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Old 01-08-2017, 11:42 AM   #1
zaq123
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M52 crank machining

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

Last edited by zaq123; 01-08-2017 at 11:54 AM.
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Old 01-08-2017, 12:07 PM   #2
digger
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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/show...4&postcount=46

John (forcedfirebird) will probably chime in and give his numbers that he recorded last time he build one
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89 E30 325is Lachs Silber M20B31

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Old 01-08-2017, 12:35 PM   #3
zaq123
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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.


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Originally Posted by digger View Post
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
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Old 01-08-2017, 12:53 PM   #4
digger
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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)
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Old 01-08-2017, 04:52 PM   #5
redlightpete
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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.
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Old 01-09-2017, 12:46 AM   #6
digger
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Quote:
Originally Posted by zaq123 View Post
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)

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Old 01-09-2017, 03:23 PM   #7
zaq123
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Quote:
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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.

Last edited by zaq123; 01-09-2017 at 04:26 PM.
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Old 01-09-2017, 04:07 PM   #8
digger
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Check out the cost of IE Mahle MS 85mm 10:1 forged pistons about $1200 iirc vs crank work, piston mods, etc

You’ll have to check Bentley for specs on bores etc and id trust advice on machinist for measurements and evaluation. Many people reuse pistons with nothing more than a hone if the bores are decent.

A 3L it won’t breathe well at all especially with stock head, stock intake manifold, and very mild cam which is fine if you just want more bottom end torque/midrange for a street engine. IMO a 3L and 2.8L will make about the same/similar peak hp numbers if there is nothing else different beside the crank but the 3L will make more power/torque below its peak and peak at lower rpm and fall over faster. The 3L probably suits the stock intake better which is good from 3500-5500rpm
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Old 01-09-2017, 04:28 PM   #9
zaq123
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A 3L it won’t breathe well at all especially with stock head, stock intake manifold............

The 3L probably suits the stock intake better which is good from 3500-5500rpm
Did you mean 2.8 suits the stock intake better?
So IE Mahle should work with M52 crank with no interference?

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Old 01-09-2017, 06:00 PM   #10
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Did you mean 2.8 suits the stock intake better?
So IE Mahle should work with M52 crank with no interference?
no, the bigger the crank will peak at lower rpm, typically mild engines around 5200-5500rpm, id expect closer to 6000rpm with a 2.8 it does vary depending on exact specs

those pistons should yes, that is what they are intended for
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Old 01-09-2017, 08:11 PM   #11
redlightpete
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Another advantage of those I.E. pistons is that you can run a bigger cam that's more appropriate for a stroker, without having to cut deeper valve pockets in the pistons. I think the conventional wisdom is that you can run up to a 288 cam with stock valve springs.
Regarding the bore size, if all you do is flex-hone it, the bore shouldn't increase appreciably.
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Old 01-10-2017, 03:15 AM   #12
zaq123
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My machinist uses one of those auto honing machines, no flex honing.

I think IE/M piston route is the way to go. Price sucks but I will know for sure that all bores are perfect and the short block is acceptable.

I don't really want to push it with 288 and oem springs plus my understanding 288 is too big of a cam for 2.8-2.9L. Maybe 284/272 max?

How bad is M52 big cw crank's interferance with m20b25 block? What areas in the block are know to need some clearancing? I've seen a sticky with the cut out for b27 block.. any info on b25? Would intermediate shaft be a problem?
Also anyone knows if M52 crank is still balanced when you take the sensor ring off?
That would be nice....if so, I will just need to do only last crank/flywheel+everything else balance and be done.

Last edited by zaq123; 01-10-2017 at 05:01 AM.
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Old 01-10-2017, 01:21 PM   #13
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With the b25 block the inference is less but it might be to do with the year of the block rather than if its b25 or b27. Basically any issues are pretty simple and straight forward. Things to check are:

- Crank counterweight webs to sides of block including intermediate shaft support bearings
- Bottom of cylinder bore and rod shoulder (24V OEM rods should be ok , some aftermarket rods cause more issue than others, the bigger the stroke the bigger the issue)
- Intermediate shaft and rod clearance (normally only an issue on big stroke crank e.g. 89.6mm)

Note to drop the crank in you need to orient it correctly (spin it on its axis)when lowering it down

The 288 isn’t a great cam with the OEM manifold as the 288 in a 2.8L wants to peak at an rpm much higher than the manifold is intended to be intune at. This is because the runners are a bit too long and restrictive so you end up being not fully utilizing the full benefit of the cam. With the RHD intake the 288 is a great balanced design.

Also if you’re still using motronic then the idle and drivability is a lot harder to get nice than with other systems once you introduce a fair bit of overlap.

It would be nice if schrick did a 284/284 instead of the 284/272…i think it would be a better option generally speaking (dbilas do a 284/284). however with OEM intake and exhaust and ECU a 272 might be better to live with on pure daily driver.
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Old 01-10-2017, 05:22 PM   #14
zaq123
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My plan is to use MS for this build with larger injectors (any recommendation on the size?). Will have to live with its stock intake and exhaust for a little while. Next mod on the list will be the exhaust and at some point ITB (hopefully). Any way to improve the oem intake? Should I look into into dbilas 284? I probably should pick up the adj. cam gear for the bigger cam and take care of head/block skim

it won't me my daily driver, just a weekend toy to have some fun in the twisties.


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Old 01-10-2017, 05:29 PM   #15
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I wouldn't turn the crank, I have always stayed away from under sizing them - specially on a 2.8, IMO when stroking an m20, you are creating a horrible R/S ratio and don't need any more of a failure point. I have always just had the journals polished, and scrap the ones that need a .010" under - I wouldn't even consider a .025" under.

The m52 crank does not need a lot of meat removed with the flat bottom short skirt pistons. The two center throws are larger than the rest. They will need about 5-6mm (guessing) and the outer ones will only need a 1-2mm. $450 to cut and balance the crank isn't overly expensive if you don't have it ground. I have actually notched the piston skirts just enough to clear the throws and left the m52 crank full circle, so that's an option too (this was with Mahle +.5mm stock replacement pistons)

BMW has extremely tight piston to wall clearances with a 83.98mm piston and 84mm bore (.01mm per side ~.0004", 4 tenths of a thousand of an inch - 1/10 the thickness of human hair). I have rarely have seen a decently kept m20 go out of round, and usually show sign of almost no wear. If you can see the cross hatches still, you will probably get away with a deglaze, we tend to go over size more for the added compression and small bump in displacement. Have your machinist measure to confirm.

Since you already have the crank, $450 for them to prep it up, spend $600 on +5mm OEM pistons, have the rotating assy balanced (~$250 here locally), and whatever your bore/hone is going to cost ($250 here), and take .010" off the block deck ($120 here). This will give you a very strong m20 for not a lot of money. I prefer to recondition the stock rods as they are seasoned and really strong already. If you use aftermarket rods, make sureyou have them checked for round and use the ARP 2000 stamped bolts. A lot of rod companies use the cheapest ARP, so you may need bolts and a big end resize even if they are aftermarket.

I wouldn't trust used stock springs unless they are tested for stiffness, never mind for a cam with extra lift. New OEM springs may be suitable for the 288, but if that's the minimum suggested, that's too close for my comfort.

Finally, shoot for a .045" head to piston clearance. The m20 really likes this number for some reason.
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