1978 Evinrude 175hp Crossflow Rebuild Question

[Breilly]

Hello All -

Thanks for reading my post. I have taken apart my motor as described above and found that the #1 and #2 pistons are scarred up a bit as well as the cylinder head. I have three questions.

One - Should I replace all the pistons or just the two that have scoring? All the other pistons look perfect besides some carboning. I spoke to the marina and they tell me to replace everything including the barrings but they all look descent as if they were brand new and the barrings spin perfectly without any rust? What do you think? Replace everything including barrings or just the pistons or just the two bad pistons?

Two - Does it make a difference if the Cylinder head is scored or can I just leave it since it really doesn't have anything to do with the compression?

Three - Why does a piston ring shed in the first place? The marina said it is because the pistons get old and sloppy and catch which is why you should replace all the pistons. What are your thoughts?

Thanks,

Breilly

 

[emdsapmgr]

Re: 1978 Evinrude 175hp Crossflow Rebuild Question

You can replace just the two pistons. Replacment factory pistons weigh the same as the others. If your pistons are the factory high-ring type, common in this timeframe, I would recommend replacing the other 4 also. These early high ring pistons made good compression, but are prone to early failure with today's poor fuels. You replace pistons when they wear along with the bore. The bore will get oval in shape, over time and the piston skirts will wear unevenly. The machine shop can measure these parts for you and help you determine if they should be condemed or are reuseable according to the factory manual. In any event, you will want 6 new ringsets. Reusing a pitted head can be a problem. You may find bits of rings imbedded in the head which can break loose later, causing problems. Also, the pits cause hotspots-the factory will suggest replacing the pitted heads (both.) Broken rings are a common failure on these crossflows. Moreso with today's fuels. You may find your pistons have carbon buildup around the rings. When the carbon builds up, the oil/fuel mixture can't get to the rings to keep them cool/lubricated. They tend to crack when overheated, then catch on the intake or exhaust ports-then rattle around in the combustion chamber till they exhaust out the engine. You may even find ring bits inside the lower unit behind the prop.