M20 - No compression (broke/replaced timing belt)

[jeffnhiscars]

I missed that pic. Not worth fixing for sure

[13Stripes]

It's not even worth it dude. A used good head is like $100-200.

That's basically what I was thinking. Just trying to determine "good" head from online posts.

[Kershaw]

It's not even worth it dude. A used good head is like $100-200.

[13Stripes]

Pics aren't very clear but don't assume the head is beyond use. Often..but not always...the impact from the piston hitting the valve flexes the rocker shaft and breaks the rocker towers. If there is no terminal damage to the casting, then guides, valves and the usual refresh could be enough.

A good machine shop can tell you whether you need to replace it or not. Even a good used head will need machining so whether you choose to spend your $$$ on a used head in unknown condition or to improve the one you have is up to you.

Agreed, the problem is that the head broke at the rocker shaft. Image1 in my original post has a pretty clear picture of that break (also attached here). I'm taking it to a machine shop today or tomorrow to see if they can weld it, but also preparing for the reality that I may need to replace.

[jeffnhiscars]

Pics aren't very clear but don't assume the head is beyond use. Often..but not always...the impact from the piston hitting the valve flexes the rocker shaft and breaks the rocker towers. If there is no terminal damage to the casting, then guides, valves and the usual refresh could be enough.

A good machine shop can tell you whether you need to replace it or not. Even a good used head will need machining so whether you choose to spend your $$$ on a used head in unknown condition or to improve the one you have is up to you.

[13Stripes]

Hey everyone, I got into things a bit further this past weekend and wanted to provide an update (for those who may be interested).

I disconnected all of the fuel lines, hoses and wires from the head. Unbolted the exhaust manifold from the head (this was a PITA, but only one broken bolt in the process). Removed the head and inspected the pistons.

Good news is that the pistons and cylinder walls look to be in pretty good condition. No damage at all to the top of the pistons. There was some carbon buildup (more on #2 and #5 than others), but taking a brush and cloth cleaned it up pretty well. No scores in the cylinder walls. Interesting that it appears the exhaust valves are more bent than the intake valves, but there are visible issues with both which clearly explains lack of compression (as many of you suggested).

Next step is to locate a decent head and have it rebuilt. I'm going to replace the clutch and A/C compressor (never worked) while it's all apart. Based on recommendations here, I'll get a new timing belt as well (they're cheap enough). Starting to compile my list of parts to order now. I am still on the fence about keeping a stock cam or going with the Bimmerheads 274.

Pics below of the pistons and valves.

[jeffnhiscars]

The timing belts are not one time use. Age and miles are the only thing that effects timing belts. They do not stretch.

I don't recall anyone saying they stretch since surely they would jump a tooth eventually if they did. I do recall guidance that once they have been tensioned they should not be reinstalled.

[Cabriolet]

From what I've heard you want a new timing belt too, those are one time use. The belt alone is pretty cheap.

The timing belts are not one time use. Age and miles are the only thing that effects timing belts. They do not stretch.


As said above ARP studs are great but you don't need them. I use them on my builds but those are not budget builds. Bmw head gaskets and bolts are hands down the best for our needs.

OP you don't need to worry about the pistons. 99% of the time they are fine.

[E30-325iS]

I suggest ARP head bolts.

Complete overkill on anything but a highly boosted engine.

[Old525i]

I suggest ARP head bolts.

[modernist]

In any case use quality gaskets and rebuild parts. I use all OEM BMW gaskets and head bolts in my rebuilds.

Yup, especially head bolts. Elring gasket sets are excellent as well. Stay away from Victor Reinz head gaskets.

[101]

Agree with everything said above.

The M20 engine in my E28 snapped a timing belt at low engine rpm's. The pistons had some minor valve "kissing", but were otherwise OK for reuse. I did not try to do anything with the head - because eta. Found a used head and went on with life.

Depending on your budget and the amount of engine damage you find once the head is removed - you can replace the head with a used head (after having it checked at a machine shop), swap the complete engine with a used one (if pistons / bores are damaged), or go for a higher performance head such as the one offered by "Bimmerheads".

In any case use quality gaskets and rebuild parts. I use all OEM BMW gaskets and head bolts in my rebuilds.

[turbo55]

From what I've heard you want a new timing belt too, those are one time use. The belt alone is pretty cheap.

[13Stripes]

My dad's timing belt skipped 2 teeth. I thought maybe I could set the timing back 2 teeth and hope the valves were not toast. The engine would turn over slowly and made a whirling sound also. The engine had no stroke/compression sound like a normal engine. When cranking a healthy engine over you can hear the number of cranks/revolutions of the engine. This engine had no such sound. I took the head off and found 6 bent intake valves. The pistons bent every intake valve just slightly enough to have zero compression on all 6 cylinders.

Got it. That's insightful. I most likely have bent valves in each cylinder then also, and explains no compression. Thanks!

buy a bentley's manual
the head is toast
you will need another good head. have it decked and pressure tested.
you will need new BMW head bolts and head gasket.
then replace the timing belt, water pump, get bmw coolant and an oil change.
it takes about a week for most first timers and will run you about 800 to fix it correctly.

I have a Bentley's manual. It's fantastic.
Timing belt and water pump are brand new.
Already started shopping for a head, however, sounds like there is strong potential for piston damage.

Starting my shopping list now. Not building a race car, but while I'm spending cash, I'd like to make it a little faster. I'll hit the search function today for more info, but if anybody is scrolling through here and has recommendations/experience with swapping in a replacement engine with some upgrades, I'd like to hear what you've done:

Replacement head (or complete engine)
Rebuild head with bimmerheads cam and better-than-stock rocker arms/valves?

[Cabriolet]

buy a bentley's manual
the head is toast
you will need another good head. have it decked and pressure tested.
you will need new BMW head bolts and head gasket.
then replace the timing belt, water pump, get bmw coolant and an oil change.
it takes about a week for most first timers and will run you about 800 to fix it correctly.