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!
(IMG:style_emoticons/default/ninja.gif)

[technicalninja]

I don't see how there could be a difference between TDC or Krusty.

For each method the valve opposite the one you are adjusting is on cam, making it impossible for the adjusting valve to be on cam no matter what version you use.

Or are you saying that your cam itself was wobbling .002 in its bores, and it really didn't matter what version you used?

Back when I had steel pushrods, my "zero" was more like .001 off. I'd roll it in till it just kissed, back off and tighten the nut 9knowing the nut was going to tighten the adjuster slightly.)

Did you account for the nut tightening the adjuster a smidge?
Zach

The base circle on the cam was made/cut improperly, off center.
Nothing wrong with the engine, 31K 120 comp on all, leak downs of 5% cold (and sitting for 2 months).

The point of this post was to make members aware of a possible problem.
I didn't say "All will do this".

If you use steel pushrods you should check base circle run out before you do!

My adjusting method account for everything including lock nut torque.
Most lock nuts pull .0007-.001 out (or sometimes add! L-series Nissan) to the adjustment.

I have also gone through the BS of setting up a dial indicator to verify how accurate my feeler gauge method is. I did not do this on my T4 as it is in-chassis.

Most folks adjust too loose...

When I'm good -.001 gauge is floppy, +.001 will not fit at all, and correct is tight with a certain amount of drag. This drag will be the same on all of the valves before I'm happy.

Many engines list a range. I choose a single number inside the rang and hit that.

I have adjusted hundreds of sets of valves over the last 40 years...

And it's a little funny. Most of the time, when I can see the cam, I set the LCA directly away from the rocker or bucket.

So, for ALMOST everything I'm going Krusty automatically!
I almost never do TDC for each cylinder. Seldom is TDC marked on the crank pully for more than two cylinders.

Especially on shim over bucket this is the BEST way as it's easier to remove the shim with the cam lobe pointed directly away...

Final point, when the valves are adjusted this way, I will commonly end up with compression test that absolutely match.
First time I did one that went perfect I didn't believe it.

The single biggest reason for compression test variation is intake valve closure point.

Each .001 of adjustment variation produces 3 degrees of camshaft duration change.
I have used this trick to see if 10-15 degrees more duration would help an engine.