OMC 4.3 broken rings?

[Grouchybear35]

I have finaly found time to remove the right side head from my OMC 4.3. I am not sure of the year of the engine but it was installed in "94". prior to this I did a comp. test to find that the rear cyl. on this side had no compression. The others looked okay. Now that I have the head off, I can find nothing wrong. I broke the valves down and still see nothing. The cyl. looks pretty good as well, no major scratches etc. I am thinking that I have a broken ring on that cyl. I am thinking of pulling the pan and pulling this piston to see what is hiding there. If this is the case, I plan to pull the others as well, hone the cyl. walls, and install new rings. When I do this should I replace the rod bearings or reinstall the origonals? I would think replace them but I wanted to run this by everyone.

 

[DJ]

Re: OMC 4.3 broken rings?

Grouchy,<br /><br />It certainly will not hurt to roll new bearings into it.

 

[rodbolt]

Re: OMC 4.3 broken rings?

hello<br /> a 0 reading wont be a broken ring, take that piston out first but I will almost bet it was a valve problem. ie it was stuck till ya pulled the head.this is were doing a cyl leakage test would have pinpointed the problem to rings or intake or exhaust or headgasket.<br /> any time I have low compression I use my leakdown tester to futher isolate the problem.<br /> if the bearings look good and you dont mix them up as for orientation, then clean everything and plastigage the surfaces. if the clearences are ok then oil and reinstall the bearing.<br /> good luck and keep posting

 

[LubeDude]

Re: OMC 4.3 broken rings?

RodBolt is right, there is no reason that you cannot use the old bearings provided they are not worn! Plastiguage will tell you that! Bearings are relativly cheap though, Its your call!!<br /><br />Be sure to use a quality assemply lube when you reassemble the engine and do not forget to prime it before you start it for the first time, you can buy a primer at your auto parts store or use an old distributor with a nut welded to the top, and the gear removed!! Chuck a socket of the aproprite size in a 1/2 " Drill and turn it clock wise untill you get full oil presure, them turn the engine over by hand and do it again, one more time if you want!! This will insure you will not have a dry startup!<br /><br />As to the assembly lube, there are several assembly lubes you can by over the counter made just for this purpose! I have over the last 5-6 engines Ive built used a black moly grease! I know there are controversies about this practice, but I have NEVER lost and engine on startup following this procedure! Just a light coat on the bearing sufaces, cover the whole cam with it before you install it and put a dab on the lifters when you install them, also a dab under the ball on the rocker arms!! Fill the crankcase with a cheap oil of your choice, (a Lighter weight) and a cheap filter, start the engine, run it at1,500-2,000 RPMs for about 10 minutes while checking for leaks and watching the oil preasure and temp! I use this procedure even if I am not putting in a NEW cam because it alows a lot of oil splash into the cylinders to wash out the first bit of breakin metal! Then shut it down and drain the oil and change the filter and install whatever oil you are going to use in the engine! Its necesary to do this to because the oil filter may be clogging with the assembly lube and there will be a LOT of small fillings in the oil from the breakin!! (No synthetics at this point as they are not good for breakin!! Then start it back up and do your final tuneing!<br /><br />Used to use a medium grit hone in a good 45 degree crosshatch paturn, but this has become old practice in the last few years, Now the engine builders are saying to use a fine final honing and not to use very much oil in the cylinders when you install the pistons!<br /><br />I think I about covered it!! Even though you didnt ask!!<br /><br />LubeDude

 

[Grouchybear35]

Re: OMC 4.3 broken rings?

Thanks for the info guys. I think I will go ahead and replace the rod bearings, Have the heads gone through and replace the rings. Hopefully, this will cover it all the way around. I had planned on going through the outdrive this winter, but this may have to wait until next winter now due to this.

 

[andrewgroup]

Re: OMC 4.3 broken rings?

This drives home the point of doing wet and dry compression checks. No rings at all on an oil wet cylinder will create compression. I once watch an old mechanic in the early 70' remove the valve rockers and using a rigged up air pressure contraption to inject air pressure into the cylinder to test for valves being stuck open. I suppose this would certainly blow the oil off the walls, but I can't see that it would damage anything.

 

[rodbolt]

Re: OMC 4.3 broken rings?

hello<br /> andrewT<br /> that is similar to a leakdown test and has been done I guess since there have been valves and compressed air. . we dont use the old oil trick anymore. its to messy and a fire hazaed. not to mention the enviromental issues. so now we use a leakage tester and a good tech will tell you exactly what and where its leaking <br /> good luck and keep posting

 

[andrewgroup]

Re: OMC 4.3 broken rings?

I'm always trying to learn more, and it's unfortunate that Grouchy has ended up in the position he is in. I hope when I'm faced with trouble, I have the sense to post it here and get some good advice before I go off and make a few subtle mistakes in troubleshooting.<br /><br />Hopefully he'll post the final results and some of us can learn from his mistakes or successes.

 

[Grouchybear35]

Re: OMC 4.3 broken rings?

Hey everyone,<br /> I will post what I end up finding but it will be a while. I am working in Canada for a while and don't have much time when I get back to Tennessee to work on this. Two weeks from now I will probably have an afternoon to get back in to it. Thanks for all the help thus far.