Take the minimum off you need. 1/8" is plenty clearance, minimum is typically 0.060".
check all to make sure you account for the biggest counterweight
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2.85 (2.9) Stroker with S50 Crank - it lives!
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Quick update: I have gotten an S50 crank and gotten it mocked up. As expected, they are snug... ( Once Again, S50b30US crankshaft, Eta 130mm rods, and B25 short skirt pistons)
[IMGhttp://imgur.com/jzmg3K2.jpg[/IMG]
They do clear with no modification! Clearance looks to be about 0.5mm between the counterweight and the skirt. Not exactly what i want by bottom end to look like but that is what lathes are for! Next step will be to get the crank inspected to verify that it is good to use, and take the pistons to a tool shop and hopefully shave off about 5mm off the skirts (until the large boss on the inside). I will keep this thread updated as info rolls in. Fingers Crossed!
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its only 0.9 mm extra throw, the rod beam-shoulder transition may touch bottom of bore slighty, nothing a die grinder couldn't fix. ive seen some 89.6 strokers with really wide H-beams that need a huge chamfer on the bottom. Also the rod side /shoulder might kiss the intermediate shaft but probably something a lathe could remedy in 5min.
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Originally posted by digger View Postif 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
Originally posted by E30-325iS View Post
I put the crank in a M20B25 block and the counter weights clear the bottom of the bores and intermediate shaft easily.
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About a week late, but here are the S50B30US crank counter weight radius measurements.
I used the same technique that sert57 used, but with a shitty set of digital calipers.
They are about the same as the 67.75mm quoted by Metric Mechanic.
From front of crank to rear, measured in millimeters.
1 67.75
2 67.93
3 67.93
4 68.02
5 67.85
6 67.93
7 67.85
8 67.91
9 67.88
10 67.92
11 67.88
12 67.85
S50B30US crank weight, no sprocket, spacer or bolt. 22.5Kg
I put the crank in a M20B25 block and the counter weights clear the bottom of the bores and intermediate shaft easily.
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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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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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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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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).Last edited by sert57; 08-17-2016, 10:48 AM.
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Originally posted by downforce22 View Postwhy 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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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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Originally posted by digger View Postthe 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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