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coblue 03-04-2013 09:07 AM

OEM rod weight and balance
This is a question for the experienced builders. Getting ready to reassemble my stock ej255 turbo block and of course the piston/rod combos got messed up. All the pistons weigh the same and I have the 2 size A and 2 size B's sorted out. The pins all weigh the same. Two of the rods each weigh 5g more than the other two (595g vs 590g). Two of the rods are stamped with a code containing the letter A and two with B. I assume they match up with A and B pistons. From a picture I took I know one of the 590g rods is number 2. Does the other 590g rod go to number 1 and the two heavier rods to 3 and 4?
Subaru built it with these rods so I assume it's in their tolerances. Is it worth balancing all the rods to 590 for stage 3 (bnr16g with possible e85)?

fahr_side 03-04-2013 09:31 PM

I would assume the stampings A and B do not relate to the pistons, but the dimension range on the big-end bearing diameter, or something like that. I would think in terms of balance that it would be better to run the rods that weigh the same opposite each other. If you're using the old bearings you could just plastigage the rods on the crank and that should clear things up pretty fast.

BlackHole 03-10-2013 07:23 PM

A and B are the piston & bore diameters. The block will also have markings for A & B bore - more info here: The most important part is to match the pistons and bores to keep the piston-to-wall clearance within spec.

coblue 03-10-2013 08:22 PM

Thanks Blackhole,
I just came in from the garage after assembling the case halves. I never got a definitive answer on the rods, so I just went with what I proposed and Fahr-side concurred- the two lighter rods on 1 and 2 journals, the two heavier rods on 3 and 4 ( and of course the A pistons in the A bores).

fahr_side 03-10-2013 08:34 PM

Did you plastigage the clearances?

coblue 03-11-2013 10:53 PM

Yes, they were within spec relative to the accuracy of the plastigage scale. I couldn't tell if the rod ends varied in size. The crank was polished so the clearances are on the high side, but I had the journals mic'ed twice and they are ok.

fahr_side 03-12-2013 12:52 AM


Originally Posted by coblue (Post 4335379)
The crank was polished so the clearances are on the high side, but I had the journals mic'ed twice and they are ok.

Be sure to check oil pressure once you're running again and be ready to shim the oil pump check valve spring if needed.

coblue 03-13-2013 10:41 PM

Thanks. Researching which gauges are a good match for the '05 dash.

fahr_side 03-14-2013 01:07 AM

Not a bad idea, but you can also install one temporarily to check pressure at start up to be sure you're okay to go ahead and seat the rings.

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