Broken valve with 284 cam from stock regrind. m20b25

[dolby]

At this point I am a bit perplexed and why it happened. On the rebuild every thing on the engine checked out. The head is at stock height, the block deck is to spec and pistons, rods and crankshaft are correct. Even with a +0.3mm head gasket the valves touch the piston on closing.

The cam timing is dead on.
No issues with a stock cam.
.290" lift on the cam.
Valve lash was .010" cold.

Anyone seen this before? At this point I am thinking it is a issue with the cam profile and the cam is being replaced.

What could I be missing to check?

Dolby

[dolby]

Anyone have an idea?

[SpecM]

not sure, maybe Nando will chime in. He knows m20 cams.

hope to see you at the Vintage at the Vineyards in May!

[lennon]

never had an m20 with 290 cam but it doesnt surprise me that the valves would hit depending on the exact cam specs. a 284 was very close when i tried it in my m20.

also wtf is .290'' lift haha? i assume you meant 290 degree duration. lift is probably ~11.5-12mm. both are your problem im guessing.

[nando]

whats the lift supposed to be at TDC? a schrick 284/272 will touch pistons on a stock engine. doesn't mean something is wrong with the cam, usually you have to notch the pistons.

[lennon]

wait i just read again.... you already broke a valve?? you didnt turn it over by hand and check before running it?

[Mike]

prolly need custom pistons

[dolby]

Sorry, I meant .290 between the high and low spots on the cam which comes to 11.34 mm when you multiply by the rocker arm ratio of 1.54.

And yes I did turn the engine over before starting it. it was not until the engine warmed up that the valve broke.

From what I am told the 284 cam should not be a problem on a stock head and especially with a +.3mm head gasket.

dolby

[nando]

It will be a problem if it's similar to the schrick profile. you're not the first person to find out the hard way. doesn't matter that much if it's been shaved, you have to measure.

rotating the engine by hand is not an appropriate method of checking valve clearance. you need to measure with a dial indicator at TDC.