Question on valve adjustment for head before installing onto block

[mike87]

Hi all,

This is probably a dumb question, but please bear with me. I recently R&R'ed my M20B25 head and adjusted all the valves to near-perfect spec (0.25mm feeler gauge) with the head off the block. After I bolted the head down, I went to check the valve clearances with the same exact feeler gauge (0.25mm) and found that every single one of them is way more snug that when I had previously adjusted them with the head off the block... so snug that I can't even get my feeler gauge in most of them.

I must be going crazy... I know the short answer is to probably just re-adjust them with the head on the block, but what a PITA...

Anyone knows what happened here? Running brand new rockers and brand new camshaft too.

Thanks,
Mike

[Old525i]

The cylinder head studs compress the head a bit, thus the change in clearance gap. The same happened to me.

[ForcedFirebird]

^^^ yup the head slightly distorts when clamping down. Also, it's best to do a "loose" valve adjustment off the car, then let it warm up and cool down a few times before re-doing the lash. The heat cycles will also cause the head to move a little until it "seasons".

[digger]

70 tonnes of clamping will move things around especially with a squishy composite style gasket and moreso and/or if both surfaces havent been resurfaced.

[mike87]

Thanks everyone, very helpful