lucas98wolvy
Well-Known Member
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
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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