Cam timing - what to trust ?

[acarlson]

I have my crank and cam installed. The keyways line up correctly - cam at 180 degrees and crank at 270 degrees. However, the timing marks are slightly off. See picture. If I move the cam 1 tooth left or right the timing marks would not even be close. I've seen arguments both ways - FlyCorvair suggests only using the keyways since the timing marks are not always accurate. Others say the timing marks must line up. Your thoughts ???

[acarlson]

I have my crank and cam installed. The keyways line up correctly - cam at 180 degrees and crank at 270 degrees with #1piston @ TDC. However, the timing marks are slightly off. See picture. If I move the cam 1 tooth left or right the timing marks would not even be close. I've seen arguments both ways - FlyCorvair suggests only using the keyways since the timing marks are not always accurate. Others say the timing marks must line up. Your thoughts ???

Cam.jpg

[terribleted]

That looks right to me. The actual marks on the cam and crank never exactly line up that I can see. If you moved that gear one tooth either way it would be even farther "off". I go by keyway locations and if I can see the mark thru the bolt hole like your photo:)

[66vairguy]

Actually the "O" was not stamped accurately on some GM cam gears and is usually incorrectly stamped on the tooth of the aftermarket cam gears (a mistake that is also in the shop manual diagram). The "0" should be just CCW of the tooth (between tooth and valley) and you'll see that would make you crank gear notch line up.

The best procedure is to accurately mark a line centered on the keyway and that should line up with the "notch" on the crank gear, but yes it may be off a little as the line has to be very accurate and most don't have the tools and approximate (the line should be slightly CCW of the tooth).

You did not mention what parts are new. I installed a new cam, crank gear, and cam gear. This obviously allows for a number of things to be "off".

The best practice is to "degree" or "index" the cam with a degree wheel once it is on the block. Since the Corvair engine turns reverse vs most GM engines you'll need a degree wheel marked both CW and CCW. You'll also need a dummy lifter and the cam timing card as well as an analog dial indicator. Don't be surprised if your results are off a little. If the error exceeds one tooth on all measurements you are off a tooth. Do use the piston/crank to find TDC. There will be a few degrees of "null" (no movement at TDC) and the center of the null is TDC.

[martyscarr]

Since the Corvair engine turns reverse vs most GM engines you'll need a degree wheel marked both CW and CCW.

Here's a link to a Corvair degree wheel that goes on your harmonic damper or crank pulley: http://www.n56ml.com/corvair/degree_wheel.pdf

And some instructions for using that wheel: http://www.n56ml.com/corvair/degree_cam.html

If by chance you already have a conventional degree wheel, you can put it on the flywheel end of the crank.

HTH
Marty Scarr

[acarlson]

You did not mention what parts are new. I installed a new cam, crank gear, and cam gear. This obviously allows for a number of things to be "off".

Ahh, I should have mentioned that. This is a new cam and failsafe cam gear installed by Clark's.

I am pretty sure the alignment is correct since moving the cam one tooth either way would make the offset worse. I was going to index the cam anyway but I figured I'd see what ya'll had to say before going through setting up the dial indicator and timing wheel...

Thanks -

Alec

[66vairguy]

I should not have said the aftermarket cam gear "0" is incorrectly placed. Terrible Ted correctly pointed out you can see it through the bolt hole of the crank gear. With the proper GM "0" location it lines up, BUT you can't see it through the bolt hole!!! By moving the "0" mark up on the center of the tooth it is clearly visible to check even if it is offset a little.

[skipvair]

If you are worried about it, degree it.


Sent from my iPhone using Corvair Forum mobile app

[Bill MC]

https://www.youtube.com/watch?v=GAyOnnNrJlM

Dave has a great youtube video on degree'ing a Corvair cam.

[notched]

If it is an aftermarket cam I would check it with a degree wheel. My custom grind Crower was 16 degrees because they cut the key way slot in the wrong spot! Dave has had similar experiences.

[Bill MC]

I went with an Isky mild 260 grind cam. It clocked in spot on the wheel

[acarlson]

If it is an aftermarket cam I would check it with a degree wheel. My custom grind Crower was 16 degrees because they cut the key way slot in the wrong spot! Dave has had similar experiences.

I have an OTTO-20 cam and I did degree it. Except for the intake close setting it was pretty much on the money. However, I have seen 3 different OTTO-20 cam sheets - all of them different by a few degrees here and there. I found that curious. However, I was really just checking that I had the cam and crank in sync since the timing marks were a little off. I torqued the case halves together and started the reassembly. Ran into a question about the oil slinger - that's another post.

Alec