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M90 engine overhaul

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janusz200
Posts: 34
Joined: Mon Mar 11, 2013 6:09 am
Location: Warsaw, Poland

M90 engine overhaul

Post by janusz200 » Sat Sep 29, 2018 5:09 pm

I have to take off the engine and fix the valve tightness and crankshaft axial play. This is also the opportunity to check the rest and probably to overhaul the block – machine the cylinders and replace the pistons. Do somebody know whether factory block overhaul manual exists as a separate publication beside of general repair nmanual? My engine is M90 (3453 cm3/bore 93,4mm – motor up to 1981 models). I appreciate all your advises about motor overhaul.

Drew
Posts: 27
Joined: Mon Mar 11, 2013 7:37 am
Location: Hiding in the garage, South Bucks, England

Re: M90 engine overhaul

Post by Drew » Sun Oct 14, 2018 10:48 am

Can't help with your question, just a reminder to get the correct CH gasket on rebuild. The 3453 engine has a very different gasket to the 3430

bpoliakoff
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Re: M90 engine overhaul

Post by bpoliakoff » Tue Oct 23, 2018 6:38 pm

Janusz I had my M90 rebuilt at an engine machine shop. Standard engine building tolerances were used nothing other than M30 fitment. Pistons were made to spec and fitted as true 10:1 giving required clearances at cylinder walls and valve interferences. valve adjustments were provided by the cam shaft grinder at ten thousandths at the cam. There was nothing out of the ordinary about rebuilding the motor other than making sure all the specs were right on and again we used standard BMW specs as used for an M30. The engine was carefully balanced and all ports were cleaned up and matched. Engine builder estimates at least 45 HP over stock but that will be found out when the car is running and goes on a dyno
81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
flickr.com/photos/bertsphotos.

88m53453
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Joined: Mon Mar 11, 2013 12:44 am
Location: Butler, GA

Re: M90 engine overhaul

Post by 88m53453 » Mon Nov 05, 2018 1:56 am

Pretty much everything Bert said, My 79 M90 was extremely strong, hi comp forged pistons, cam and Stahl headers and it easily handled e30 M3's at Summit Point 30 years ago. Also a e9 with supposedly 12:1 and triple side drafts that the builder claimed well over 300HP... I doubt it, but it felt good anyway. M90 is a great engine properly built. It's been a long time, but I think my cam builder spec'd 12 thou. I got the chance to drive a track prepped m635 a few years later and it spoiled me lol

bpoliakoff
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Re: M90 engine overhaul

Post by bpoliakoff » Mon Nov 05, 2018 2:38 am

My cam grinder specd 10 thou but that could be at the cam not sure. Probably. My cam is a regrind. It is a 284 and I am using oversized eccentrics
81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
flickr.com/photos/bertsphotos.

janusz200
Posts: 34
Joined: Mon Mar 11, 2013 6:09 am
Location: Warsaw, Poland

Re: M90 engine overhaul

Post by janusz200 » Sat Dec 15, 2018 5:30 pm

My engine is in pieces now. Cylinder head - I have to change the valve’s guides and sealings. Crankshaft – rod journals are scratched – have to be grinded.

The block - cylinders 1…5 are worn of 0,03-0,04 mm (0,0012-0,0015”) measured across the crankshaft below the "fret" at the top (I mean upper end of top ring travel). Pistons lost max. 0,015 mm (0,0006”) 13 mm above the skirt’s bottom (measuring as recommended by shop manual). Total wear between piston and cylinder is max. 0,12-0,14 mm (~0,005”). Not very good but no disaster.

But the cylinder no 6… Max. wear at the top measured across is 0,08 mm (0,003"). But is twice bigger measured at about 45° to the engine axis. This means the rod is bended most probably. Anyway – I don’t like to leave it like it is.

I have the following possibilities:

1) Buy custom oversized pistons (original Mahle are not available no longer) and machine all cylinders. Proper solution according to the state of art but… Piston manufacturers (JE Pistons/Wiseco and Ross) recommend clearance of 0,004” (~0,1 mm) between piston and cylinder. In factory manual there’s half of this (0,045-0,055mm). Maximum allowable wear clearance is 0,15 mm. I’m not a specialist but it looks for me like their alloy gets higher thermal expansion than old BMW’s. Does it mean that engine will be noisy working cold? Can it cause higher wear of piston’s skirt low part? I suppose their pistons are very good for sport engines but not for classic car driven occasionally. Last but not least the price of custom pistons are beyond my budget (see viewtopic.php?f=7&t=30466).

2) Leave the original pistons, polish them (some small scratches available), replace the rims and put the cylinder sleeves into the block. Looks attractive due to clearance of forged custom piston’s problem as well as price-wise .

3) Do as above but only for 6th cylinder. I feel that it’s not the good idea.

All your opinions are highly appreciated.

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