Thoughts about root cause?

[safe]

A friends engine, happened on the track but no over revving. It started to sound a little different and right after that a rattling noise, shut it down immediately.

What can happen that would cause a timing chain to brake?
We are wondering if the broken idler is the cause or just a result?
Also the idler gear is very worn, but if that happened when the chain broke or before?

Cam is spinning freely, but all six rockers are sheared, obviously...







The other side is fine.

[docrodg]

Hard to tell, but I would think that the idler arm breaking could be the cause of the chain break. Broke, loss of tension, chain siezed then snapped. Worn teeth probly did not help.

[safe]

The wear is strange, the engine only has 3-4000 miles on it.

Never heard of a idler that broke before, can't be that common?

[docrodg]

Misalignment of the sprockets? That could certainly put a side force on the idler.

[KTL]

Magnus,

The rebuild previous to my current one was caused by a seized idler arm. The tensioner (Carrera oil fed style) was in a highly extended position and the chain was strongly tensioned. The seizure was caused by the idler arm bore galling against the chain box post. Despite the high tension, the chain and idler did not break.

My guess is possibly the breaking chain "whipped" the idler arm hard against the tensioner and broke the arm? Just a guess.

A telling sign of high chain tension may be on the countershaft sprockets down in the case. My countershaft sprocket that was connected to the seized idler bore was very blued from the heavy load. Check to see if you find anything like that in the case.

chain tensioner problem?

Are the idler sprockets some special size for chain slack takeup? Those are not stock-looking sprockets. I know that performance shops have made special tooth counts to account for narrow/wide engines that have trouble with chain length.

[HawgRyder]

I have seen the effect on motorcycles...caused by a worn sprocket.
The teeth on the sprocket get so worn...that the chain can "climb" the teeth.
When that happens...the chain sort of gains a 1/2 tooth and forms a "hump"...and then the chain drops the hump and snaps...or starts jumping every other tooth.
Because that sprocket is an idler...it is not powered except by the chain...so for all the teeth to be broken...the chain must have be intact long enough to break the sprocket.
The final break would be the chain...when the last of the teeth disappear...and the rockers are nicely broken!
There will be tons of metal in the engine now...so a complete tear down is necessary.
Sorry about your situation.
Bob

[Flat6pac]

The idle sprocket doesnt look stock.
Bruce

[safe]

I seriously hope that the gears wasn't misaligned, the engine was built by a well known Porsche engine builder here in Sweden (and probably in other places too).

The idler is not stock its a 21 tooth gear from Jerry Woods. The engine is a 3.0 liter built on an early 65 aluminum block, don't ask me how, but I guess i'll find out soon...

Unless the idler gear was faulty and wore quickly, high tension sounds likely.
We were thinking that if the idler arm broke first, it could mess up the chain, It was quite a large piece of it we found in the sump. But breaking the arm wouldn't explain the wear on the sprocket so that must have come first.

Kevin, I'll check if the idler arm and bushing, if it was seized it should be a bit hard to remove, right?

[Lapkritis]

Can you take a look down into the Case to see how the chain lays on the intermediate shaft? It is possible for the chain to jump off during assembly and ride a single row of teeth which would saw all teeth on the cam and idler.

[safe]

Originally Posted by Lapkritis

Can you take a look down into the Case to see how the chain lays on the intermediate shaft? It is possible for the chain to jump off during assembly and ride a single row of teeth which would saw all teeth on the cam and idler.

Can't say, it's just a bunch... We just got half the chain out, the other half is somewhere in the case.

Checked the idler, its not sized and moves freely. It's the updated turbo idler arm.

The more we look at it the more we think the chain has been out of alignment. The gears are so badly worn on the sides.

I looked into the case and the intermediate shaft gears appeared to not mesh more than ~50% and one gear eating into the case!!!???
Can they move about? or be made to move?

[safe]

Hard to get a good picture from a "true" angle, but the IS chain gear is not lined up with the chain rails.
The drive gear does not align with the gear on the crank shaft, its of by ~4 mm.
IS shaft has worn a deep cut into the IS shaft cover.
Hard to tell what damage came first...




A few pictures of 95 mm cylinders mated to an early case. Not much meat left...

[rsscotty]

If you turn your head side ways and look at your picture of the intermediate gear, it appears the lower chain rail is pushed rearward. So maybe the engine was assembled with only one half of the chain engaged on the sprocket as Lapkritis stated.

[safe]

Originally Posted by rsscotty

If you turn your head side ways and look at your picture of the intermediate gear, it appears the lower chain rail is pushed rearward. So maybe the engine was assembled with only one half of the chain engaged on the sprocket as Lapkritis stated.

Yes, that's a possible scenario. that would maybe explain why only half the gear is stripped.

[Lapkritis]

That's what it would look like. Sorry to see that, tough break.

[AlfonsoR]

It is possible for the chain to jump off during assembly and ride a single row of teeth which would saw all teeth on the cam and idler.

Two questions, on what came first

If the chain is assembled on only one row of teeth at intermediate shaft, would it be possible to break the idler arm?

On the other hand if the idler arm broke first (arm could have been damaged/ cracked during tensioner installation), the excess slack could have allowed the chain to jump to be driven by a single row of teeth?

Either way, shouldn't this be covered by the engine shop?

[safe]

Originally Posted by AlfonsoR

Either way, shouldn't this be covered by the engine shop?

The engine was started for the first time almost 3 years ago. The owner who commissioned the engine build is no longer with us, the current owner bought the car from the widow.

A lot of things is unclear, but the shop will be contacted.

[Flat6pac]

It does cause a hard time to load the tensioner when there is 1 row off the intermediate shaft. When everything doesnt load smoothly on the build, its time to look.
Bruce

[Walt Fricke]

Idler wheel arms can break all on their own. When wondering why one of mine was so close to the chain housing case I noticed a crack in the arm. It looks to be a forged steel part? I have no idea why it cracked (but didn't break, though perhaps in time it would have.

I once stripped the aluminum teeth on the IS gear. Instant breakage of all 12 rockers. No damage I could see to anything else, other than some bent valves.

For what those observations are worth. Long enough ago that I can't recall if I spotted the broker idler arm before or after the tooth stripping episode.

But having the chain not on both sets of teeth on the IS can't be good. You might suspect that tooth misalignment led to tooth decay which eventually led to the chain binding somehow and breaking. Perhaps if, before the chain broke, tooth skipping or chain bunching put so much stress on the slack side of the chain that it forced the idler (and thus the arm) up so hard that it caused the tensioner to bottom, and then some, leading to the fracture. Hard for me to see how this would happen, since this is the slack side and the tensioner's main function is to keep it from vibrating too much, like a shock absorber. The work is done on the tension side, with one toothed wheel pulling another around with the chain straight.

[safe]

We got the case split open today.

Some damage to the case from the chain:


IS cam gear:


This doesn't look good??




IS gear been eating into the case:



When the IS is "all the way out" the gears line up.


The intermediate shaft issues looks "interesting", can it be the wrong IS or can the gears move on the shaft?

[safe]

There is also this issue.
Uneven wear on the bearings. The crank was hard to turn.