How can valve clearances get TIGHTER since rebuild??? Options

[Dudleydaftdog]

Hey all, just wondered please if someone could help me get my head round this...

After rebuilding my st205 head and working to Toyota spec (0.15mm - 0.25mm inlet / 0.25mm - 0.35mm) all of a sudden the head developed a loud knocking and a leak at the crank seal... After checking all of clearances (forged rebuilds now done 2000 miles) no1 cylinder exhaust valve nearest the cam pulley has shrunk to 0.20mm!!!. After turning the crank by hand it gets harder to turn as this valves opening...

I rebuilt the head with JUN valves and springs and HKS 256 cams. All was in spec after the rebuild. Thought clearances opened up as the engine broke in not got tighter! The engine was stone cold (left overnight) when I checked. The head was also proffionally rebuilt...

Could this be a worn valve seat possibly? Any thoughts/advice or previous experances pike this would be really appreciated.

Thanks

Chris : )

[Special_Edy]

The cam lobes and lifter shims are subject to slight wear. The majority of the wear to the valve occurs at the valve face.
The exhaust valves are especially prone to valve seat wear due to the heat of the exhaust gasses. Intake valves are cooled by the fresh air and fuel flowing in and generally wear slower.

I have a picture of a burnt exhaust valve from my old 5S head, the valve will literally shrink in diameter, causing it to sit farther into the head and the valve stem to be closer to the camshaft.

Try to reshim the valve into spec first and check it after a couple hundred miles. If there isnt a thin enough shim then a new valve and possibly a new valve seat will be needed.

The worst thing you can do is continue to drive it because as the clearance shrinks the rate of wear is accelerated (because the valve is open more and therefore cooled less by the head)

This post has been edited by Special_Edy: Mar 19, 2014 - 11:41 PM