998 Valve seat distortion
Turbo Mini Forums

Has anyone experienced this? Happening on two heads that have had inserts fitted by a respected builder and after a couple of races the compression drops off and head has to come of to regrind valves to restore compression.

Both fitted with Rimflow valves and bronze guides.?

Spring pressures, may be a Tad high. For the camshaft installed and or the seats may have not been cut correctley in the first place?

Reving to 7700 Reverted it back to single springs and it valve bounces if it revs any more. Pointing more at either seats or how they have been fitted....

The only time I've seen a seat move was whe there was a crack between the valves which expanded when hot.
If the seats are installed properly with the correct interference fit there's not usually an issue.

Phil.

WWW.TURBO-MINI.COM

I've only ever had one seat drop or break up and drop that was because the guy cooked it completely. This one goes really well, does have a tendency to spit some water but most oval racers do reving the nuts off them in second isnt what they were made to do even with a alloy rad and a 16 pound cap. If the valves are ground in again its fine for a couple of meetings and does it again. It happens across all cylinders, which would be hard to put down to a crack, so Seat inserts you think?

sounds like the inserts are a bit on the soft side

When it reaches the point you have to re-lap them and start lapping, is there any symettry or pattern as to where the metal is being removed from first (ie, the bit of the seat which has gone "high").

For example is it allways the bit next to the inlet valve, allways the back, allways the front etc., or is it just random ???

If there is a pattern it might give a clue as to why.

Schrödinger's cat - so which one am I ???

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Album with a load of pics in here, I helped them pull of the head and a buddy pulled the valves and labelled them with a dry board pen by mistake! so couldnt tell which was from which! first time I got a look at the head tonight, we are not allowed to blend the seat into the port at all, but this seems to be using a very thin seat, and there is a very sharp edge on the port below the seat and sticking out almost a mm in places, could this be causing the area to get too hot ? two valves one linlet one exhaust look like they have suffered microwelding, some of the exhaust seats look similar, the oilway modification job across the face of the head is interesting to say the least, it was still weeping from what look like set screws where the fire ring seals when I looked at it. They have used bronze guides which I have always considered wrong for Rimflows, but will check with G and S in morning. The guides have been undercut down as you can see but this extends into the head, you can slide a scalpel blade up 3 or 4 mm between guide and head. Guides internally look to have balled a little and have plenty of play to the valves.

Thoughts?