M20 - No compression (broke/replaced timing belt)

[Old525i]

I suggest ARP head bolts.

[E30-325iS]

I suggest ARP head bolts.

Complete overkill on anything but a highly boosted engine.

[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.

[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.

[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]

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]

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.

[Kershaw]

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

[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.

[jeffnhiscars]

I missed that pic. Not worth fixing for sure