Piston sizes: A, B & C?

[soldave]

Have to state first that according to the USDM CAPS system this particular issue doesn't occur with 1G or 2Gs, but I thought you guys might still be able to offer some light on the situation. Even with the Mitsubishi CAPS parts system, this particular one is kicking my behind. Am trying to find the part number for a set of Evo I pistons (although the same is true for all Evos), and the CAPS is giving me 7 different sets:

STD A: MD300034/MD313285
STD B: MD300035/MD313286
STD C: MD300036/MD313287
O/Size: 0.25: MD300040
O/Size: 0.50: MD300041
O/Size: 0.75: MD300042/MD313291
O/Size: 1.00: MD300043/MD313292

What I want to know is: what is the difference between the the STD A, B and C pistons? Am assuming there are just tiny differences in size in the pistons that occur from day to day in the factory, but could I put, for example, a set of STD B pistons into a car that previously used STD C pistons? Am hoping to put a set of Evo 8 or 9 pistons into my Evo I but want to know if I should worry about whether they are A, B or C, or whether it doesn't matter.

All very confusing.

 

[Street Surgeon]

Suby's have different "standard" sizes for their pistons as well, not quite 10 over, 20 over, just a tiny bit from different manufacturing flaws or whatever you want to call it. I assume (hate to say it) that this is something similar. I would just go .20 over on whatever pistons you choose and not worry about it but that's just me. That should get you in the safe zone as far as whether or not you might have a, b, or c size pistons.

 

[soldave]

Well, when I went to pick some parts up from Mitsubishi today they had the answer from me in the form of a fax & diagram from mainland.

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The Japanese characters at the top left say that the diagram is of the bottom of the cylinder block. It then refers to 4 sets of numbers which appear as Roman numerals. In the table below the left column reads "Cylinder bore size mark" and on the right is the standard piston size, so there definitely are 3 piston sizes depending on the cylinder bore size if anyone didn't believe it before.

Now what exactly happens if I put a type A piston into a mark II or III hole... well I'll find out in about 7-10 days when I fire this engine up! Have been told that on an old engine it shouldn't really have any big effect.

 

[jrohner]

Huh, I thought they only used the A,B,C piston sizing on stuff with nikasil cylinders, like snowmobiles (because you can't overbore them).

 

[donniekak]

On the 3000gt's they have select fit cylinder bores, main bearings, and rod bearings. Mitsubishi machinists must be garbage. After you run a cylinder hone through the cylinders it wont make a difference. I've seen people use different sized pistons then the markings called for and it worked. If i recall there is only a couple .0001" difference in the pistons.