Piston Oil Ring Damage Found - relacement ?

[Mike Jenkins]

Hi Folks,

This summer my late model,1978, Atomic 4 started much more blow-by than normal. I have the Indigo PVC kit but started smoking up the cabin. I therefore decided to remove the engine from the boat and I have taken it apart in my basement. I reffered to MM's overhaul section in the manual. I discovered on piston 1 (closest to prop) the bottom oil ring was broken into several pieces as well as a few small pieces. A section of the piston was also broken between the oil ring groove and the next ring groove above approximately 1" x 1/4" long segment almost as deep as the ring groove. I believe this may have been my problem. The engine operated ok except while under load when it smoked terribly.

My question is can I replace a single piston? Or should I replace all 4 and if so what other work should be done; honing cyclinders, re-boring, or simply confirm measurements and install a new one in?

I also noticed on main grank journal beraing sheels(bottom and inside) front and back some pitted sections. Can these shells be simply be replaced?

I hope to take all the parts to a machine shop for measuring, testing and cleaning prior to reassembly in my basement.

Regards,

Mike

[Don Moyer]

Mike,

I'd defer judgment on piston replacement until you have the measurements in hand relative to bore diameter, etc. If (as per Part 4 of our manual) you can machine-hone your cylinders and have them clean up "reasonably well" by .004" over standard, you can reuse your original pistons. In this case, you can buy one used standard piston from our catalog to replace the bad one. I would recommend replacing the rings on all pistons.

Don

[Mike Jenkins]

Thanks Don,

I bought an electronic micrometer, accuracy .001 inch, from Canadian Tire and checked pistons and cylinder difference I think I am within the .004 tolarance but will have a machinist check it.

The crank-shaft journal bearings are also pitted (series of irregular gouges 1/8" wide and 1/32" deep) on inside half of bottom shell for both forward and aft bearings. I expect to replace these with standard size bearings. Would the bits of broken rings cause this? I assume the solution is a new set of bearing shells regular size if crank shaft journals have not been reduced much.

Regards.

Mike

[Don Moyer]

Mike,

Sorry, I missed the part of your question relating to bearings.

The series of irregular gouges you're seeing on the lower half of your main bearing shells is quite common (I'm attaching a photo of what I think you're reporting). According to Federal Mogul technical material, it's caused by "excessive idling". I have always had a difficult time relating the indications to too much idling, but on the other hand, I don't have any other explanation either - and neither apparently does anyone else.

We have seen these gouges progress across most of the width of a bearing shell on occasion and to our knowledge (as gross as the bearing shell looks) we have never heard of an actual bearing failure from this phenomenon, or even low oil pressure, which speaks well for the amount of safety built into the bearings.

You should, of course, replace the bearings when discovering this deterioration during a rebuild. As per Part 4 of our service manual, as long as you can retain a bearing clearance of .0035" or less, you should be OK to stay with the standard bearing size.

Don

[Mike Jenkins]

Thanks Don,

Thats exactly what I'm seeing on the bearings. I'm relieved its something common. Its odd that is only on the inside.

Mike