oversized pistons for M90 engine

[bdorey]

Hi Ralph,

I'm going to look at a 1991 BMW 535i 5 spd this week and if I go that route I will have some M90 parts I don't need. Unless I choose to rebuild this unique motor. I will get some pics of the exhaust manifolds.

Byron

[tschultz]

I wonder where I could get a used crankshaft for my M90? The threaded end of mine broke off. Otherwise, the motor ran great with oversize JE pistons, a 284 cam, 320i intake runners and a 3.73LS diff. I want to get my car back on the road but can't find a crankshaft. Also, will an M30B35 head work on this block? Thank you for any guidance.

I have one but I have to make sure I will not need it for my M90. My motor had an oil pump problem so rod bearings took a beating

B35 head will work but compression will be reduced as the B35 chamber is supposed to be larger. How much larger I’m not sure. Good luck

Ralph

Ralph,

My 635 had a similar problem. Oil presure blew out the filter seal and I apparently didn't have a working oil pressure warning light. Seized up and I am going to have to pull the engine to inspect. I suspect the rod bearings are gone but am not sure if the crank will be. No bad sounds except a light pingint before it slowed in power and simply seized. I am hoping the crank will be ok but haven't pulled it yet as am working on a b35 build first.... Though I did find a M90 crank that I have headed my way just in case...

Can you speak to your experience and the possibility of a damaged crank/connecting rod?

Thanks!
-Tom

[Ralph in Socal]

Sorry about your situation Tom.

The motor I have was from a scrap yard car. I bought it because the bores and pistons were good. Further inspection showed the oil pump bolts had loosened and backed out so it suffered from low oil pressure and took it out on one specific rod journal. Others journals are likely damaged but I have too many other priorities to get farther with the M90

The blown out oil filter oring is the primary issue that caused your M90 failure. Early, single pressure relief valve oil pumps have a history of causing extremely high oil pressure at cold start up. Klaus in Vegas had a similar issue but he noticed while it was happening. Solution was to retrofit a later dual relief valve oil pump into his M90. He's been happy with the fix so I'll be retrofitting newer oil pumps from now on

Ralph

[bpoliakoff]

My engine is the original M90 with custom pistons, I ordered mine from Diamond Racing pistons. The parent company, I found out, is JE pistons. The difference is JE wanted 2-3 months to fill the order, Diamond less than a month Prices very similar and the pistons are the same. You can order them for what ever compression ratio you want. Mine are about 10.5:1 I ordered 10:1, but because the head has been cut, we figured they are somewhere around 10.5:1. I am running on hi test gas, which inn AZ is crap, but do not have any pinging. I am running open loop with a stock 002 ECU, is't year Motronic. Also a 284 cam, reground by Schneider Cams in San Diego and my throttle body has been opened up about 2mm. Because I don't have many miles on it I haven't gone much over 4K RPM. But after2K RPM it comes a live and goes liker a scalded cat. As soon as I get more miles on it, I will get a dyno pull on it. Best part is it cost a bit less than $4300 plus the cam cost for the rebuild all blue printed. Luckily labor costs in Phoenix are very righteous