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Discussion Starter · #1 ·
I recently acquired a 1988 E28 M5 (with S38 Engine), with 129K miles, and low compression on the #'s 5 and 6 cylinders. I have torn the thing down....head removed, block is still in the car. I was checking for burnt valves on the #'s 5 and 6, but found none. I did however find that valve seal condition on the exhaust valves was not what it should be, so I think this is the cause of my low compression. I have been lapping these valves, which seems to be working. I have been testing the valve seal by using brake cleaner in the exhaust and intake ports, with the springs installed.

Any reason why I should be able to reuse these valves?

I'm not new to M88/ S38 scene...I recently completed a 1985 E24 M635 engine rebuild (M88/3 engine), so I know my way around the motor pretty good. I replaced ALL (24) of the valves on that rebuild, because the previous owner had buried all 4 of the #5 cylinder's valves into the piston...which I still have, and can see daylight through....Different circumstances, so I wondered if I could get away with re-using the valves in this case. Their condition doesnt appear to be too bad.

Thanks,
Daniel
 

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I'm not new to M88/ S38 scene...I recently completed a 1985 E24 M635 engine rebuild (M88/3 engine), so I know my way around the motor pretty good. I replaced ALL (24) of the valves on that rebuild, because the previous owner had buried all 4 of the #5 cylinder's valves into the piston...which I still have, and can see daylight through....Different circumstances, so I wondered if I could get away with re-using the valves in this case. Their condition doesnt appear to be too bad.
Exhaust valves-and seats have a hard time in any S38/M88 and therefore I concider them as consumables. Therefore IMHO, it is wise to replace all (12) of them. The intake valves run rather cold and can be reused.

During my M88/3 rebuild, I replaced all 24 valves as well. The exhaust valves for the above mentioned reason; the intake valves because of their relative low cost, @30 Euro/pc (ex VAT) it didn't make sense to cut cost in an 8k (Ex VAT) project. That was my simple reasoning. I also specified the replacement of the valve-seats by sintered types.
 

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Discussion Starter · #3 ·
Raymond, Thanks for the response. I read your rebuild of the M88/3, and was hoping you'd see this post, and chime in. I sent the head off to get hot tanked/cleaned last night, cause I dont have enough cans of brake cleaner and toothbrushes to do it myself! The machinist is supposed to grind in the intake valves in...they looked fine after I wire wheeled them clean, and I'm going to get a new set of exhaust valves. I used a different machinist when I rebuilt my M88/3 (about 2 years ago now), and I was able to get intake and exhaust valves for $20 apiece! I am hoping that I can do that again.

I am ordering a new set of timing guides to do the chain while I'm inside the engine. you have any thoughts on piston rings? I did a leakdown test, but since I was not able to get the exhaust valves on the #'s 5 and 6 to seal, there really was no way to check the ring condition. The bottom end has 129k miles on it, and cylinder condition looks good(block still in the car). You see any real good reason to fool with rings? Oil left in the cylinders for over a week hasnt drained. If you hadnt discovered it, BigCoupe.com is a great site for 6 series stuff, but there are alot of M5 owners there, since the engines are the same, and there is alot of valuable technical info there.

Thanks for the help.
Daniel-
 

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The machinist is supposed to grind in the intake valves in...they looked fine after I wire wheeled them clean, and I'm going to get a new set of exhaust valves.
That already is an excellent approach; the intake valves aren't highly stressed. Will you replace the valve sprins and collets as well ?

you have any thoughts on piston rings? I did a leakdown test, but since I was not able to get the exhaust valves on the #'s 5 and 6 to seal, there really was no way to check the ring condition. The bottom end has 129k miles on it, and cylinder condition looks good(block still in the car).
The only method to determine actual wear and thus condition is by measuring the cylinder-to piston clearances. When I dropped my short block at the machinist, his first analysis was that the cylinder walls looked fine and no new pistons were needed. However, that changed after he performed his measurements, forcing me to leave my original plan of just replacing the bearings and piston rings combined of a rehone of the cylinders. I could have insisted on a second measurement of the pistons but that would not have changed anything as I already had decided to buy a new set of pistons (first oversize). These come with a new wrist-pin and a new set of piston rings.

At 129k miles, the short-block usually is still within the first halve of its expected life. It's the head that needs major work first. However, I wouldn't feel myself safe without inspecting the bearings and piston rings before making such decisions. This also applies to the waterpump
If you hadnt discovered it, BigCoupe.com is a great site for 6 series stuff, but there are alot of M5 owners there, since the engines are the same, and there is alot of valuable technical info there.
Thanks, I wasn't aware of that site. I will take a look at it later this week.
 

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Discussion Starter · #5 ·
Sounds like the same thing happened with your pistons and mine. I was going to replace only the piston that had 4 holes in it from the valves. (Supririsingly the cylinder condition was fine...there was no scoring). We measured the clearances on the other 5, and found that they didnt meet the standard. I've attached a link to a few pictures from the rebuild, and another link to the story. It was a learning experience...having never rebuilt an M-motor, it was a great way to cut my teeth.

http://s1228.photobucket.com/albums/ee447/spankeysM635/

bigcoupe.com :: View topic - M635 Rebuild Story=
 

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Discussion Starter · #7 ·
Thats neat! At least those pistons are doing good work now! I (and my machinst) were both surprised, because we didnt have to do any grinding on those things to balance them all. All 6 of the pistons came from the factory perfectly matched in weight. I threw alot of parts at that rebuild. You can see where I had to take a die-grinder to the head to fit the Schrick 272 cams....I would have used my old BMW spec cams, except that they were chipped and I didnt want to risk spitting out one of my shim discs. Incidentally, you may be interested to know that the 31mm shims that we have in our cars are exactly identical to the 31mm shims that Volkswagen uses...thats how I shimmed my car. They work fine, and are significantly cheaper than the BMW shims. Same stuff really. For anyone else reading this thread, the S38 engine (US spec) uses 33mm valve shims in the buckets...these are identical to Volvo 33mm shims. Much cheaper and easier to get a whole kit than a BMW shim kit.
 
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