S38B35 Piston Running Clearances

Specific conversations and info for the BMW E28 M5 and M535i.
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S38Mike
Posts: 864
Joined: Jul 01, 2011 1:03 PM
Location: South Bay, SoCal

S38B35 Piston Running Clearances

Post by S38Mike »

Greetings group, long time lurker/seldom poster.
I’ve enjoyed reading all of your posts since joining, they’ve helped me dive into rebuilding my fairly stock S38B35 engine. My goals: balance, good ring seal, longevity, reliability and learning how to blueprint properly.
I’m keeping costs down by running Maxsil pistons so I’m seeking your real world experiences and recommendations for piston/bore clearance with your projects.
Memphis Motorworks has a good reputation so I had them check, bore and deck the block.
I’m measuring the piston clearances by assigning piston to specific bore then repeatable measurements of the skirt 15mm up from the bottom perpendicular to wrist pin bore per repair manual then setting up dial bore gauge for each respective piston/bore to measure clearance using only Mitutoyo and triple checking readings. Is there a better method?

Factory S38B35 piston running clearance spec: 0.03-0.06mm (.0011-.0024”) Wear limit 0.15mm (.005”)

My measurements are:
1) 0.058-0.068mm (.0022-.0026”)
2) 0.063-0.080mm (.0024-0031”)
3) 0.058-0.068mm (.0022-.0026”)
4) 0.063-0.081mm (.0022-.0031”)
5) 0.058-0.071mm (.0022-.0027”)
6) 0.050-0.068mm (.0019-.0026”

I’d like to see my clearances a little bit tighter to have years of good service but am I splitting hairs? Any advice and patience are greatly appreciated.
more questions and a build thread to come as the project continues...
Preyupy
Posts: 245
Joined: Jun 21, 2012 12:48 AM
Location: Issaquah WA

Re: S38B35 Piston Running Clearances

Post by Preyupy »

I don't know anything about Maxsil pistons. What do they recommend? Is it a forged or cast piston? Depending on the alloy they used the growth is different from one type of piston to another. Under normal circumstances the forged pistons I have used take a much larger clearance than the stock cast pistons. You also need to make sure you warm up the engine before you start leaning on it very hard for that same reason. I would go with the recommended clearance as specified by the piston manufacturer not necessarily the stock "BMW" number.
S38Mike
Posts: 864
Joined: Jul 01, 2011 1:03 PM
Location: South Bay, SoCal

Re: S38B35 Piston Running Clearances

Post by S38Mike »

The pistons are cast, and based on their recommended clearance, it appears my cylinders have been opened up too large. I’m trying to decide if I should start over the larger (possibly forged) pistons and more optimal running clearance.

From the website: https://www.maxsilpistons.com/specifications.html

“.0015"(+/-.0003") piston to cylinder wall clearance
MaxSil™ Pistons for BMWs are manufactured using a special aluminum alloy blend that includes the optimum amount of silicone. This aluminum alloy offers more strength, less weight, less heat expansion, and less friction than other alloys - while avoiding the brittleness of hypereutectic. This allows installers to hold a .0015 (+/-.0003") clearance between piston and cylinder wall without risk of seizure.”
Preyupy
Posts: 245
Joined: Jun 21, 2012 12:48 AM
Location: Issaquah WA

Re: S38B35 Piston Running Clearances

Post by Preyupy »

Was the block bored before you had the pistons in hand? I would be surprised if a good machine shop would do that.
S38Mike
Posts: 864
Joined: Jul 01, 2011 1:03 PM
Location: South Bay, SoCal

Re: S38B35 Piston Running Clearances

Post by S38Mike »

No, I sent them the block with pistons new in their boxes along with the crank, head and cam housing.
To cover my bases, I also printed out a copy of every factory specification page from TIS.
Regrettably the only thing I didn’t do was tell them exactly where I wanted the running clearance to be but I figured based on the reputation they would set it to what was appropriate. I guess I would like things to be a little bit closer to the tighter side of the spec to give the pistons/rings more of a shot at longevity but maybe I shouldn’t be so concerned. I tend to overanalyze :roll:
EuroShark
Posts: 1429
Joined: Feb 12, 2006 1:00 PM
Location: Colorado

Re: S38B35 Piston Running Clearances

Post by EuroShark »

Mike, a customer of mine recently recommended a place called Line 2 Line Coatings (https://www.line2linecoatings.com/) that will apply an abradable coating to the piston skirts to take up excessive clearance. Pretty interesting stuff. I was skeptical at first but there are quite a few guys who have had great results with these coatings.

The S38 I'm building now has a clearance of 0.0045 - 0.005" with original pistons after having the cylinders honed, so it's below the 0.006" wear limit but not by a whole lot. For ~$30 per piston I'm tempted to give it a try. If it's as durable as they say it is then it'll be a huge cost savings over buying a new set of pistons. If I was seeing clearances like you measured I wouldn't worry about it. You're probably in pretty good shape if you've been able to repeat your measurements.

I've also talked to MaxSil and it sounds like they offer a nice product for a pretty reasonable price. If you want to start over with brand new pistons I think I'd go with them over a forged set.
M5BB
Posts: 1549
Joined: Jun 13, 2007 10:13 AM
Location: Peachtree Corners, Georgia

Re: S38B35 Piston Running Clearances

Post by M5BB »

It's been a while.

I am running CP pistons in my built S38 B35 (see below)
I went to 94mm.

Seems like by the time you go through all these steps you could go with forged pistons and be done.

Mine have been in motor for 10 years now with 11:1 compression.
Great way to make some power even with stock cams and timing.

Not sure what clearances are and would have to look up but trusted machinist to do what was specified.

Just saying.....
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