Evaluating piston condition on Merc 500

[hohnstadt]

I have low compression on 1 cylinder of 1969 Merc 500. I removed the intake covers, and saw that 3 of 4 cylinders looked clean and the rings are loose enough to depress when pushed on (compression in them was 120, 115, 120). <br /><br />The piston in the cylinder with lo compression (30) had a darker film on it (somewhat oily, somewhat hardened), and the rings did not depress when pressed on. It also seems to have some small, horizontal grooves near the rings (scoring?). When pushing on the rings, I noticed the whole piston moved laterally a small (but visible) amount. Does this imply I might need something more than a new piston (new block and/or crankshaft)?<br /><br />Are pistons swappable (e.g., move a good one to this cylinder to see if the compression improves)? My thought (being a cheapskate) was that if a used piston generated a decent compression, I could search for one of those vs. buying new, and/or would know whether the cylinder needed boring. Or is this not a good plan?

 

[Ben Konopacky]

Re: Evaluating piston condition on Merc 500

for,sere,you might as well take a good look at the inside of the eng.if you are willing to take it all apart replace a piston w/used one replace all gaskets and might have to do over if compression doesn't increase... Doesn't make sence .Do the job the correct way,i beleive when it's for me it should be the best and right way..<br />SO when you take a look at it,know what needs to be done,is it wirth the time , effert and money to do it right.<br />IT does sound like a piston.bore,& ring Job (.015 oversize). GOOD LUCK

 

[Barnacle_Bill]

Re: Evaluating piston condition on Merc 500

First off, I agree with Ben. To go into an engine that far and not do it right is a complete waste of time. From your description, it sound like a motor that has sat for a long time and the piston with the stuck rings was up on compression stroke. Try decarbing the engine (see FAQs) and I think a product is made that may free the rings up. I can't remember the name but perhaps someone else can. IMOHO Its worth a try before you tear the engine apart.

 

[Hank 911]

Re: Evaluating piston condition on Merc 500

I don't know if it's what Bill is referring to, but a product called Marvel Mystery Oil used to be the thing everybody used to free up an engine that had set up for years. Pour it in the cylinders for a few days before cranking it over. It worked for me on a '59 MG. It might free up your stuck rings. Good luck.

 

[emckelvy]

Re: Evaluating piston condition on Merc 500

Nothing's going to help at this point but a complete teardown. It's already past the point of no return; been operating with stuck (carboned up) rings for quite a while obviously, and the exhaust gasses have blown past the rings and burnt/scored the piston. You won't know the extent (if any) of scoring in the cylinders until you tear into it. <br /><br />You can get some idea how bad (or good) it is by moving each piston down to the bottom of its stroke and checking out the exhaust side of the cyl by peering thru the intake ports. <br /><br />You may only have to deglaze the cylinders, replace the one bad piston, and re-ring/re-bearing/re-seal the motor.<br /><br />HTH.......ed

 

[hohnstadt]

Re: Evaluating piston condition on Merc 500

Thanks for the advice - I'll look for the new parts to do it right.