340doc
Well-Known Member
Would anyone know the weight of a factory 10.5 to 1 piston?
trick is to get a good big end weight with any DIY jig. The varience is unbelievable with the smallest bit of pivot movement on either end. I bet that pro jig in all the videos would give a different weight with the slightest amount of skew in the horizontal plane. I ended up using a ball bearing in the little ends rod bushing so it would find its highest point horizontally and using 3 ball bearings under a metal plate to support the big end at a perfect 90 degree on the scales plate. That way the lateral position was always the same with no x-axis force, and still could only get 2-3g repeatablilty. Cheap scales dont handle side to side force well when looking for repeatablilty. straight weight is eazy, highly repeatable on even the HF .1g scale. 4 of 5 weigh ins on a piston will give me the same .1g result with 1 being .1g off. sometimes just tapping the table will get it to fall in line with the others...I just weighed up the pistons and pins and ring and locks and bearings, combined that with the rod end weights listed on the SCAT box, used a standard bobweight formula from the Eagle site, handed the resulting bobweight number and the crank to the machine shop, and asked them to balance to that bobweight....and I enjoy doing that type of work.
Yep, I tried all sorts of techniques to get the side load error out of my $50 scale.....I tried a sub-plate on the scale on oil of various weights to relieve the side load error... 75W90 gear oil was the best. But it still had several grams of variance in repeatability. Finally, finding the area to tap on the machinist's flat that supported everything to make thing 'settle in' did the trick. Ended up with a +/-1 gram consistency on the big end. Even with that, the sum of the small and big end weights average about 1 gram below the total rod weight by itself so there is probably a small uneven-ness somewhere.trick is to get a good big end weight with any DIY jig. The varience is unbelievable with the smallest bit of pivot movement on either end. I bet that pro jig in all the videos would give a different weight with the slightest amount of skew in the horizontal plane. I ended up using a ball bearing in the little ends rod bushing so it would find its highest point horizontally and using 3 ball bearings under a metal plate to support the big end at a perfect 90 degree on the scales plate. That way the lateral position was always the same with no x-axis force, and still could only get 2-3g repeatablilty. Cheap scales dont handle side to side force well when looking for repeatablilty. straight weight is eazy, highly repeatable on even the HF .1g scale. 4 of 5 weigh ins on a piston will give me the same .1g result with 1 being .1g off. sometimes just tapping the table will get it to fall in line with the others.
I keep remembering 740 grams when I think back.
Maybe that's the 340 rod I'm remembering
Well. Your pretty darn close! The shiny ones are a 'balanced 'set. All taken off the little end pad...? The rusty one is untouched. All the balanced caps weigh the same so the must have hit the big end with a flat belt sanderI keep remembering 740 grams when I think back.
Maybe that's the 340 rod I'm remembering
Im thinking anyones jig is going to give a different reading and that includes shop to shop. We have both seen the slightest variance move the scale 10g (~1/3 oz) in either direction. Im starting to believe that the balance you get with the shops bobweight (1X4) is gonna get you close, but there is nothing short of a dynamic balance in the block that's gonna get you nuts on (at one RPM!!) ...and with under and overbalance practices and moving balance numbers throughout the RPM range...its just needs to be close. For example , there is an added 'oil assumption' value, "...Eagle uses 5g for small block assemblies and 15g for big block assemblies. (all balance shops use an estimated oil weight) Since it is impossible to accurately represent this value, it is just an estimate.." so that right there is saying 10g out of balance isnt going to make a noticeable difference. Road kill garage had a 360 assembly 'balanced' at a machine shop and they said it 'shook like a paint shaker' on the dyno. I think those guys know the difference between a neutral balance and a Detroit balance in their parts... holy crap! ...It seemed like the slightest tilt of the rod would change the reading drastically.Went back to my flat plate jig.....got a 1 gram variance putting the same rod on a coupla times.