Stock camshaft base circle deviations Options

[technicalninja]

I ran into a problem with my stock 75 1.8L that I thought I should make others aware of.

The base circle of my stock camshaft 31K motor had more than .002" run out.

I figure this out when setting the valve adjustment on my engine.

There are multiple ways of adjusting the valves.

The "Krusty" style where you set the opposing valve (across the engine) at full open and the TDC method are the more common.

Both SHOULD have the same play if the base circle of the camshaft is concentric with the cam centerline.

I did mine Krusty style and then re-checked TDC style. Big difference!
Like as much as .002" difference.

Perplexed, I mapped base circle on all lobes.

I had .002"+ run out on all 4...

The lobes were different offset for the run out as well.
Krusty method OR TDC method did NOT equal same offsets.

All this doesn't mean anything if you are using the stock pushrods that require .006-.008" valve adjustment. That much clearance makes .002 run out insignificant.

But if you're using steel pushrods that require zero you will be doomed!
Burned valves will be the result.

Checking base circle run out is NOT a normal thing to check.
I was NOT planning on doing it...

I adjusted Krusty style and merely re-checked during the leak down test (which has to be FLAWLESS TDC).

My first thought was "Dumb ass got the valve adjustment wrong".
I blamed myself!
Nope, not me, original parts deviation!

Had I not been doing the LD tests at the same time I wouldn't have caught it...

There is more than 360 degrees of "base circle" crankshaft movement in any cam design.

IF YOU ARE USING STEEL PUSHRODS WITH ZERO LASH YOU MUST CHECK THIS!!!!

This could be a "built in" Reaper just waiting to kill your engine!
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