M52 crank machining

[zaq123]

the left and middle clear comfortably with eta crank best i can tell, the one on the right not so much (its pretty minimal)

digger, thank you again for your help. My pistons are just like in the center of your pic. I guess they should clear.

I picked up the head from the shop today. Spoke with the machinist about the crank work etc and I have a dilemma right now.

M52 crank work will be about $450 (cut, balance)
I would like to hone cyl. bores even for stock pistons since new rings will be installed. This will increase bore even more for old oem pistons and I don't even know if all bores are perfectly round at this point (160k miles). The price to bore to 85mm and hone will be only $70 more than just hone.
I want to get new rods since the price for the reconditioning of old +bolts is very close to new rods + bolts. So 130mm or 135mm will be about the same $$$

I already invested into M52 crank (not good) but......

1.Should I just sell M52 and get S52 crank (my understanding that no machining needed except may the intermediate shaft ) and put $450 towards 85mm custom pistons?
I guess I could find someone cheaper to grind the crank and just have him machine it. He want $275 to grind the crank ~0.25" and $175 to balance it.

2.Do you think I will be ok with new hone job and still be using stock oem pistons (granted bores are still under 0.02mm out of spec)? What's the minimum that the honing can remove without getting major piston bounce in there?

3.Will 2.8L go better with stock valves and schrick 272 cam than 3L? I'm using new oem valve springs so don't want to go more aggressive than 272.

[digger]

Is there any need to shave late i pistons when doing 2.7 with eta crank and eta rods?
I was actually planning to use my late '90 i pistons and shave m52 to the same size as eta crank.
Obviously I'm going to mock everything up to check for clearance but I'm just trying to put this all together in my head right now.

the left and middle clear comfortably with eta crank best i can tell, the one on the right not so much (its pretty minimal)

[redlightpete]

I had 0.20" taken off the radius of my 2.8 crank. I still notched the piston skirts a millimeter or two by hand to ensure there was clearance with the edges of the counterweights. I wanted to err on the side of less crank modification.

[digger]

there are quite a few different skirts so I'm not sure which ones you have. If you post a pic I might be able to mock it up quickly. I have 81 and 84mm cranks lying around here plus some b25 Pistons ( not every skirt combo though because there are two manufacturers and each has a couple versions atleast)

[zaq123]

Is there any need to shave late i pistons when doing 2.7 with eta crank and eta rods?
I was actually planning to use my late '90 i pistons and shave m52 to the same size as eta crank.
Obviously I'm going to mock everything up to check for clearance but I'm just trying to put this all together in my head right now.

not all counterweights are the same size i dont recall the numbers when i measured last time, some are bigger radius than others. from memory i think you need to take about 6mm off radially as minimum but depends a bit how much you take off piston skirts.

when doing a mock up there is minimal clearance using a 135mm rod between the largerst counterweight and piston boss, this emulates the same clearance that yopu would have by removing 5mm radially. so i dont think it would have been enough to keep me happy but that was done a few years back

John (forcedfirebird) will probably chime in and give his numbers that he recorded last time he build one

[digger]

not all counterweights are the same size i dont recall the numbers when i measured last time, some are bigger radius than others. from memory i think you need to take about 6mm off radially as minimum but depends a bit how much you take off piston skirts.

when doing a mock up there is minimal clearance using a 135mm rod between the largerst counterweight and piston boss, this emulates the same clearance that yopu would have by removing 5mm radially. so i dont think it would have been enough to keep me happy but that was done a few years back

http://www.r3vlimited.com/board/showpost.php?p=4008354&postcount=46

John (forcedfirebird) will probably chime in and give his numbers that he recorded last time he build one

[zaq123]

I checked MM specs and it looks like eta crank has counterweight radius of 67.5mm when M52 is 72.75mm
Can I just machine M52 to match ETA crank (take off 5.25mm)? That's only 0.2"..
If ETA crank and 130mm eta rods clear everything with 325i pistons, that should work....am I correct?
Some people are reporting 0.25" or even 0.3" needs to come of. Are they talking about the radius or diameter?
Also if I take off 5.25mm from around M52 crank, that would leave only 3mm (45mm journal with 84mm stroke) for rod journal shoulder..is it sufficient?
Did I calc it correctly: 45mm rod journal, 84mm stroke: 22.5+42=64.5mm to the outer side of the rod journal.

Obviously it will have to be balanced but I don't see how it would be difficult even with that much removed since the crank is three 120' plane design