I have a set of identical looking pistons, I'll measure them up tomorrow and see what compression ratio they figure out to be and post some dimensions and volume.  I didn't know what they were out of either.

 

A Massey Harris 33 rod has a 2.25 crankpin and is 8.312 long.

 

A Perkins rod from a 3-152 engine is 9" center to center and a 2.25 crankpin diameter.

 

Another good cantidate is a Cockshutt, Waukeshaw rod.  I can't rember dimensions though, 2" or 2.125 crankpin sticks in my mind and a 8 or 8.5 length, but the rods are not real common.

 

David Brown also, again I can't rember but I measured up a bunch of rods one day at the salvage yard and filed it away as a good cantidate in my memory.

 

None of these rods are set up to hold common .990, or smaller wrist pins, all need to be bushed or plugged and offset bushed

 

Your block is 12.565, your crank has an offset of 2.25" and your Allis rods are 7.5".  Now you can run some numbers.

 

I you wanted to offset grind a crank, the H rod will  likely work with an allis piston, but now you need to cut retainer groves in a piston, or buttons to hold the wrist pin. 

 

Are the sleves good enough to use?  Start running some rings into out of round, tapered sleves and it won't seal, push them into the unworn  top part and you'll have junk in a hurry. 

 

The Massey rod and an automotive piston are cheap.  A set of 1.56 compression height pistons on the massey rod, and you'll pick up a littel stroke offset grinding it.  $80 will buy you a set of 8 hyperutectic pistons from Summit, add some rings and your sleve boreing, crank offset grinding, etc.  It'll soak up several hundred bucks no matter what.

 

You really aren't going to notice .100" inch of stroke, it just makes the rod fit.  It takes a set of rods and a crank grinding to get it.  If the crank has to be ground anyway then the additional cost isn't much to offset grind, but if you don't need to grind the crank than it's  a waste.

 

The other cheap thing you can do is use your crank and rods, use a WD type piston with a higher compression height.  You'll have to figure out the comp height of the piston and run some compression numbers for yourself to make sure you are safe, and or trim the pistons.

 

I have a set of 8.5" long rods I made, they use an Allis berring, allis bottom end, chev top end and boxed, balanced and sized for a .927  pin or a simple hone to .928 piston pin.  There's a good selection of automotive pistons available to make some 9 to 13 to 1 compression combinations.  I made the rods thiking I was going use them with an automotive piston in 4.125 or 4.155 bore.  I have a real nice set of sleves that need to be bored, new O rings for them and these rods.  There's several pistons available to make a 9 to 12 to 1 comp.  I would sell the rods if you are interested, I changed plans.

 

I have a set of M&W pistons from an WD, the compression height is high enough that they are .015 above the block deck when you add up the numbers.  THe head gasket is .045 thick and this gives me a .030 squish band in theory.  The bowl of the pistons is 70cc, this puts the compression around 13.5 to 1 which is to high.  But there's plenty of room in the piston bowls that I can trim them in the lathe and cut some valve reliefs in them.  I'm targeting 11.5 to 1 compression in a "quench" engine, a mild carb and a decent cam.  My head and manifold are worked over well and I would hope it can make around 70 hp on the PTO. 

 

A few weeks agoe someone posted pictures of  a M&W AP46 from a WD that they had on a 4.5 stroke engine.  No work done and the valves hit the piston, they also stated that the wrist pin boss was breaking.  THese old cast pistons will not support that kind of compression but to get one down under 12 to 1, and preferably around 11 to 1  I would think you could have a good running cheap engine and it will live a long life.  Compression is only an indication, a good head and cam will make up for a few points of compression, I'd rather see a good breathing 10 to 1 engine, then a poor breathing 12 to 1.