m30b35 exhaust valve snapped after rebuild. Why?

[Yakinho]

So... About a month ago, I bought a 92 e34 535i with a broken rocker arm on cyl #2. I purchased it because the car has a host of goodies in it that I couldn't turn down. I found another motor for 300 bucks that I took the head off of because I didn't want to deal with removing the PITA rocker shafts (also cracked the oil pan om the ride home due to rhe coilovers having it damn near on the ground...

Anyway, I installed the head, hooked everything up minus the maf and valve cover and discovered a crack in the head along the driver side rocker arms. I decided to take everything back off again to avoid the possibility of destroying everything else in the head and rebuild the other original head. I spent the weekend swapping the cam, rocker, and oil bar from the new engine and bolted it all back up. I then spun the crank by hand 4 times and nothing touched. I then hooked up everything else up, primed the oil filter housing, gave it a few dry turns with no spark, then plugged those in and gave it a go. It ran AMAZING for 30 seconds, until I hear an ungodly clanking noise near the rear of the engine and turn it off immediately...

Took the fan clutch off, spun it by hand again and it would only spin about half way. I took thw front cover off, hoping that I had dropped something and it was jamming the chain, but nothing. Removed the plugs and discovered some lovely pitting im cyl #6. Ripped the head off for the third time in 2 week to discover a broken exhaust valve and a nice hole in the piston.

As fun as it is to spend all free hours of your day and night working in 30 degrees, I'm a little deflated and confused as to why this valve would go. I mean I can now do a headgasket job in 3 hours flat... mostly with the 12/13mm toolkit wrench. Pictures to follow, the valve kiss on the pistom is from the previous owner finding out that (with another head) he didnt set the timing right.


Thanks in advance.

[Good & Tight]

Did you check the valves from the original head ? Or just swap the rockers out? Sounds like #6 was bent and ready to go.

[slammin.e28]

Even if it had a little little bend in the valve, the motion of it being shoved in and out of the seat would be akin to when you bend a paperclip back and forth. Eventually it breaks. Can't check this with your eyes.

Every time I do a head gasket/head, I flip the head upside down, spark plugs in, and fill each chamber with gas, closing the valves on each one, to make sure everything is sealed up.

Kinda shadetree way to make sure nothing is bent, but when all the machine shops with pressure testers are closed on Sunday.....it works.

[Yakinho]

Thanks for the responses guys. I only checked the valves by eye, so i suppose a slight bend in it would make sense. If anyone knows of a B35 head for sale, please point me in that direction.