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Old 12-30-2004, 04:52 PM
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Join Date: May 2002
Location: sunnyvale ca
Posts: 338
how hard is it to rebuild a motor?

so i'm about set to replace all the water hoses/thermostat on my 71 300sel 3.5, and if i pull all of that stuff off i might as well put a chain tensioner on it (when the oil is old it makes noise on a hot restart), and if i do that maybe i should just pull the heads, as they are 33years and 165,000miles old, and if i do that maybe fresh rings would be nice, and well, with a mazda rotary, i'm used to just pulling the engine and building it like this. the benz runs ok, but its a 33 year old motor i'm beating on.

as for tools/experience i can and have rebuilt my rx7's from the ground up, engines/tansmissions etc, is there anything wrong with my train of thought here? or am i crazy to pull apart a decent running motor?

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Old 12-30-2004, 07:58 PM
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Join Date: Jan 2004
Location: Baton Rouge, LA
Posts: 69
just make sure that the machine shop understands all of mercedes specs when it comes to machine work.....

It is not hard to rebuild an engine.. It is hard to rebuild it right!

Good Luck.
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Old 12-30-2004, 09:40 PM
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Join Date: Aug 2002
Location: near Scranton, PA
Posts: 5,125
It's probably your timing chain being stretched, and not a bad tensioner - when the chain is hotter and the oil is thinner with age, it might slap against the heads a bit on startup.

I'm going to do the heads and chain on mine (eventually) but that's where I'm going to stop. I don't want to put new rings in, especially because I dont want to have to lift the engine out and remove 50 or so allen-head bolts from the oil pan(s), first off; and secondly, I wouldn't put new rings into a worn bore and piston... they've worn together, and new rings might not "fit" quite the same.

It almost certianly doesn't need ANY bottom end work - rings or otherwise - unless it was overheated to the point where the cylinder walls are damaged. I haven't experienced it firsthand but rumor has it you can often see the cylinder crosshatching in the 3.5 and 4.5 engines at 150k miles.

If you see your cylinder walls have damage (some marks are almost certian, but not deep ones), then you might want a rebore, although I'd advise against it unless its pretty severe. You don't want to ruin the block's integrity and special cylinder wall hardening process, get new pistons and rings, and then rebalance the whole thing to original specs so you don't get horrid vibrations at 3000+ RPMs.

Basically, I'd stop at new stem seals, valve guides, and POSSIBLY a VERY slight valve seat lapping (and subsequent replacement of the 16 valves, which is quite costly). Clean the carbon out of the heads, use all new gaskets, torque it all down properly, and make sure to rematch the cams to the proper rocker arms (You may want to buy a few rocker arms if you see they have worn down through the hardened surface, before your cams suffer).

Of course, I'm going to assume you want to do all this for fun. I think I might have burnt valves, thus why I'm going to do it, but if you don't need to, and don't have $250 over budget just in case something goes wrong (IE Machine shop "drops" your head or finds out it "was cracked"), don't even attempt it!
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Old 12-31-2004, 11:33 AM
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Join Date: May 2002
Location: sunnyvale ca
Posts: 338
hmm the reason i'm condeming the tensioner is because the chain was done about 40k ago (its on chain #3!). hmm i just did the math, maybe it is due

i'm not decided on actually pulling the motor, but in theory it would be nice to have a fresh strong engine, instead of a question mark.

i may end up doing the top end just so i'm not worried about whats inside the motor, this is kind of a rotary thing, as they can run fine with no good parts inside them, and theres no machining so you can be s.o.l. with those

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Old 01-01-2005, 10:57 AM
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Join Date: Dec 2000
Location: Long Island, NY
Posts: 3,727
Motors are made (and remade) to specifications that in benz's case are of "tight tolerance" and in some cases, pretty convoluted. You can measure chain stretch, valve timing, rocker clearance and cam lobe dimensions. I'd suggest that you start there.

Also, you can check head dimensions to see if somebody's already pulled the heads and milled them. Lastly, check your gears for wear.

Armed with this information, you can make some diagnostic decisions. -CTH
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