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Old 08-16-2016, 06:52 PM   #1
sert57
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2.85 (2.9) Stroker with S50 Crank - it lives!

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.
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Old 08-16-2016, 07:45 PM   #2
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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...ne-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
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Last edited by digger; 08-16-2016 at 07:59 PM.
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Old 08-17-2016, 02:52 AM   #3
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Quote:
Originally Posted by digger View Post
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...ne-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).
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Old 08-17-2016, 03:06 AM   #4
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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.
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Last edited by digger; 08-17-2016 at 03:12 AM.
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Old 08-17-2016, 03:10 AM   #5
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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.
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Old 08-17-2016, 03:22 AM   #6
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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
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Last edited by digger; 08-17-2016 at 03:30 AM.
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Old 08-17-2016, 03:59 AM   #7
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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.
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Old 08-17-2016, 04:10 AM   #8
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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
Hmm, now i just need to track down one of these cranks.
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Old 08-17-2016, 07:14 AM   #9
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why run the s50 crank when you can get an m54b30/ s52 crank for the same price and be closer to 3 liters? You will need to figure out pistons of course.

The m54b30 runs the same bore/stroke (84 mm/89.6 mm) and is a nice engine. The s54 runs 87 mm bore and 91 mm stroke. I don't think its a huge issue on the ratio like digger said.
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Old 08-17-2016, 07:40 AM   #10
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why run the s50 crank when you can get an m54b30/ s52 crank for the same price and be closer to 3 liters? You will need to figure out pistons of course.

The m54b30 runs the same bore/stroke (84 mm/89.6 mm) and is a nice engine. The s54 runs 87 mm bore and 91 mm stroke. I don't think its a huge issue on the ratio like digger said.
Is the S52 crank incompatible with the stock pistons? I was hoping to not replace the pistons.
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Old 08-17-2016, 08:53 AM   #11
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Yes S52 is incompatable with stock pistons. Currently the only crank confirmed to work with B25 pistons is the 84mm, which (in the us) requires counterweights to be machined down and then it needs to be rebalanced, adding to the cost of a build.

The reason for looking at the 86mm option is that it could potentially work like the 84mm crank but not need to be modified. If it works then it could be an easier budget build option. As digger said, he (nor I) have never heard of anyone trying it, so be warned.

Unlike the s52 or m54b30, this has possibility to work with b25 pistons and eta rods, so it is budget friendly ( IF it works).
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Old 08-17-2016, 02:25 PM   #12
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If you are not in a hurry, I have a s50us crank and can check clearances and take some pics on the weekend.
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Old 08-17-2016, 02:49 PM   #13
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If you are not in a hurry, I have a s50us crank and can check clearances and take some pics on the weekend.
Measure the counterweight radius of you can and do all of them as they visually differ
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Old 08-22-2016, 05:09 AM   #14
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I was able to get some counterweight measurements this weekend from my 2.5 crank and machined ( but warped) 2.8 crank. These were taken using calipers from the counterweight outer surface to the bearing surface, and then the bearing diameter was measured with calipers, so take these for what they are.

2.5 2.8
65.81 64.92
65.43 64.87
66.07 65.06
65.67 64.88
65.86 64.71
65.96 64.96
65.90 65.94
65.80 64.36
65.91 65.28
65.58 65.02
65.66 64.88
65.38 64.86

Assuming MM numbers, my 2.8 was machined down ~ 0.3" ( 7-8mm) which is about as expected, normally people suggest .25" but some go over.

I then used these numbers and other MM crank measurements, as well as some from E30zone to roughly calculate average counterweight to skirt clearances at bdc using B25 short skirt pistons. The results of the seta pistons make me doubtful of this accuracy, but if nothing else it seems close to previous B28 measurements and confirms the ~4mm gain from the S50 crank. Following dimensions were done at the edge of the piston skirt, so I used some trig to adjust counterweight hight appropriately.

Crank clearance(mm)
2.5 9.5
2.8 1.0 * This is the crank I have that has been machined down
S50 -2.7
B28 -6.5
Seta 0.26

I'n not terribly confident on the E30zone numbers ( for one, their pic of a Seta pistons is different from every other one I have seen) for pistons and would like to have a physical set measured, and would also like advice on where interference is most likely to occur.

Unfortunately my B25 short skirt pistons are a state away, so I cannot get measurements readily but will try to in the next week or two.

Based on these quick calculations, the S50 crank needs 3.2-3.7 mm MINIMUM of material removed from either the pistons or crank. This is significantly less than the ~7mm needed to be removed from a US B28 crank, but still a good bit.

How much material can safely be removed from the bottom of a B25 piston?

If 3.7mm is too much ( which is a very likely possibility in my unexperienced opinion) the S50 crank would at least be cheaper to machine down since you would be cutting down half the material, so I still may pick one up and play with it.
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Old 08-22-2016, 02:25 PM   #15
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IMO just dont hack into the actual piston pin bosses and you'll be sweet that way you have about 5 mm material below the pin which is enough. if you are aworried you can shape the piston to only remove the minimum amount by cutting it into an arc shape rather than full flat bottom skirt
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