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  1. #21
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    I'd just make the timing gear adjustable if there's a stupidity tax....even if you don't use it that way. Turn out the centre, machine, drill and tap the spokes, remake the centre disc and broach, slot on rotary table for fasteners.....Bob's your aunty!

  2. #22
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    Quote Originally Posted by Tony Quiring View Post
    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
    If the engine in an interference engine where valves could be bent if the timing chain were to be displaced due to a broken cam gear, the damage could be incredible.

    In the other hand, I had a Bolens Garden Tractor for something like 20 years that had a cracked flywheel at the keyway where it fit on the crankshaft. It never caused any trouble.

  3. #23
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    Quote Originally Posted by motion guru View Post
    would something like this be a suitable replacement?

    WD Express 069 33019 280 Cam Gear | eBay

    eBay
    No - unfortunately every website and online vendor lists the 6.9 and the regular 450SEL (4.5) as having 100% interchangeable parts, but in fact there is almost nothing that the two engines share.

    Quote Originally Posted by howieranger View Post
    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?
    I was replacing the head gaskets. I have just bolted the heads back on with new gaskets, and fed in the new timing chain, that's it. (So the intake, water pump, SLS, accessories are still off.)


    I was unable to find a definitive answer, but based on the lift on the cam lobes and the dish in the pistons, I'd bet that it's an interference engine. Once I make the valve spring compressor tool that I need, I can install the rocker arms, turn the crankshaft, and find out for certain. But whether or not it's interference wouldn't affect my decision making - there is enough potential for carnage in the chain area alone. So I will get this thing fixed with 100% confidence level or buy a new one.

    I have found a new one online for $400. I've never hired a machinist for any kind of custom job like this, but if they charge somewhere between $50-100/hr, and it takes them a few hours to properly repair the old sprocket in one of the ways mentioned already, would you guys go for it or buy the $400 part? I'm leaning towards buying the new part but just want your input. Thanks

  4. #24
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    For the record, Dye penetrant kits are fairly cheap, easy enough to use, and will show you where the crack ends on any metal vs just steels with magnets and powder.
    Just be careful if non-metals are close to where you would be testing. Everything but the developer could be sprayed into a cup and brushed on if precision is needed.
    For cleaning the dye off before applying the developer you have to use the supplied cleaner, for post inspection cleaning you can use cheaper more available solvents.
    That being said I'd just get a new one... To get the kind of weld you would trust forever would likely cost more than a new one from the dealer.

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  6. #25
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    What model and engine is this... telling us that would be helpful. All I do is fix European cars for a living and have some sources for stuff. Not helping anyone by witholding info... also the key iteself doesn't take all the load, the friction created by the bolt pinching it to the cam takes the load.

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    Quote Originally Posted by kustomizingkid View Post
    What model and engine is this... telling us that would be helpful. All I do is fix European cars for a living and have some sources for stuff. Not helping anyone by witholding info... also the key iteself doesn't take all the load, the friction created by the bolt pinching it to the cam takes the load.
    Quote Originally Posted by FRporscheman View Post
    I am reassembling a classic Mercedes engine, an M100 6.9L V8.
    . . . .

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  9. #27
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    Down the alley from my shop was a Spanish gentleman who fixed older expensive ,prestige cars for owners who had no money......this was the kind of bodge he specialized in......and the monumental fails he had were great laughs at lunch break.....he did a bodge on a V8 Benz,the double row chain jumped the sprocket one row across,and bulk damage......so he bought another motor from the wreckers......unfortunately ,it blew lots of oil smoke,and the owner got pulled up by the cops on the way home ,and given a defect notice .........he did gas conversions too,and many amusing explosions when rocker covers and sumps were blown off........

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    Quote Originally Posted by motion guru View Post
    . . . .
    Man... I could have sworn the first time I read this post a week or so ago there was no mention of the model, oh well. I guarantee you can find one of those second hand, those old CIS benz's are out there all over dead in the water with CIS problems. Have you had any luck at all trying to find one second hand?

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    where in California? there used to be several German wrecking yards near Rancho Cordova.
    Bill D.

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    Quote Originally Posted by FRporscheman View Post
    I need to run this by a machinist.
    You just did that. Several, actually. All having to GUESS, of course as to how much risk an already-cracked cast item has in it, unfortunately.



    Having actually HAD to replace not only bent and shattered valves and guides PLUS pistons, rods, gudgeon pins, bearings, etc. over a similar type failure on a much less-costly motor that was still in high-volume NEW production in the day?

    I'd surely buy a new one for this riskier case - especially as it is still in the "Bents" (phonetic.."zed", after all... they got THAT part right...) parts system.



    That it IS in the parts system hints that your one is not the first-ever failure, yah?

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    This looks like a cast-iron gear. Any adequate repair will involve enough heat to anneal it, if the teeth are hardened. So, first test tooth hradness. If they are hard, buy the new part.

    If teeth are soft, I think it can be fixed. Whether the processes I suggest are cheaper than your $400 replacement is your decision.

    First determine if the crack extends out to the OD of the hub or into the spoke, or if it stops before it gets to hub OD. Make a jig so that you can damned-sure put the keyway back where it belongs.

    If there is any uncracked hub-wall-thickness, bore out to that diameter, make a keywayed bushing out of steel with OD a couple of thousandths smaller than your bore, and silver-solder it in. Heat the whole gear slowly and slow-cool or it will crack somewhere new. Did you remember to index the keyway?

    If hub is cracked all the way through, vee-out the whole crack, wherever it goes, with a die-grinder, from all sides until the vees meet in the middle. Flux, preheat, and braze-weld. Slow-cool. Re-cut keyway.

    Done right, either repair will be as good as an uncracked sprocket with however-many miles this one has on it. $400 does not sound so bad?

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    Quote Originally Posted by john.k View Post
    And the fibre gears too,but you cant compare GM car with one of the German beauties.
    Those 'German beauties' are grossly overrated. The best thing Mercedes has going for it is a great advertising department.

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    thermite, I know (and thanks!) but I meant in person. The one I spoke with turned the job down. I ran this whole situation by my buddy who is a blacksmith and his concerns over the heat-treat lead him to advise just buying a new one.

    I'm in the SF bay area. No luck finding a used one, but I have 1 lead that I'm still waiting to hear back from.

    CIS is very reliable - it's CA smog tests that send these cars to the junkyards. But only 1816 of these cars were sold in the US (of 7380 total) so they are rare (and I don't see that as a good thing).

    Thanks guys for all the great ideas, advice, and knowledge. I will give up on fixing the old part, and if I can't find a good used one in the next month I'll order a new one. At least it gave me the opportunity to learn everything I've learned in this thread, which is a LOT.

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    Quote Originally Posted by FRporscheman View Post
    thermite, I know (and thanks!) but I meant in person.
    ..as in "hands, eyeballs, and metrology applied to the actual physical part".

    I hear yah, but ...

    ..the reality is that for all our vast collective experience, PM "community", MOST among us are individually rather more specialized. Not all failures happen often. Fewer yet cross our bench very often.

    So it would have to be a person who had dealt with rather more bustid gears than average, better yet, IC engine gears, better yet, upscale Euro motor car engine gears, better-yet ones rare enough to justify serious investment in repair - else decision to fab / contract-out replacements from scratch.

    PM has several from-scratch gear MAKERS, but as to assessment of a possible repair to a relatively high-stress motor - thin-section cast gear especially - I'd say Ross (AlfaGTA) may be one of only a handful among us with much of that sort of exposure.

    Even so, it is probably the grey-haired and balding career Benz mechanic who has seen more of this than any machinist, and has the better idea of the risks.

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    CIS is actually fairly reliable... but when you do have a problem oh boy oh boy oh boy. Its either impossible to get parts or eye watering expensive. I will shoot a message to a vintage Benz guy I know in Wisconsin, he has a real hoard.

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    I'm not sure if this has already been spelled out, but if you index to another exact chain tooth to existent key angle, and put a new key slot in that location (ideally 180 degrees away), you should be fine to run.

    If there's no press fit to the camshaft, it means radial (hoop) stresses are minimal. Without radial stress, there's no (significant) force on the crack. If you make the new slot with small radiused corners, then round the edges of the camshaft key to match, you'll reduce the risk of a new crack forming.

  20. #37
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    There are several M100 guys out there, you could try Metric Motors in Canoga Park or SSF which is now part of IMC I think? Not sure if the M117 sprockets are the same, I have several I could give you. Don't put a known bad part back in the engine, M100's are rare as you know and it would likely take out a piston or two at the very least.

    Steve
    Former Mercedes performance shop owner.

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    Quote Originally Posted by Milland View Post
    I'm not sure if this has already been spelled out, but if you index to another exact chain tooth to existent key angle, and put a new key slot in that location (ideally 180 degrees away), you should be fine to run.

    If there's no press fit to the camshaft, it means radial (hoop) stresses are minimal. Without radial stress, there's no (significant) force on the crack. If you make the new slot with small radiused corners, then round the edges of the camshaft key to match, you'll reduce the risk of a new crack forming.
    If the keyway were - just for sake of example - chewed-up to the point of degraded positional accuracy, and there was NO crack, I'd support the cutting of a new one.

    That there IS a crack tells me this particular casting may not be as safe a candidate for that sort of repair as it could be. It has already cracked in one place. It could do so again. Not all that safe to cut new without also doing stop-crack and/or braze to the existing crack, too.

    And then one is back into pre-heating, heat-treat effects overall... etc., etc. as already mentioned.

    Ross can chip-in if he's onto this thread, but my guess is the better solution w/r trustworthy durability for a rare motor of sufficient value - now or as a future collector's item - might be to machine a new gear from the solid?

    Not cheap, ($1,500?? or ?? ) ..but neither is it exactly rocket insemination.

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    Quote Originally Posted by FRporscheman View Post
    I'm in the SF bay area.
    PM 'Dan from Oakland'. Beaucoup de automotive experience, a variety of hobbers and shapers and even a keyseater or two.

    ^ The guy who used to do Ross'es gears is no longer available

    Danny Borg is still in Redwood City tho, if you're too nervous to go to Oakland and D from O is too busy. The Other Dan does good work also. Background in submarine transmissions, which might be overkill for your job but nice guy anyhow. Borg Gear, Redwood City.

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    Quote Originally Posted by thermite View Post
    That there IS a crack tells me this particular casting may not be as safe a candidate for that sort of repair as it could be. It has already cracked in one place. It could do so again. Not all that safe to cut new without also doing stop-crack and/or braze to the existing crack, too.
    Bill, that crack is without doubt on the driven side of the sprocket, meaning it's been subject to the repeated drag, release, drag of wiping the cam lobes over the lifters millions of times. I.E. lots of cyclic loading, at a sharp inside corner - the perfect recipe for crack initiation.

    So sure, that part's cracked. But take away the cyclic loads (it'll see no load when no key is present) and it's just going to whiz around getting dizzy, and won't give a toot otherwise. The new slot, being rounded and prepped for use (hell, go and shot blast it for the compressive layer) will almost certainly be better than just putting a new, sharp cornered sprocket in place.


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