Cracked chain sprocket

FRporscheman · Jun 5, 2019

First of all, forgive me for posting about a car, but if it helps, the word for car in my parents' language is "machine"...

I don't know if this is the best subforum for this question but here it goes.

I am reassembling a classic Mercedes engine, an M100 6.9L V8. I just found this crack in one of the timing chain gears/sprockets. The dealer is the only place to get a new one, and they want a month's rent for it. I normally am OK with shelling out and taking good care of a car, but this engine has already cost me a lot. I wanted to know if there is any way to repair this type of thing or if I have to buy a new one. Thanks in advance for any/all help you can provide.

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memphisjed · Jun 5, 2019

might work 1 mile or 100000 miles. drill a small hole at the very end of the crack, then gouge with a thin wheel from the inside of key way to hole, fill with bronze or brass. neatly file everything flush to all original faces. then take a small round file (3/32 or so) and file into the corners of the key ways at about a 45. Put it in car, drive it like you stole it.

gouging out crack, drilling stop hole, filling with brass the entire key, recutting keyway at different point would be better - still rolling the 1 or 100000 mile fuzzy dice.

Hodge · Jun 5, 2019

Bear in mind if you change the keyway location you may have to adjust timing marks if it's more than just as idler. Do these cars have a noted problem with timing gears or was this engine involved in a violent stop/ lockup. I'm assuming these engines are interference on the valve/piston in the event of timing failure. This would play some part in my decision making process.

memphisjed · Jun 5, 2019

Hodge said:
Bear in mind if you change the keyway location you may have to adjust timing marks if it's more than just as idler. Do these cars have a noted problem with timing gears or was this engine involved in a violent stop/ lockup. I'm assuming these engines are interference on the valve/piston in the event of timing failure. This would play some part in my decision making process.

You are right, changing key would require same relationship to tooth layout. zero would definitely be different from original, that is easy compared to filling and cutting.

Richard King · Jun 5, 2019

You could take it to an engine rebuilder as they probably have seen this problem before. They may turn down the OD and make a steel bushing to slide over the shaft you turned down. I would say a shrink fit. I would not try to weld it. You need to think about balance and if you use a screw the balance of the shaft might ruin any bearing down the line.

sealark37 · Jun 5, 2019

I have never heard of a timing sprocket failing. Chain wear, tooth wear, yes. Unless you are going to race it, don't worry about it. Just my take.

greggv · Jun 5, 2019

How long would it take to replace it if you "fix" it and it fails? And what kind of damage would it cause? I'd just replace it and move on.

Tony Quiring · Jun 5, 2019

Price of brand new branded product compared to just the labor to get to it later.

Add in the cost to repair all of the possible damage and it gets nuts.

Even considering this brings the attitude of the service provider into question.

We are very cheap... but stuff like this must be done correctly or not at all.

Sent from my SAMSUNG-SM-G930A using Tapatalk

atomarc · Jun 5, 2019

It's interesting that the little crack is right in the crotch of the broached keyway. There appears to be a large number of massive spokes surrounding the ID of the gear so to my untrained eye that little crack seems pretty innocuous..but you never can tell. Does your automotive machine shop have a magnaflux setup or some other crack detection system they can use to see exactly how far that the crack goes.

To the 'purest' it would seem prudent to replace the gear but from a practical sense it may not be necessary as the crack may never migrate beyond its current length. That's the conundrum, isn't it!

Stuart

kustomizingkid · Jun 5, 2019

Did the engine crash valves or did you just find this? If there was no massive failure I would probably put it back in and not worry. What motor is it?

TDegenhart · Jun 5, 2019

Once fatigue cracks like that start, they usually go to failure. If failure destroys the engine then replace it.

Tom

FRporscheman · Jun 6, 2019

Thanks for all of the input. I'll try to respond to everything:

1 or 100000 miles - yeah. I'm not big on gambling, and this engine would be very expensive to replace.

Noted problem with timing gears? - It has been seen before but it's not common. There wasn't any violent lockup but I suspect the oil used was low-zddp, and this engine has flat tappets, and some of the tappets have some gouging. Maybe the added drag from the gouging event caused the crack, who knows.

Moving the keyway to a new spot - that seems like a great idea, which I will discuss with an experienced local machinist once I find one. I'll ask him about other ideas too like sleeving it.

This part can be replaced in an hour or two, but if it does fail, I guess I'd have bent valves and a bit of carnage in the timing chain cavity. Maybe some damage to the cams and rockers. Best case scenario, engine would have to come out for a teardown. Definitely wouldn't want to risk it so if buying a new part is the only way to solve this, I will.

My buddy is an engineer and says the "crotch" of the keyway should have been radiused at the factory, but agrees that the crack will most likely progress until failure.

So I am totally willing to cough up the dough for the new part, but I just wanted to explore the possibilities of fixing it. I am convinced that I can not use this part as is. But I am intrigued by the idea of cutting a new keyway somewhere else on it. This, and drilling a hole at the end of the crack, seems like a good plan? I need to run this by a machinist.

john.k · Jun 6, 2019

i think it will be ok with sleeving and drilling the crack............buuut.....do you want to worry every minute you are in the car the the timing will crash......one thought ,in Jappas ,the pistone are designed so valves cant crash ,even if the cam stops........My experience with Benzes.....a friend had a 5cyl diesel...300? maybe?.....anyhoo, he did something or other ,and replacing the camchain,dropped a washer.....assuming it was lying harmlessly in the sump,he fitted another washer,and buttoned up the motor.......about six months later,a bang ,and snapped cam,bent valves ,etc ,etc.washer picked up in the chain.

Kingston1100 · Jun 7, 2019

Richard King said:
You could take it to an engine rebuilder as they probably have seen this problem before. They may turn down the OD and make a steel bushing to slide over the shaft you turned down. I would say a shrink fit. I would not try to weld it. You need to think about balance and if you use a screw the balance of the shaft might ruin any bearing down the line.

Best option in my opinion

crazygoat · Jun 7, 2019

sealark37 said:
I have never heard of a timing sprocket failing. Chain wear, tooth wear, yes. Unless you are going to race it, don't worry about it. Just my take.

You must not be familiar with the nylon toothed gears used in mid 60's GM products. I have replace many of them. Replacement gear was an all steel one for a 283 chevy engine.

john.k · Jun 8, 2019

And the fibre gears too,but you cant compare GM car with one of the German beauties.

Luke.kerbey · Jun 8, 2019

Bore out, double key and sleeve and re cut the key in the same position.

It’s easily done, I used to bore and key sets of sprockets that were used in gangs on shafts where all the teeth had to line up

michiganbuck · Jun 8, 2019

Rock Auto or LKQ might offer a used part for much less than new.

failure of a shoddy repair might take out the engine.

QT:Richard King -> You could take it to an engine rebuilder ....

(also a chance an engine rebuilder might have an OK used part or know where to find one.)

yes may have to find the foreign engine rebuilder shop. likely plenty in California.

motion guru · Jun 8, 2019

would something like this be a suitable replacement?

WD Express 069 33019 280 Cam Gear | eBay

eBay

howieranger · Jun 8, 2019

Replace the timing gear. We never had to rebuild a 6.9 but did several 6.3s (basically the same motor mechanically)
At idle speed an expensive repair at high RPM you can have a catastrophic failure. Timing chain failure was easily a $10k repair 20 years ago and a high RPM failure can just trash the engine. How dissembled do you have the engine?

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