M20b20 bent valves??

[bealo]

Hi all,

I’ve just lapped the inlet valves on my m20b20 head with no problem.
Started on the exhaust valves and all 6 don’t seem to fit very well in the seats

There is a gap between the valve and the seat which if you turn the valve, the gap is present and follows the valve as you turn suggesting a bent valve

Upon lapping only one small part of the valve is being lapped again suggesting a bent valve

Tried another set of valves and they all sit fine (not lapped yet)

Has anyone else experienced 6 bent exhaust valves or am I missing something

First engine I’ve stripped/rebuilt

The engine was seized when I got it, cambelt was intact

Thanks
Iain

[Jeroen]

If timing is messed up at some point with the M20 engine, the exhaust valves are the ones that are affected indeed. Perhaps the timing belt was fitted incorrectly. Skipping a tooth usually leaves visible damage on the belt unless there was no tension on the belt.

[Duracel79]

Could also be an over rev with a bit of valve bounce.
Earlier engines only have single valve springs so don't like very high revs.
Std engines only peak at 5.8 to 6k so no real need to rev much more than 6.2/6.3.

As Jeroen says the exhaust valves get the closest so tend to get bent first.

Seen quite a few bent exhaust valves, even from reasonably running cars.

Regards

Ben

[BertjeConti]

Yep clearance between exhaust valves and pistons in a M20B20 is verry minimal, i've seen several pistons with hit damage from exhaust valves.
If the head is skimmed, clearence gets a bigger problem.

Thats why i've made exhaust valve pockets in my pistons for a new to build engine.

[bealo]

So you don’t find it odd that all 6 appear to be bent?

I’ve had the head skimmed to I’ll have to closely at valve/piston clearances

[BertjeConti]

So you don’t find it odd that all 6 appear to be bent?

I’ve had the head skimmed to I’ll have to closely at valve/piston clearances

No i don't, when the valve guide's were worn too, the bent valves might still have done their job, because they had some more play, they sealed anyway.
M20B20 engines have the tightest piston valve clearence to get a proper compression rate, because of the short stroke (66mm) .

I've got a set of high compression pistons (M20B20 LJET), and they all had minor hitting damage from the exhaust valves, don't know the history of that motor they came out:



and thats why i reworked those pistons by adding a exhaust valve pocket and made the inlet pocket a little deeper.

[bealo]

Ok thanks for the info that’s very helpful.