1 Cylinder Low Compression - New Build (M52)

[f0xy]

So I built a fresh M52B28, using an iron M50 block, after pulling the threads on an original M52 aluminium block. Everything was replaced. New mains shells, big end shells, piston rings, rod bolts. Block was honed and decked. Head was skimmed and pressure tested. Valve stem seals replaced and valves re-lapped. Vanos unit rebuilt. Built up with a new stock head gasket and new head bolts etc.

Ran the engine in for 500-600 miles on mineral oil. All working well, no issues, no leaks. Changed the oil+filter, then took it to the track. Ran all day without issue, performance was good, no coolant or oil used, as per https://www.youtube.com/watch?v=R33OFQ0PnGk

On the way back from the track, the engine had developed a missfire at low RPM. It was quite subtle at the time, but I could tell it wasn't as smooth as it should be. It had got slightly worse by the time I got home (20mi). I loaded up INPA and cylinder 1 was way off the scale on the engine roughness test. I also had a knock sensor fault code stored, but not present.

I changed the spark plug, no difference. Changed the coilpack, no difference. Changed the injector, no difference.

Next I compression tested it, cylinder 1 only came back with around 80psi. The other 5 cylinders were all 200-210psi. I repeated the test on cylinder 1, but this time wet, although the reading did not increase.

After this I decided to pull the head off, all the head bolts were the same torque to remove. The only odd thing I could see initially was the piston tops on the inlet side. Not sure why cylinder one might be different to the rest.





Leakdown on cylinder 1 OK.



Some funny marking on the head side of the gasket, on the inlet side..



Which related to some weird markings on the head itself..





All other chambers/bowls on the head are completely smooth, yet cylinder one seems to have this step/marking you can feel with a nail. The head was fine when skimmed and pressure tested before fitting..



Does anyone have any ideas what may have caused this? A bad headgasket? Wrong bore headgasket? Cracked head?

Cheers

[rich318i]

which pistons did you use? steel liner ones?

[f0xy]

M52B28 pistons + rods from an Alusil block, which the engine started its build from. M50 block was used (honed, decked) after the M52 block pulled its head bolt threads on assembly. New piston ring gaps checked and all within spec, etc.

[rich318i]

the blocks can warp also..

[f0xy]

The block side of the head gasket looks OK, no markings/damage, as does the block itself. I cleaned it up and it checks out being flat with a steel rule

I've not bothered taking the piston out of cylinder 1 yet (to check for ring or ringland damage) to try and minimise the number of parts that are going to need replacing, rod bolts etc.

I have spare M50 blocks and spare heads, but I don't want to go re-building a head/getting it running again without knowing what caused the problem this time round... Its probably done 900miles in total, with the last few hundred at high rpm all day long.

[rich318i]

Im wanting for Danthe comments, his the lad to ask but even the m50/m52 iron US blocks do warp and they blame the head at first

Nice vid by the way

[f0xy]

Cheers

Cool yeah, I think this block might even be one I got from Dan (before skimming/honing), as I went through a few months of collecting a couple of them before building after the M52 block threads pulled.

Might be worth mentioning also, my old Nikasil engine used to run 105-108C oil temperature on track, with a 19row cooler. When I ran this new engine with the iron block, I was getting a consistent 10C higher temperature throughout the day on the oil, using the same cooler setup, sensor in the same location. I was undecided whether this might be because of the iron block, or if there was a problem with the engine in the first place..

Coolant temperature seemed to remain normal throughout, same between both engines.

[rich318i]

the treads do pull on iron blocks too not just aluminium ones. What head bolts did you use?

[f0xy]

M10x95mm for M50 iron engine block. All took the tightening sequence fine when built