blown 87

The timing belt drives the exhaust cams and the intake cams are chain driven off of the exhaust cams.

the flyin' scotsman

A pic for those following along:

Mongo

Don't they usually last a VERY LONG TIME on a street car? mine have never been changed and I have 147,000 on the clock. Should have any concern?

blown 87

Yes, the chains last a very long time.
I would have concerns about the tension pads and if it were my car I would change the chains out at the same time, but I have only heard of a couple in 928's letting go with the pads intact.

fraggle

Again from motorcycle experience, remember that in the mid 80's all the literbikes used 630 sized drive chains for strength. Now this isn't done because of advances in metalurgy. Gains in weight and acceleration is a result.

the flyin' scotsman

Fraggle jolts my memory.

From my motorcycle experience as a owner, mechanic and racer I never experience cam chain failure in any configuration. Most DOHC engines use a one piece chain either in the middle or end of the crank up to the exhaust cam over to the intake and back down to the crank via a tensioner. The chain was typical link chain but in some manufacturers (Honda come to mind) used HyVo.

The tensioners very either semi auto or full auto adjust with long slipper blades.

Some bigger engines, Honda CBX for example, used 2 chains with the crank driving one cam and a seperate chain driving the other cam.

When I say failure I mean that never saw a cam chain break although have seen chains that were beyond tensioning. These cases were typically in engines that were neglacted so the chain had been allowed to 'rattle' around in a loose state for many many miles before being tensioned. Of course auto tensioners eliminated the requirement (cant trust the human owners to remember).

M/cycle drive chains are another matter as they're exposed to all sorts of conditions..........I have seen a 630 chain that broke.

blown 87

What are they using now?
I ran 630 o-ring chain for many years and 530 on the Sporty.

the flyin' scotsman

Many manufacturesr are now using belts or shafts. The racing/sporty bikes still use chains for lightness.

fraggle

Lots of the big superbikes now running 530s.

As mentioned, the conditions are totally different. The cam chains are in a nicely sealed environment.

Has anyone observed a cam chain break that wasn't tensioner pad related? I can see that plastic part wearing as being the real weak link here.

the flyin' scotsman

Not me............some m/cycle engines use a 'blade' plastic piece thats anchored in the crankcase and top of the barrels. The tensioner then presses on this blade typically through a roller bending it onto the cam chain. I've seen this blade break although not completely as it has a metal backing strip.

mark kibort

They look idential to the stock chains but just a little beefier

mk

blown 87

Next time you have the cam covers off take some measurements and some pics Mark, if you would.

I showed the new chain and the old chain to a mechanical engineer friend of mine yesterday.
Most of what he said went way over my head, but the cliff notes were the stock one was way better.

He said that there must not be much pressure on them for mine to have not worn or for the replacement ones not to be breaking in the field.

Brings up another point, does anybody have a reason that you can only run a used chain one way?
I have been told this many times, but it would seem to me that is should not matter.

the flyin' scotsman

There's no arrows on the OEM or new chain are far as I can see so they can be probably fitted any way when new.

Used, I would fit as they were as the chain and sprockets find a running sweet point.

My OEM cam chains were fitted from factory so the timing plates (the silver coloured ones 7 plates apart) face the front of the engine, both banks.

Didn't Mark K. have an issue with breaking teeth off the cam sprockets; don't recall the cause.

blown 87

I was talking about used ones.

As far as breaking the teeth off, the only time I have ever seen that is when the chains stretch (wear inside the rollers and pins) and that changes the pitch length which in turn makes the chain ride higher on the teeth.

It is weaker at the top of the teeth and they break.

I have no idea on Marks, but that is a good guess.
His got turned up a bit in RPM on a regular basis.

the flyin' scotsman

I responded to new and used chains orientation.

There are others who run high rpm but haven't reported cam teeth breakages, hope MK chimes in.

Back to the m/cycle analogy.........drive chain sprockets get hooked from wear as the chain wears and stretches but I've never seen this on cam gears. Must be the environment diff the chains live in.