TOPIC: Crossflow piston fitting question

I’ve been meaning to discuss the subject of piston to cylinder wall clearance for a long time. I’m getting to do a full overhaul on a 1968 Evinrude S100, which I plan to have bored .040” oversize. I have NOS pistons & rings on hand for this overhaul.

The OMC shop manual shows piston to cylinder clearance as .0025” min - .004” Max

Every crossflow I’ve ever taken apart has around .022” piston to cylinder slop. This includes a couple of NOS factory RPH (replacement power head) engines that I have acquired and inspected. These engines were new old stock from the OMC plant and were new replacement engines, not rebuilt. Pistons had between .015 and .022 clearance – almost falling out of the block. I’ve seen this on Chrysler crossflows as well.

I am thinking this extra clearance is to allow for the expansion of the cast aluminum pistons when they are up to operational temp. Cast aluminum expands a lot, so this seems likely the answer.

Can someone explain to me in detail how to account for this extra clearance when re boring a cylinder and fitting oversized pistons, and why this is not covered in the factory shop manual?

Thanks -

Measure a new piston

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ALLOVER

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the place and that will be your answer

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They have lots of taper when new.

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Factory manuals clearly show how to determine piston size !!!

Its possible(?) the specs are when using new pistons during a rebuild, you are correct- new cast pistons will expand when running and will take a "set" after a while, and arrive at the clearances you measure. At my job, heat treating 6061 to T6 does affect the size, so our print specs are inside the actual finished specs. How much depends on each individual profile. Hope this helps.

I have the 1966, 67 & 68 factory shop manuals and they all say to use a caliper to check to be sure the piston skirt is round. In the table of clearances, the bore is shown a 3-3/8" + .020 or + .040 . They don't cover machining the block or fitting pistons other than checking piston skirts for roundness, measuring correct ring gap and providing the clearances in the table.

Is there a manual that discussed machining the block? because these don't discuss that at all.

Thanks

They clearly state that the piston is to be measured about a 1/4" from the bottom of the skirt at 90 degrees from the wrist pin.

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Measuring the piston anywhere else is

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A WASTE OF TIME !!!

I have the 1966, 67 & 68 factory shop manuals and they all say to use a caliper to check to be sure the piston skirt is round. In the table of clearances, the bore is shown a 3-3/8" + .020 or + .040 . They don't cover machining the block or fitting pistons other than checking piston skirts for roundness, measuring correct ring gap and providing the clearances in the table.

Is there a manual that discussed machining the block? because these don't discuss that at all.

Thanks

You can not measure with calipers !!!

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Micrometer maybe.

Bruce

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Tell me the EXACT measurement is of your new O/S pistons at the bottom of the skirt. Then the exact measurement at the piston rings.

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Tell me if you know what the

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co-efficient of expansion is for aluminum and the same value for cast iron.

My micrometer only has a 1.5" capacity. I have a dial caliper, but I don't trust it for this. I'll have to get ahold of my friend who is a machinist. I'm hoping he has one that will measure the piston. I haven't a clue on the expansion rate for the piston, or the cylinder.

I scored big time on a beautiful set of NOS .040 pistons. Late production with the oil grooves cut into the upper 2/3 skirt. These look like a much better casting quality than the other NOS pistons I,ve found. 40 bucks each.

Any measurements ???

No, sorry. I've been out of town & working every day. The boat stuff will have to wait for a while.

Bruce