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Thoughts about root cause?
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... http://forums.pelicanparts.com/uploa...1380138884.jpg http://forums.pelicanparts.com/uploa...1380138941.jpg http://forums.pelicanparts.com/uploa...1380138995.jpg The other side is fine. |
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.
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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? |
Misalignment of the sprockets? That could certainly put a side force on the idler.
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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. http://forums.pelicanparts.com/porsche-911-technical-forum/641167-chain-tensioner-problem.html 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. |
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 |
The idle sprocket doesnt look stock.
Bruce |
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? |
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.
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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? |
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... http://forums.pelicanparts.com/uploa...1380229198.jpg A few pictures of 95 mm cylinders mated to an early case. Not much meat left... http://forums.pelicanparts.com/uploa...1380229380.jpg http://forums.pelicanparts.com/uploa...1380229592.jpg http://forums.pelicanparts.com/uploa...1380229737.jpg |
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.
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Yes, that's a possible scenario. that would maybe explain why only half the gear is stripped. |
That's what it would look like. Sorry to see that, tough break.
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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? |
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A lot of things is unclear, but the shop will be contacted. |
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 |
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. |
We got the case split open today.
Some damage to the case from the chain: http://forums.pelicanparts.com/uploa...1380390824.jpg IS cam gear: http://forums.pelicanparts.com/uploa...1380390995.jpg This doesn't look good?? http://forums.pelicanparts.com/uploa...1380391325.jpg http://forums.pelicanparts.com/uploa...1380391433.jpg IS gear been eating into the case: http://forums.pelicanparts.com/uploa...1380390628.jpg When the IS is "all the way out" the gears line up. http://forums.pelicanparts.com/uploa...1380391800.jpg The intermediate shaft issues looks "interesting", can it be the wrong IS or can the gears move on the shaft? |
There is also this issue.
Uneven wear on the bearings. The crank was hard to turn. http://forums.pelicanparts.com/uploa...1380392448.jpg |
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What intermediate shaft number is on there? What case #? Looks like the intermediate shaft may not be original to the case. Edit: looks like you have a 64-68 intermediate shaft installed. What crank is installed?
Think you would need intermediate shaft from the later engine here. one similar to this: 0510 PROJ964 03 Z Photo 3 |
I believe you have to run the old style IS in a 65 case? Early aluminum cases have no IS bearings and the shaft runs in the plain bore. So the IS has to be sized to run in the plain bore, or else the bores are enlarged for bearings and you use the later shaft?
There's a picture of an early aluminum case in Bruce Anderson's book with a caption that says "Inside the sand-cast-aluminum crankcase, the layshaft runs in the crankcase w/out insert bearings. This particular layshaft was only used in these early crankcases" My book is the 2nd edition and the picture is top left on page 53 |
Sounds like this special engine build allowed the previous owner to run in a particular racing class. The shop will have no liability. Sorry to hear about your trouble, but thanks for sharing as it is very interesting. Good luck in your new build!
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Haven't checked the year of the case, but it is a rare early 65-68 sand cast alu case. As a side note it has MFI reconfigured to 3 liter and S-cams. It had a lot of torque and a really good throttle response, a bit chocked an the top above 5500 with SSI. The heads are 2.4S (Y-alloy) or possibly 2.7 carrera , slightly ported I think. |
The sand cast cases did not use bearings for their intermediate shaft. Don't really need them, in fact. So that's the IS shaft in this motor.
End play is set with gasket thickness under the IS end cover (that plate you show with wear) and there is a spec for that. The end plate for these motors is cast iron, not mag or aluminum. I suppose it is cast iron because Porsche thought that the shaft end and the plate might occasionally come into contact? On my sand cast motors I found I could use the softer metal end plate without wear issues. Pressurized oil is in that area, and I have supposed that it helps push the IS shaft toward the flywheel end of the motor a bit, offsetting what the diagonal cut of the gear teeth want to do by way of shoving it this way or that depending on whether you are accelerating or decelerating. The IS gear on the IS shaft is different for these early shafts/cases. If you put on the gear from a later engine, it will be offset like this one is. I ran into that when I talked a guy into having some straight cut gears made for this application (stripping the teeth off an aluminum gear was an incentive here to have something stouter, though not everyone has such problems with aluminum). I gave him dimensions from later IS shafts, and when it came time to put the ones I bought into my motor I found I had an offset. I was able to change something (turn the gear around?) to minimize the offset, and (perhaps because both are steel) have had no issues at all with that setup. You can't just put the later IS shaft into a sand cast case. You can machine the case to accept one of the bearings for the later cases (I think the non-thrust end, but may have that backward), but not the other. The bearing seating surface of the case is too large for the later bearing. So you either have to weld up the case and rebore, or make some kind of trick insert to fill the gap between case and bearing. Since this case has no bearings, it is running the early IS shaft. But it makes sense that having the chain for the left bank (isn't that the one?) only on one row of teeth at the IS shaft was pressing the shaft toward the crank pulley end of the motor, thus causing the scoring of the cover plate. And maybe exaggerating any gear mesh offset there might be. Having the chain off like this is one of the things Bruce Anderson's book cautions against! Along with installing the cylinders wrong side up. Hard to fathom this, and the angularity should have made noises which could not have been adjusted out with the cam shims. Even with no shims you still would have had noise which shouldn't be there. And removing shims would have thrown measured gear parallelism way off, since it is based on the gears (which probably were in OK parallelism) and not the chain angle, so a mechanic would probably not have gone there. It isn't all that hard, when reassembling a motor, to get a chain off like this. But it also isn't hard to notice this, and to correct it on the engine stand: stand the engine on its side, drop the chain down so it is loose, and move it over so when you pull it back up it is where it belongs. You can see this before you put the chain boxes on, and you can use a mirror to see if all is right after they are on (or, nowadays, a borescope too). |
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Wrong IS gear, I'll give it a good look to verify that. I also think it would have rattled like crazy, but I have no experience with such an error. |
Reviewing the pictures, no question that this is the early IS shaft. Note that there is a large flange on the crank pulley side of the shaft, but no corresponding flange on the side toward the right side chain gear.
http://forums.pelicanparts.com/uploa...1380490740.jpg This is one reason you can't use the double flanged bushing (thrust bearing each direction) with these early IS shafts. So the IS flange holds the shaft in position in one direction, and the cover plate in the other. The setting spec for the cover plate provides the right clearance for the thrust surfaces in each direction, with the pressurized oil coming out of the bearing surfaces of the shaft at that end providing the oil cushion end to end. You can see from this picture: http://forums.pelicanparts.com/uploa...1380490929.jpg why you can't adjust out the offset between the crank and the IS gears. Way too much movement to deal with using a couple of shims. Worse, you'd put chain wheel parallelism way way out of where it should be, and way too much to compensate for with camshaft shims. And you might put a special strain on the splined coupling tube connecting the oil pump to the IS shaft. I don't think there is a spec for this, but if you had only a short length of splines matching on the two, that would be weaker than the normal, longer spline engagement. So my vote is wrong aluminum IS gear for that part of the issues. Though that offset might be acceptable. You can take a look at the aluminum gear faces to see if they look especially worn where they contact compared to the side where they do not. And try flipping it, or its replacement. Early sand cast IS gears (and didn't the "no separate bearing" system continue on into the earliest mag cases? So those use the same IS gear) may be hard to find new. One thing you might consider for the rebuild is the steel large IS gear from a 964 (I think from that, if not then a 993 - I'd have to look up what I purchased). If flipping it gives you no more offset than you have (or improves that), it will bolt right in and you won't have to worry about offset there. Don't know what to think about the crank bearing wear. Worth having the crank checked for straightness, and the case bearing saddles for the same. Those of us running the sand cast cases take it as an article of faith that they never go out of bearing alignment, and never need line boring. But, as with most things, there can be exceptions. |
To be clear, you have the right IS shaft for this case. But may well have the wrong IS gear.
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This engine doesn't have the original 66mm crank, but a 70.4 to take the capacity up to 3.0 liter. Is it just the IS gear that has a different offset, or is the crank gear different to? We will probably have to check the crank for straightness. The engine was really hard to turn. I don't think it was that before. That said, the case might be tweaked, a lot of welding has been done to it before. It has probably been in some accident and/or thrown a rod. |
More pieces of the pussle that we missed until we lifted the crank....
http://forums.pelicanparts.com/uploa...1380562742.jpg Never seen a broken crank before. Might explain the wear on the bearings. Hard to tell but the case looks tweaked, will be sending it out to be measured. Not sure if this is related to the chains, can't imagine how. |
Can't believe the chains caused that! Wow, takes a lot of force to break a crank. Typically you will destroy rods, pistons, the case, etc. before breaking a crank. The chains jamming would likely just break them and the gear/is shaft at most. That crank must have been messed with somehow to weaken it.
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wow.. thats depressing. sorry you're going through all this.
probably not what you want to hear but at this point i would just look for another motor. |
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Instantaneous shock loadings can work nasty miracles inside of an engine!
I have seen cranks in small block chevys do this same thing. They broke on the center main just like in your picture. I believe it happens when the hardened crank is subjected to seizure on one end and the point of the break is at the weakest part. Bob |
Yes indeed Bob. I've seen the same with valve springs binding and shattering a camshaft. The crank jamming at one end would be the same. In this case it appears to be the intermediate shaft locked and transmitted the load through the gear to the crank. The inertia of the flywheel likely did the work shearing the crank.
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It's hard to tell from the picture but if you look at the surface of the brakage you would see that the edges (counter weight parts) are glossy, smooth and "hammered". The center part is raw and dull.
I would guess that the crank was flawed when it went into the engine and the rest broke along with the chain. |
Probably ran for a few revolutions after the break. That'll hammer things down a bit. Usually when you find this kind of carnage it's with a rod that had windowed the case/block making it real hard to tell what happened. I think the relatively high number of webs/bearings in the 911 engine kept the crank revolving in two separate pieces through contact on the shatter joint. Not all are so fortunate.
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The 70.4 crank is not, I think, quite as stout as the 66. A friend had the crank break on his '74 911S some years back. They track the car, but there was nothing special one could point to as the cause. I think they, and those of us kibitzing, thought a chain tensioner had failed, so suggested they take it easy driving home from a track. They got home all right, broken crank and all.
I once drove a VW with a broken crank for three hundred or so miles. After the first appearance of some problem I saw that the crank pulley was broken. A welding shop along the way fixed that. A couple of hundred miles later it broke again and it was time to stop. I'd be inclined to blame the chain failing due to improper installation for this, though you can probably tell from the fracture surfaces whether a crack had developed and was getting bigger, or whether it all cracked at once. The older surfaces would be a bit smoother (unlikely to be rusted or discolored given that they live in an oil bath), and would have a semi circular tree ring like pattern as the crack grew larger. That all ends with the final fracture, which is of a more uniform appearance. Cranks breaking aren't all that common, but that is not unheard of either. |
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