Cam Sprocket Alignment

[Matt7c]

Why do the marks on my cam sprockets not align when at TDC?

I'm taking the cylinder head off. There is a nice photo in Wilkins that shows the marks in perfect alignment. On mine, when the marks on the crank pulley are at TDC, the mark on the inlet cam sprocket is perfectly level with the top of the timing case, but the exhaust sprocket is about 5-6mm lower.

Is this normal? If not, what is out of kilter? Cams, chain, crank pully?

Given that the inlet mark is perfectly placed against the timing case, I don't see how the 2 cam sprocket markings could ever be aligned with the current chain, no matter whether the crank pulley is mis-aligned or not. Could the chain be the wrong one? It was replaced by the PO about 3000miles ago (about 16years!).

I started this job to renew the valve springs because of timing issues (despite the timing having been well set, I still get spit-back through cyl2 and backfiring between 2-3000rpm in the overrun). Could this cam mis-alignment be the culprit?

Any tips gratefully received, as ever.

Matt

[Matt7c]

Forgot to say that the sprockets are marked with different numbers. Inlet is 3, exhaust is 9.

[rgh0]

Matt

Tolerances in the original or replacement part fabrication and / or wear in the components can lead to this sort of apparent error in cam timing from the original factory timing marks.

The only way to get the timing right and know it its right is to measure the actual cam maximum opening point on your engine versus the cam specification using a degree wheel on the crank properly set to TDC using a dial guage in the spark plug hole and then using a dial gauge to measure valve opening via the bucket movement. Then use offset dowels or vernier cam sprockets to adjust this maxium opening point as needed.

5 or 6 mm error between the 2 cams though is close to half the cam sprocket tooth pitch of 10mm out and thus the maximum possible error. While this amount of aligment error is unusual between the 2 cams it is not impossible given the right combination of cam sprocket and chain wear, variation in relative cam positions in the head and variation in accuracy of the marking on the cam sprockets versus the target MOP.

regards
Rohan

[Matt7c]

Having studied the cam sprockets and tensioner more closely, there are plenty of signs of wear on the teeth, so the wear theory works for me. I guess I should renew the cam sprockets and tensioner, but should I also assume the crank and jack shaft sprockets are also worn (and also the chain?). I wasn't planning on quite so much rebuilding, but thats life!

[rgh0]

Matt

The crank sprocket wears at least 4 times as fast as the cam sprockets because it has fewer teeth carrying more load at higher speed.

If you have signficant wear on the cam sprockets then the crank sprocket unless its been replaced separately will also be showing sigificant wear.

Its a pain to replace as you have to pull the whole front cover / water pump assembly. You should at least pull the sump so you can see the crank sprocket and assess its condition.

regards
Rohan

[Matt7c]

Ho-hum. The water-pump bearings need renewing anyway... Normally, this should be an easy job as I have an AKS front cover with a Sierra water-pump that can be extracted without removing said front cover. I think the sprocket wear issue has just removed this advantage!

Many thanks for the tips, Rohan. I guess it will be a while longer before the Elan gets back on the road again.

Matt

[steveww]

Fit a new chain while you are in there. They are only about ?12 a lot less than a new engine if it breaks. Go for vernier cam sprokets while you are at it as well. It will make getting the cams spot on so much easier.

[twincamman]

I have found the best way to line up the cam sprocket marks is to set one cam and sprocket in the correct positon then take the second sprocket put it in the chain with the marks aligned and move the second cam into place with vice grips --when the cam falls into place put the bolt on---ed

[freddy22112211]

I use offset dowels.
Gordon