Timing Belt Questions

[ibmer]

I recently purchased a 1990 BMW 325is which ran fine, and needed only
minor cosmetic care. I am now learning about "interference" engines.
I am also learning why when I asked my mechanic for an estimate to
repair the damage (caused, no doubt, by age and a backfire after
I power washed the engine.), he advised me that the car was "toast" .

- $1500 labor to replace the belt and head.
- $500 plus parts to rebuild the head.
I thought that too much to spend on this old timer, so I'm doing it myself.

Chilton says the Torx screws and bolts can not be reused.
Haynes recommends cleaning and chasing the holes and bolts.
I did the latter.. Comments?

Both manuals refer tangentially to a sping belt tensioner.
The tensioner in the car has no spring.
None of the replacement tensioners come with a spring, but I have found
a tensioner spring and spring bolt in a parts list.
Chilton recommends that the tensioner be replaced when changing
the belt, and that it be upgraded to a compatible tensioner.
The roller bearing in the tensioner looks fine to me.
I hate to replace it just because I don't know how to set the tension.
My gut says it should be snug enough to take up any slack after
I've spun the crank shaft a few times. ( About what a spring might provide.)

Can anyone offer advice?

[SRR2]

Yeah.

1) Get the Bentley manual.
2) Don't reuse any fasteners that are either a) self-locking or b) holding something difficult to replace, having the potential to cause damage, or holding something important to engine function. Fasteners are cheap compared to the damage their failure can cause in critical areas.
3) For parts specific to your vehicle go to realoem.com and plug the last 7 digits of your serial number into the configurator. You will be presented with all the details of what's in your car and all the part numbers. realoem.com is one of the truly great (and free, to boot) resources available to BMW owners.
4) Never, NEVER power wash an engine!

[Kalevera]

You might not want to waste your time rebuilding the head. It would probably be cheaper to drop in a good used engine....HOWEVER...it's hard to find a low mileage M20/one that probably won't need head work in the near future.

I think your mechanic is charging a bit higher than the norm, so good for you for taking the initiative to do the work yourself: I don't have anything to do with the administrative side of my business, but I believe we charge something around $500 inclusive of parts to rebuild an early M30/M20 head, and head replacement (including the used head) is a $1000 affair. The belt costs ~ $40 from most retail places, and a novice could install it in an afternoon (but run the potential risk of ruining the head by not following the directions).

As Steve said, get the Bentley manual. I suggest that the fact that the tensioner didn't have a spring helped to destroy your engine (not uncommon, and I've found worse). Have a look at this diagram, the spring is #18:

http://www.realoem.com/bmw/showparts...85&hg=11&fg=25

The tensioner does need to be replaced, and should have come as part of the timing belt kit; just good preventative maintenance. It's a bearing that can fail, and they do fail (but usually not before the belt does).

Before the belt is installed, the tensioner and spring are pushed until the spring is at maximum compression (the only way to get the belt on), and then the 13mm headed bolt should be tightened to hold the tensioner in place. The belt should go on tight -- the slack should come up through the tensioner side, not the crank to cam side (the engine always rotates clockwise). I manually rotate the crank twice (one full revolution of the engine) and check timing again (hopefully, the engine was set to TDC before the belt was removed, if not -- be CERTAIN that the thing is in time before it's put back together). Then, the 13mm headed bolt is loosened, the spring+tensioner tighten the belt, and the bolt gets tightened again to set the tensioner. The engine gets rotated a few times to confirm that the thing is still in time, and by then you're probably good to go.


best, whit