1 5/16" conrod variants

[JonE]

Having found a 15g disparity in conrod and cap across the 4 rods in a running engine, I surmise that there are two variants of the same part number, early and late; the early ones having the fancy Austin script and the later heavier ones not. 

Presumably engine builders notice this when evening up weights for the units, whereas those trying to fix an engine don't; they just repair and replace... which may lead to the situation I was in in the first place.

Any thoughts?

I consider I probably won't find 15g removal on a rod and cap and thus may have to add washers to the other three... or find an earlier variant that has perhaps been used on a reground crank whose whitemetal could be made good for my unground crank? (I'd like to use aerolock nuts so I don't have much option for shortening the bolts on the heavy one, either).

[John Cornforth]

Hi

When I rebuilt my 2 bearing engine many years ago, I found that I had rods (with caps) of 318, 361, 363 and 372 grams. One of the many joys of the occasional repairer who has to learn things the regular repairers will probably be familiar with. I assumed that the 318 was a light pattern and there was no way of adding about 47 grams to bring it in to line with the average of the other three. I put the 318 aside and luckily managed to source a 370.

By assembling the engine with 370 and 372 on Nos 1 and 2, and 363 and 361 on Nos 3 and 4 I figured that balance would be acceptable. It feels OK for a road engine that rarely goes above 4000 RPM.

The density of cast iron is about 7, so 15 grams is about 2 cc. Quite a lot to grind off, but not impossible if it is done over a wide area. A 5/16 inch 40 thou thick steel washer is only about 1 gram, so you might struggle.

Maybe slimming down the sides of the white metal would help ?

[bystander]

There are also quite a lot of aftermarket rods kicking around that are considerably heavier. I've no idea what they actually weigh, but you can see just by looking at them how much chunkier they are.

[Chris KC]

15g is a lot to remove from an already slender rod, and too much to correct with washers. I'd acquire a few spare rods and endeavour to match the others more closely without grinding.