 85mm 10.5:1 piston to see how the dome looks to get a better understanding of this. The best picture I have been able to find is this one:

2012-05-28_21-47-12_977.jpg


So I can see there is still a significant flat area at the edge of the piston to create the necessary "squish", I pictured more of a rounded dome which is why I was confused. So it should be possible to remove some material from the dome at the centre of the piston to get the compression ratio down, either that or increase the chamber size, 4-5cc's if the CR calculator is right.

It sounds like you milled the dome of your piston, did you figure out a way to determine exactly how much material to remove, or did you just mill down to the same level as the flat edges? I think milling the piston might be the simplest method as I'll be sending that along with my cylinder to get the bore to match.

It makes sense to me to have the crank rebuilt while the cases are split but Yamaha does not list the main bearings or the big end bearing separately for the Wolverine or the Big Bear. Have you rebuilt the crank? Do you happen to know the part numbers?
 
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I have a burette for doing cylinder heads, I used the wolvy cylinder popped the JE piston in it brought the piston to the top of the sleeve, with the comp ring installed and loaded with petrolum jelly to seal it, slapped on the lexan plate added 50mls to burette and opened the valve. Subtracted the amount from my total, than I milled off about half of the dome, and repeated the procedure. I end up taking off 3.5~cc's, which is why my comps a little on the high side.
 


My chamber
This was the rough finish before it went to the machine shop for a shave, so by polishing it I made it larger, than smaller again by milling the head lol.
 
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I have a burette for doing cylinder heads, I used the wolvy cylinder popped the JE piston in it brought the piston to the top of the sleeve, with the comp ring installed and loaded with petrolum jelly to seal it, slapped on the lexan plate added 50mls to burette and opened the valve. Subtracted the amount from my total, than I milled off about half of the dome, and repeated the procedure. I end up taking off 3.5~cc's, which is why my comps a little on the high side.

Ah, that's smart. I was thinking I would have to submerse the entire piston in alcohol or a thin oil and check displaced volume before and after, either that or some crazy math formula that I could probably never figure out ;)

I have been trying to figure what the best compression ratio for me would be, I definitely don't want to run race gas, I would be happy using 91 but ideally I could still run with 87 in case I end up having to use it. I realize there are multiple factors at play so it isn't as simple as saying 8.6:1 = 87 and 12.0:1 = race gas, but I can't find an answer to what compression ratio would still allow me to run 87.

The Wolvy has a 83mm bore, 64.5mm stroke and 9.2:1 compression ratio, the Big Bear 400 has a 83mm bore, 71.5mm stroke and 8.6:1 compression ratio and the Kodiak 400 has a 84.5mm bore, 71.5mm stroke and 10.5:1 compression ratio and all three still call for regular unleaded gasoline.

The Kodiak 400 has the same bore and stroke as your Sneaky 401cc but it has a completely different head and cam than the Wolvy and Big Bear. I was surprised the Kodiak still calls for regular gasoline with that compression ratio though. Would a final compression ratio of 10.5:1 with Big Bear 400 crank and modified 85mm 10.5:1 JE piston run on 87 octane in the Wolverine?
 
Yes, it can be made too, your cranking compression has to be kept to 165-185 for 87; if you run 93 you can run as high as 210-215psi. I removed the bare minimum from my piston, my compression is higher because I milled the head .010 after I smoothed out, and polished the chamber I was 2cc's over stock, mine had some particularly odd waded gum castings in the chamber. To run 87 octane...I would enlarge the chamber 2-3 cc's and not shave the head, in conjunction with a light dome shave. If you ever have the opportunity to look at a Kodiak 400 or 450, even though it's liquid cooled, it's a copy of the air cooled motors. 2 valve head, same valves, as our 350. The cam is identical, same bearings same space between the lobes, same overall size. They literally took the air cooled motor and transferred as many parts as they could. Deck height on the 400's is the same from air to liquid. Yamaha was in a word cheap lol.
 
I read on the Wolverine BigBear Gear Swap thread that you have the motor in and it sounds like it works great, good to hear!

Did you happen to keep a note of your measurements during the build? I'm still planning my build on paper and it would be nice if I had some accurate numbers to work off of. I have been in contact with JE but they didn't give me the dome volume for the 85mm 10.5:1 piston or answer my question about deck correction. I have read that the pistons are deck corrected but I think it is probably more accurate that their calculations are based off zero deck clearance, rather than the piston having a taller compression height. There is a statement on their website for their automotive pistons saying Compression ratio calculated at "O" deck and .040" head gasket thickness but nothing on the powersports pages.

If they do use zero deck clearance and 0.040" gasket thickness for their calculations, that would put the dome volume at 3.25cc in order to hit the 10.5:1 compression, but that seems low. You estimated removing about 3.5cc from your piston and you left some of the dome right?

I haven't been able to find accurate numbers for the Wolverine, but according to users on another forum, the Warrior has 36cc combustion chamber, .030" deck clearance, .043" crushed head gasket and .020" base gasket. The head, head gasket and base gasket are identical so those values should be the same, the deck clearance might be different though.

As a sanity check, I used those numbers to calculate the compression ratio for the 350 Wolverine and 400 Big Bear. I got 8.58:1 for the Wolvy and 9.41:1 for the Big Bear. In order to hit Yamaha's numbers the Wolverine piston would need a dome with 3.45cc volume and the Big Bear would need a dished piston with a volume of 4.9cc's, that seems like a pretty big difference (8.35cc total).

Those values would also mean that the squish area would be .073" which is nearly twice the optimal size.

I'm thinking my numbers must be off, do they jive with you?

If those numbers are accurate, it sounds like I should be planning to deck my cylinder as well. Even if I eliminate the base gasket the squish will still be .053". Did you have to deck your cylinder?
 
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Factory Piston to dome clearance a.k.a. quench height is around .050 if your lucky. All of the yfm motors pistons sit down in the hole to some degree. Things that add to the height are the lower cylinder gasket, which can be left out all together, if you wish because its all sealed via O-rings. However, its only a .010 paper gasket, and it adds about 1-1.5cc's to the overall volume. Next the compressed factory gasket is about .030 thick. The piston sits down in the hole when its all assemble about .5mm give or take once you fully torque it down. I clayed my piston top then torqued the head down, then checked its thickness. I was at .057, so I pulled my bottom gasket, that dropped me down to .048, I used a cometic head gasket which was .003 thinner than the compressed factory gasket I used for initial mock-up, before I even squished it down. Other than the fire ring, it doesn't compress down much, thou. So, I did manage to get a decent .045 quench, but not ideal quench.

Also the dome volume on the JE piston is about 6cc's +/-5 cc burette accuracy. Really, because of manufacturing tolerence you should always clay your motors when you build them. Lol, had a never been apart pickup motor with a piston .180 down in the hole that according to factory blueprint, was only supposed to be .07 lol.

Also just to shed a little more light on actually compression, my 1mm+ Kodiak has 185psi cranking psi with a big dish in the piston and all the factory gaskets in place. My 366 with a 2mm+ domed piston only has 170 psi cranking compression. According to yamaha the kodiak comp is 10.5-1, according to JE their piston should provide 10.5-1. The psi on the kodiak is within factory spec of 180-215psi. It's interesting how cylinder pressure can vary so much on two motors with the only difference being actual displacement.

Also, if you measure the piston dome, I marked my measurements with black lines, which will be small because the from them the dome chamfers at about a 45 degree angle all the way around, down to the top of the piston.
This is an 85mm JE for the record and the dome measures 3.5mmx33.5mmx47mm in size. Which equals 5.51cc's, again this is smaller than the dome actually is, but it gives a starting point.
*figured out that the corners cost about .256 cc's and the chamfer around the dome adds roughly .590 cc's, I traced it on graph paper, which is roughly 6mmx6mm squares and did a bunch of basic geometry. So the math gets very close to the burette in the end.


 
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Also as I learned you will need the cylinder head bolts from a big bear, I found mine on ebay for 14.99 with free shipping lol
 
This was a 350 warrior with a ported head and 10-1 piston in it, no cam, no pipe, no air box mods, no other trickery, on a pretty stingy dyno lol. Also it had 22" BKT's with stock gearing, so not the lightest or the best for not sucking up power.

 
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