Lapping Piston Rings

[HydraSport179V]

Hi.

Doing an OMC crossflow build of a 2.6 GT. Since I have the time I thought I might lap the piston rings to the bores. Reduce break-in, improve heat transfer, etc.

Could someone go into a little more detail about it? How much lapping is enough? Number of strokes? What should I look for to know when it’s good? I imagine I would see the ring gap open up a smidge as the rings lap in, but how much is too much? Would you use any kind of a compound on the bores or just plain oil? These are ProV pistons and rings.

Thanks

[BarryStrawn]

None. Modern rings are lapped when manufactured.

[oldschoolltv]

Use to use marvel mystery oil and comet, we would cut an old ring so it had half an inch gap and lap the piston then lap the new ring on a piece of granite and 1000 grit sand paper.

[BarryStrawn]

Bon Ami is much better than Comet for antique engines that get unfinished rings.

Neither is appropriate for any outboard. If new rings need lapped, return them for undamaged ones.

[CUDA]

Better spend your money on a lap-dance.

[toyomirage02]

Sealing rings on the crank

[FMP]

Crank to flywheel taper

[HydraSport179V]

Crank/flywheel taper - yep, doing that.

Lap the crank sealing rings? They are all new. How would lapping them make them seal any better?

Looks like I won't be spending time unnecessarily lapping parts though. Pistons came yesterday. Apparently the parts guy didn't know the difference between big bore and small bore pistons. Back on the piston hunt again. Can't believe I'm having such trouble finding 3-5/8" bore pistons for this engine. Cast ones.

[seahorse]

Bon Ami and 30 grade oil was they way it was done generations ago. The rigs were first lapped on a piece of glass, then to the ring lands, then the piston and rings, to the cylinder wall to expedite break-in.

[racer]

Done several the way Seahorse mentioned only used XD30 two stroke oil. My race engines were done this way and then only run 15 minutes before raced. I lapped the crank seal ring sides on a piece of glass with WD40 and 400 grit paper.

[oldschoolltv]

There was a very old omc service bulletin that discussed this, my grandfather use to do it to all our race engines.

[spybot]

Done several the way Seahorse mentioned only used XD30 two stroke oil. My race engines were done this way and then only run 15 minutes before raced. I lapped the crank seal ring sides on a piece of glass with WD40 and 400 grit paper.

Maybe a stupid question but
If i were to do as seahorse and your self have done to lap Rings, then lands and finally the cylinder will this cut down on break in time after the rebuild ?

I am not building a race engine just a really good better than factory finish to my OMC V4
I have picked up bits and pieces of info from all this site. I am now in the process of reassembly. Any tips i can pick up like the one mentioned here are invaluable to me. I stay in the east of Scotland. There are no johnson mechanics near me to pick there brains so everything i have learned on the subject is from here.

If i can make the engine break in easier by doing the lapping it shall be done

[rude tim]

Maybe a stupid question but
If i were to do as seahorse and your self have done to lap Rings, then lands and finally the cylinder will this cut down on break in time after the rebuild ?

I am not building a race engine just a really good better than factory finish to my OMC V4
I have picked up bits and pieces of info from all this site. I am now in the process of reassembly. Any tips i can pick up like the one mentioned here are invaluable to me. I stay in the east of Scotland. There are no johnson mechanics near me to pick there brains so everything i have learned on the subject is from here.

If i can make the engine break in easier by doing the lapping it shall be done

Doing this process is not easy and if not done correct it can cause problems. In a full on race motor that piston life is a concern this process is a good idea. Proper clean up after is paramount.

[spybot]

Doing this process is not easy and if not done correct it can cause problems. In a full on race motor that piston life is a concern this process is a good idea. Proper clean up after is paramount.

Is the clean up the cause of problems ?
Or are there other factors ?

[rude tim]

Is the clean up the cause of problems ?
Or are there other factors ?

Bon Ami is a form of lapping compound. If you do not get ALL of it off the rings, pistons and cylinders you will wear out the engine immediately. If you have never done it before I would not suggest it. Also the time spent doing this will end up being about the same amount of hours or more than it would take to brake the engine in in a standard fashion.