Sunday, July 10, 2005

Weights of the pistons and conrods

This is for the new motor, I should probably start a new blog for that and keep the running car entries separate. For now, I just want to get this in.

On 10/31/2004 I did big-end journal sizes of one rod, to get some practice in on using my caliper (analog, thimble-type). I concluded that it was 2.244, which is about 4 thou above high-limit for a std rod.

On 7/3/2005 I measured by weight the rods and the new JE pistons I got. The rods have numbers on the outside of their caps (so you don't mix/match them--why two of mine have the same number(!!) is beyond me.

700 -479g
742 -480.5g
204 -481g
204 -480.5g

Because there are two 204s, but their weights are different, I'm referring to one as "204(h)" (for "heavy"). It is a pig, well, really, the 700 is 1.5g too light and shouldn't be in this set, but what are you going to do?

The pistons are a little close than the rods, only 0.5g off, rather than a full gram (unacceptable).

Piston pis. +ring










I have to do some dressing to get this gang of parts within spec. If I start by matching the 411.0 piston with the #700 rod, I can reduce that big diff between the 204(h) and the light 700 rod. I was hoping I would only have to dress two rods, to be a 1/2 gram out on the high on each throw. But the rods were two grams different high to low, and the 0.5g of the pistons doesn't cover that gap. If I took the heavy #204(h) rod down to 480.0, that would bring its total to 890.5 and within a 1/2 g of the two "middles. But that leaves the "low" pair still a gram out. Does that 1.0g put too much whip on the crank? Which journal should the low pair of rod #700 go on if there is a difference?

Mix and match table
Rod No. Rod wght Cyl. Wght. Total
204(h) 481.0 410.5 891.5
742 480.5 410.5 891.0
204 480.5 410.5 891.0
700 479.0 411.0 890.0