HG replacement question ('90 535i)

[Dash01]

Preparing to replace the HG on my '90 535i, I contacted a local expert. He says the engine also needs new crank bearings and piston rings, as stated in the following:

"Your cylinders are close to round with the head off....then you tug at the four corners of each hole with head bolts that distort the circular shape.
Your piston/ring assembly has to "machine itself" to fit this unique configuration.

New pistons, with new rings on freshly machined cylinder walls do a good job of this if "seasoned" properly.

Once you have unbolted the head, the block relaxes a bit from the "button-hole" shape....then you replace the gasket and tug again, but the new distorted shape is different than before.

Even if you do not touch the pistons, the contact between the piston/ring assy is different and it can be impossible to get the "used" rings to seat into the glazed cylinder walls.

To avoid this, experienced mechanics always pop the pistons and install "repair rings" (if the grooves in the piston are still servicable) and run a "dingle-berry" hone down the cylinders to "break the glaze".

After the cylinders are cleaned, a lot of debris has contaminated the crankshaft, and the bearings are old, so replace them.

Without pulling the pistons, you cannot inspect/re-radius the oiling holes in the pistons.

...A proper valve job can double the life of the motor...a botched one will do more damage than if nothing had been done..."

[B]So, has anybody with a 535i engine found this to be true? Do others also undertake this extra work when doing a HG replacement? Have others also found trouble by NOT replacing the bearings & rings? What is the consensus view on these questions?/B]

[Martin in Bellevue]

Sounds like somebody need to make his boat payment.

The m30 bottom end is pretty robust.

A good machine shop can replace the valve guides & true the head's surface in an oven, if needed. Do clean out the headbolt holes in the block thoroughly.

[Dash01]

Well, since nobody that I recall here on Bimmerforums seems to have had this issue, and since Bentley does not mention it, I'm skeptical too.

Still, I'd like to hear of the insights/experiences of others with HG replacement in the M30 engine.

Whatdya say, guys?

[zygoteer]

Having dealt with this engiine, I believe that it is probably classified as 'innovative income generation'

[Mr Project]

I agree. I had a shop straighten the head slightly (less than 7 thousandths) and clean it up before I re-installed it on the untouched (other than a good scrubbing of the piston tops) bottom end. This was at 180k, the motor has 200k now and runs better and smoother than ever. No loss of compression, or increase in oil consumption.

Maybe on a 25 year old SBC I'd want to pull the pistons and do a quick hone, but not an M30 if it's in otherwise ok shape.

[bill g]

There may be some sense in the theory.
However I have replaced at least a dozen head gaskets in my time on various engines, including my M30 about 9 months ago. As the engines were not noisy or burning oil or lacking power beforehand I did not think it necessary to carry out all that other work and I never had problems with ring sealing, poor compression or failed bearings afterwards.
Be meticulous when cleaning block surface etc. and dont allow scraped off material and dirt etc. to fall into cylinder bores, oil and water passages.


Bill G Melbourne Australia

[joshua43214]

Sounds like a nice way to make some extra money for a shop.

The only time I have ever worried about the bottom end on an engine when doing a head gasket, is when the car has been driven arround for an extended time with coolant in the oil. This will cause rod and main bearing failure after a time. I have done several hundred head gaskets and only seen it a few times, and each time I was able to determine before hand that the risk for bearing failure was high, or they already had a knock.

Unless the engine has been driven for quite a while with coolant in the oil, I would just replace the gasket and be done with it. I also only machine heads when they are warped, just to prevent the added variable of machine shop mistakes.

Also by the logic of the shops statement, honing the bores would not be enough, you would have to rebore them round so they would properly fit the pistons. My guesse is that they got burned on an engine once and now every one that needs a head gasket gets bearings as well.

[Dash01]

The only time I have ever worried about the bottom end on an engine when doing a head gasket, is when the car has been driven arround for an extended time with coolant in the oil. This will cause rod and main bearing failure after a time. I have done several hundred head gaskets and only seen it a few times, and each time I was able to determine before hand that the risk for bearing failure was high, or they already had a knock.

Unless the engine has been driven for quite a while with coolant in the oil, I would just replace the gasket and be done with it. I also only machine heads when they are warped, just to prevent the added variable of machine shop mistakes.

Mine was driven ~1/2 mile in overheat condition, just to get off the road. I think some water did mix with the oil, though, as the oil cap has chocolate colored crud on it. When I drained the coolant, it looked normal and no oil whatever. The dipstick and oil filter oil looks dirty, but otherwise normal.

I'm gonna have the head checked by a good local shop, and re-heated or resurfaced if needed. May also have the valves and guides done, etc..