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]

Something I should have added in the first post.

CNC machining has massively improved since my camshaft was cut.

I would expect perfect base circles in modern camshafts.

If you have a modern cam and steel pushrods, I believe the chance of you having a problem with this is nearly zero.

Now, after seeing this crap in mine I do know what I would do at different points in my build.

Brand new camshaft BEFORE installation: Place on V-blocks and check base circle run out and total lift.
You can make V-blocks out of anything hard. Don't have to be fancy here. Oak V-blocks would do in a pinch...

New camshaft installed in case: Using a dial indicator directly on the lifter with a dialed in degree wheel on the crank nose. Base circle run out, valve open and close points (angles) LCA (angle/lift). I'd do this on all 4 cylinders.

I'd "degree the cam" and check lift and run out basically.

Any deviations and I'd be speaking with the camshaft manufacture.




I HIGHLY dislike ZERO lash! (IMG:style_emoticons/default/headbang.gif) (IMG:style_emoticons/default/headbang.gif) (IMG:style_emoticons/default/headbang.gif) (IMG:style_emoticons/default/headbang.gif)

This gives you ZERO safety cushion for any valve wear at all.
Any wear and you're hanging it open, the valve overheats (if the car still runs), and it burns the crap out of the valve, seat, and maybe the head.

I'd set a zero-lash engine at .001-.002 initially. Run it at least an hour "loose" before going back in and taking lash to zero.

This is ONLY after I did my "full monte" hand lap job on all of the valves.

Fresh from the machine shop (which I NEVER do anymore, they don't get the springs with me) I'd UP those clearance +.002" and run it longer or have two valve adjustments. Run for an hour at .004, tighten to two, run for 10 hours, tighten to zero.

On an engine that is really running zero lash this is the care you have to exhibit.

Now, the zero lash portion of the running is ONLY cold. Once it heats up you should actually have measurable lash. I would expect more than .004" and less than .010".

Has anyone ever verified this?
Checked lash at full operating temp?

I would after that first hour!