U20 Upper Timing chain Slack

Post by **itsa68** » Sun Oct 23, 2005 9:39 am

While turning the engine at the crank in the opposite of running direction, the upper chain tensioner collapses and extends in pulses during the rotation.
At this time the chain on the opposite of the side of the tensioner (Evil-L side) tightens and slackens.

When the tensioner snaps out and is extended, the chain does not touch the Evil-L when depressed.
During the further rotation in the opposite running direction, the upper tensioner then becomes collaped by the chain tightening up, and excessive amount of slack is produced in the chain on the other side, which would definitely whack the hell out of the Evil-L.

A) Is the compression/de-compression of the tensioner and chain tightening/slackening a typical condition when rotating the engine in the opposite direction?

B) When the tensioner is extended (during opposite running direction), is that the time to check the chain on the Evil-L side?

C) What the heck is causing the chain to tighten and slacken when the engine is turned in the opposite direction? (stretched chain section, eccentric gear, other.......)

Gordon Glasgow's article doesn't mention this type of condition when the engine is rotated backwards to check the chain slack.

????????????????

Ray B.
(I now know why I've stayed with the R16)

spyder · Post by **spyder** » Sun Oct 23, 2005 12:04 pm

A common practice is to put a spacer on the tensioner shaft to limit the chain rattle when the engine is started. This is a copmensation for chain wear or head shaving. In extreme cases of head shaving, you may need cam tower shims. If you remove the cam towers it is said they need to be aligned bored. Also, shims change the cam followers geometry and you may need taller lash pads.

Post by **itsa68** » Sun Oct 23, 2005 6:39 pm

Not being familiar with the U20, I'm trying to diagnose whether this engine is in good shape or not before using it.
Supposedly it was rebuilt a short time ago.

Thanks for the heads up on the corrective action side of things.

Ray B.

Post by **dbrick** » Sun Oct 23, 2005 11:38 pm

I think the loose slack thing is the engine compression as the piston passes TDC and pushes the piston down. Are you turning the crank or the cam? Probably would not happen with the plugs out.

As far as slack, I did the math and figured about there's about 20 -25 lbs pressure on the upper tensioner at full oil pressure. I had a spring from an old oilpump relief valve that I cut down to fit and replaced the stock spring in the upper tensioner. It gives me about 22 pounds pressure all the time, oil pressure or not. Death rattle is gone. I also added the oil filter tube at the same time, so it may not be the tensioner spring that solved it.

Has anyone ever played around with advancing or retarding the cam relative to the crank to either make up for slack or to alter the powerband? Common practice on American V8 was to advance the cam 5 degrees to lower the powerband a bit. Just curious.

spyder · Post by **spyder** » Mon Oct 24, 2005 1:12 am

Yup! The more you advance it the more low end torque you get. Keep in mind this is an interference engine!

In regards to the chain slaping, this is the valve spring tension against the cam. In other words, the cam rotation is not smooth and it will smack the chain around if it is loose.

Post by **itsa68** » Mon Oct 24, 2005 9:50 pm

The engine is being turned by hand at the crank in opposite direction when the slack occurs.
But it does tighten up to not have any slack and not touch the Evil-L, I think after each compression stroke.

I will have to re-visit the engine and give it another whirl to be sure.

Ray B.

Post by **dbrick** » Mon Oct 24, 2005 11:04 pm

Well, it just wouldn't be any fun if you didn't risk putting a skylight in the pistons. I have a piston from my X-1/9 with a 1 by 2 inch hole in it on top of my toolbox as a reminder.
Is there a procedure to get a specific advance/ retard. I'm assuming even with a slack chain, one tooth is too much either way.

spyder · Post by **spyder** » Tue Oct 25, 2005 2:08 am

With an "A" cam you can move it a tooth up or down without piston contact. I did not try it I intentionally. Ran like crap! As I drifted twards the higher performance direction there is a smaller margin of advance/retard where you start contacting piston/valves, read hi lift long duration cam, pop up pistons and shaved head! This is when it gets spooky!

Post by **dbrick** » Tue Oct 25, 2005 10:27 pm

I can live without the extra 3 horsepower.

SLOroadster · Post by **SLOroadster** » Tue Oct 25, 2005 10:47 pm

Every U20 that I have intentionially spun backward has had the same issue. Its like spinning a bike chain the wrong way, there is a slack side at all times, it moves depending on which way its being pulled. Putting a shim in the upper tensioner is easy, just pull it out, go find a delrin washer that will take up as much slack as needed and reassemble. When I built my current engine, I shimmed both tensioners just for good measure. The chain says tight at all times. You will know the death rattle when you hear it. I'd say go for it.

Will