2.85 (2.9) Stroker with S50 Crank - it lives!

[sert57]

Wow that metric mechanic .pdf is a fantastic resource! I think I will be reading that over the next few days to learn what I can.

I think it is good to note that the 84mm crank 2.8 option normally sites about .5mm lower in the block than the stock B25 setup, meaning ideally the 86mm option would only stick out .4mm above stock, where I imagine a head gasket would be sufficient and I would not worry about any piston components going farther than they should. Obviously this is all 'Ideal' and the only real way to see is to get a crank. I think I may give this a shot, seems like it could potentially be a nice option for the North America guys.

[digger]

haha, fair point. I imagine that the wear is just as much of a function of the extent to which it is under-square rather than the angle alone. Increased surface area leading to lower pressures and all.

the difference in angularity between the proposal here and an eta is about 1.1 degrees and compared to the popular 84 stroke 130mm rod its 0.4 degrees which is not very much to worry about unless your goal is to get 300k miles out of it. you might think running a 135mm rod would be better and it is but by only 0.8 degrees. when the rod angle is greatest the cylinder pressure is very much lower than the peak pressure so its not like you get full combustion pressure pushing into the wall at a large angle

[validius]

when its worse than what BMW sell with a warranty id expect to see reduced longevity

haha, fair point. I imagine that the wear is just as much of a function of the extent to which it is under-square rather than the angle alone. Increased surface area leading to lower pressures and all.

[digger]

At what point do you think the force pushing the piston against the bore at 90 degrees ATDC becomes an issue with strokers? I'm in no hurry with my build, this could be an interesting thing to try (referring to my thread on the subject).

when its worse than what BMW sell with a warranty id expect to see reduced longevity to the point that's probably not worth it. You can get a 1.43 rr BMW. It's not as big an issue as the Internet says.

[validius]

according to MM data the 85.8 stroke crank has a CW of 67.75 (basically same at ETA) and the 84 mm is 72.75 mm so its 5 mm smaller. the throw is 0.9 mm longer to there should be 4.1 mm (5-0.9mm) NET clearance improvement if you can trust those numbers it might just be a skirt mod alone which is simpler.

due to the longer throw the piston will stick out a further 0.9mm so you will probably need a thicker gasket but with the 2.05mm gasket you might sneak it in if the block and head are close to original thickness. also the rods will get slighty closer to bottom of bore and intermediate shaft etc but nothing that isn't insurmountable if it does end up being too close. pistons pin comes further down but should still be enough skirt in the bore

http://metricmechanic.com/wp-content/uploads/2016/07/M20-Engine-Booklet.pdf

ive never seen anyone do this build could be an interesting investigation and cant think of any show stoppers, you;ll need to get a crank and do a mockup etc

At what point do you think the force pushing the piston against the bore at 90 degrees ATDC becomes an issue with strokers? I'm in no hurry with my build, this could be an interesting thing to try (referring to my thread on the subject).

[digger]

according to MM data the 85.8 stroke crank has a CW of 67.75 (basically same at ETA) and the 84 mm is 72.75 mm so its 5 mm smaller. the throw is 0.9 mm longer to there should be 4.1 mm (5-0.9mm) NET clearance improvement if you can trust those numbers it might just be a skirt mod alone which is simpler.

due to the longer throw the piston will stick out a further 0.9mm so you will probably need a thicker gasket but with the 2.05mm gasket you might sneak it in if the block and head are close to original thickness. also the rods will get slighty closer to bottom of bore and intermediate shaft etc but nothing that isn't insurmountable if it does end up being too close. pistons pin comes further down but should still be enough skirt in the bore

http://metricmechanic.com/wp-content/uploads/2016/07/M20-Engine-Booklet.pdf

ive never seen anyone do this build could be an interesting investigation and cant think of any show stoppers, you;ll need to get a crank and do a mockup etc

[sert57]

I was wondering what the sizing difference is between the S50US crankshaft counterweights would be versus the M52B28 US Counterweights.


I was originally planning to do a nice straightforward 2.8 build, but the ( admittedly cheap) 2.8 crankshaft I purchased turned out not to be very straight nor balanceable, so I am now being creative and looking into other potential options.

It looks like I can simply use a thicker head gasket on the block and get away with the S50US crankshaft ( ~86mm, i read 85.8 somewhere also) and a thicker MLS head gasket. With this I would hopefully not have to machine down the counterweights and this save a decent amount of money on a stroker. Obviously rod ratio would worsen some (from 1.54 to 1.51, still better than s52US's 1.506) but I imagine it would work.

I've read multiple places that the S50/S52 US counterweights are smaller than the M52b28 US counterweights, but am curios if it is enough of a difference to affect the previously mentioned application. It would need to be at LEAST 2mm smaller diametrically just for the increased size, not to mention the normal machining that is needed.