sc cam timing query
 

aloha, i'm just finishing up an ovwehaul on a 79 big valve 3.0 engine and am having an issue with cam timing. i have done several other engines and have never had this issue. i time the cams (08, sc cams) to 1.55 lift at z1 with the chain tightly tensioned and tighten the camshaft locknut. leaving the dial indicater on i test the timing after tightening the nut. still spot on. however, when i replace the tensioners the cam timing advances about .35 to around 2.0 whats up??? (the chains are new and the gears don't have any visable wear) any ideas???? cheers, geoff

What do you mean "replace the tensioner?"

Maybe you need to prime the oil feed tensioners first before checking.

Here is how I did my 81SC in 2007 with photos. Hope it helps.

Seattle Engine Drop III: reassembly - Rennlist Discussion Forums

That is a must!

aloha all, by replace the tensioners, i mean i use a c clamp to tighten the left one and a factory tool clone to tighten the right chain and after adjustment put the tensioners back in. these are the original tensioners and not updated oil line fed. i'm wondering if maybe they don't have enough spring in them. i did soak them but maybe they're getting air in them when i compress them into the holder tool as i've been using a horizontal vise?

Maybe just prime them anyway so there is no air. Just submerge in oil and move the plunger in and out so there is no bubbles. Then install them both. Then reset the timing without the supplemental tension. That's my guess. (It's good that you rechecked the timing after you put the tensioners back in.) Btw, is that a stock Sc can? If it's an upgraded 964 can, 1.85mm is wrong.

Actually the timing is what it is regardless of the the tension type. When the engine is running and the cams are being pulled by the chain the measurement you are getting with the tight chain will return. The chain will lose any slack that is being shown using the factory tensioners when it is running. Do not reset the cam timing using the factory tensioners. Set it using the method you have already used and put it back together.

Lindy

hmmmm
 

i think i'm leaning towards lindy's view. the chain will tighten with the engine running (thus correcting the timing) sometimes i think too much. thanks to all, gb

The chain will stretch with heat and the valve spec will become a lower value.
Bruce

If you can push in the tensioners by hand they are toast. Buy new ones or rebuild yours. The non oil fed tensioners are hydralic in the sense that they have oil sealed inside them with o-rings. It should take a strong c-clamp or bench vise to compress them. Do it slowly. Insert pins to hold them down and when installed pull the pins. This would be a good time to install the Carrera tensioners(oil fed). I can not stress enough not to install the tensioners if they are easily compressed. You can blow the motor. Good luck.

+1
The old style tensioners need to be rebuilt every 50K miles or so. Think about how old the oil is in those old tensioners. It's also a good idea to install "chain savers" on the tensioners. The old style tensioners work just fine as long as you don't expect them to last for ever.