timing chain question

[ficke]

Do timing chains get looser or tighter from cold to operating temperature?
I guess the guys who run mechanical tensioners would know the most about this?
Thanks in advance!

[ficke]

Specifically to an engine with aluminum cylinders.
Does the chain stretch to match the cylinder, heads, case, cam towers?

[Steam Driver]

The chains tighten up as the engine warms because they do not expand at the same rate as the case and cylinders. This is true even in the case of the old cast iron cylinders (911T/914-6 etc.). I used mechanical tensioners in a 914-6 long ago and when it was cold, especially really cold, you could hear the chains rattle on start-up Ned this persisted for a few minutes. So it behooved one to keep the revs down far awhile until that went away.

The hydraulic tensioners, regardless of variant, are somewhat locked at any temperature so the chain is tight when cold. Then as the engine warms they "give" a little, keeping the chain from being too tight. This latter is a big issue as it increases both chain and sprocket wear. I wouldn't use them now but this was back before the tensioner issues had been "solved" to the extent they are now. Various reputable sources were selling mechanical tensioners in those days. I made mine from a failed set by drilling and tapping the bleeder screw boss to take a capscrew that tightened down against the plunger to jam it. Worked great for quite a few miles.

[chris_seven]

Chain tensioners are useful in speeding up the build of new engines as they accommodate production tolerances and as has been said they deal with the growth of the engine during warm up.

I am not sure that the difference in growth between a cast iron and aluminium cylinder is very significant at about 3.5thou. The impact of this difference on the preload in the head studs is much more relevant.

If you plan to run an mechanically fixed chain adjuster then you also need to be aware that the initial bedding it of the chain will significantly influence its length.

It would be important to re-adjust a chain after a few hundred miles or it will be slack on the adjuster side and could jump on the overrun.

Once you have re-adjusted chains will probably run for 10-20k miles before further adjustment is needed.

Again as has been said they will tend to rattle when cold.

[ficke]

Thanks,
Very helpful.
My chains rattle when cold and go away when warm.
I have about 10,000 miles on the engine since I had the heads done.
I have new 930 tensioners, The mileage is low (82,000) on this engine.
I did have the heads cut when the valves were done. I did not believe I need more spacers under the cylinders, because the cams lined up in the cam tower nicely.
I reused the chains rather than go with a split chain or splitting the case. Maybe just new chains?

The issues one has taking short cuts..... am I really saving money in the long run playing these games?
I think I know the answer to the last question.

[KTL]

When one cuts the heads, the chain case can be cut the same amount. Doing so will ensure the cam housing aligns with the chain case.

The tensioner will "automatically" account for the slack in the chain, even with wear. The problem is, should there be more slack in the chain than the tensioner can address (tensioner fully extended) then you have a problem.

JB Racing has a set of instructions they made for the fixed tensioners, based on their long term experience with using them.

[chris_seven]

2000 miles seems a bit excessive.

If you build a new race engine with a 40 hour life between rebuilds then to run in for this distance would be costly and use a significant portion of its life.

I would tend to break-in a new engine on a dyno for about 30-40 minutes with varying loads and engine speeds and then re-adjust valve clearances before doing the power runs and set ups.

All you are doing with the chains is loading the pins and rollers and burnishing off the high spots which should happen very quickly.

I think break-in distances of 1000 + miles is a very old-fashioned concept and just too long.

[ficke]

Once again, thanks all for the responses.
But to clear up a little confusion and maybe my experience will help others I will explain hopefully a little more clearly what happened.

My car, 1968 911S, had 72,000 miles on it when I removed the cam towers and heads for oil leaks and while they were off had a valve job done on them.
I reinstalled them and at that time put in brand new 930 hydraulic self adjusting tensioners, I could not easily go with oil fed tensioners because of the sand cast chain box's or I would have.
I shimmed the cylinders up to make up for the heads being fly cut. they matched the cam box's perfectly.
They engine ran great, but cold it rattled with a sound that was at half crank shaft speed and the rattle went away when warm and then the engine sounds great.
There is no material in the oil.
I drove it like this for over 10,000 miles.
But it bothers me, maybe one of these wore out chains can snap?
I now doubt it had 72,000 original miles or I believe the oil was changed in frequently by the PO, because my chains or so wore out the tensioners do no take up the slack.
Before I pull the engine to replace the chains I wanted to verify that worn chains make sound that will go away with a warm engine, I knew an 911 engine grow more than the chains. but I just wanted a second opinon, before I remove the engine and go through all that work.
I know that people who run or ran fixed mechanical tensioners would know about chain sounds, that is why I asked them specificly in my first post.

I do not run mech. tensoniers or care about them in any way other than for advice on chain stretch as related to heat from the experienced owners of mechanical tensioners. I do not run solid/mech. tensioners or wish to.
I only use mechanic/ solid tensioners as a rebuild tool.

I need new timing chains.
I will split the case this time and do it right and not cut corners like last time.

[cmcfaul]

I believe you can change the chains with the engine installed in the car.

Cut the existing chain, connect the new chain to the old one and rotate the engine pulling the new chain through. The valve side will need to rotate as well to avoid valve piston contact.

Not too difficult I think. Timing the Cam afterwords is not too difficult with an installed motor either.

Chris