I would be inclined to buy a new pair of matched pistons and rings, since you don't know the provenance of the old ones and they are clearly mismatched, are they even the same size? Even 2 different pistons of the same bore size can have varying weights etc. I myself feel it is false economy at this stage, pistons are not very expensive and you don't want to have to strip it again if it vibrates - or even worse!
Hard to turnover after new rings
- Thread starter rx7171
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dave M said:
The pistons look exactly the same including cutouts at oil rings and both with STD stamped on top and since it was put in no later than 77 I'm thinking it was probably a Norton replacement part and hopefully it was weight equalized to match the earlier ones to allow for single replacements caused by valve strike, etc.
If I went to new pistons, since these are standard size I would want to go up in size which would mean a bore job since I'm guessing 10-25k miles on engine with 5 years of use.
Boring, pistons, rings is adding up and I'm on a budget.
I'll run it and see if there is a feel that seems unnatural.
Thanks for your thoughts
Bob
We can all understand keeping an eye on the budget in these times of austerity, why not at least weigh the pistons before putting them back in. you can always grind a bit from underneath one to equalise weights if necessary, this is what happens if you send all the parts for dynamic balancing. You could also have a machine shop cut the ring groove a bit deeper in one piston, assuming that the piston wall thickness is the same.
you can use your rings to measure bore wear. Great explanation here:
http://www.hastingsmfg.com/ServiceTips/ring_gaps.htm
The whole Hastings web site "technical tips" section is a great resource.
Good luck.
http://www.hastingsmfg.com/ServiceTips/ring_gaps.htm
The whole Hastings web site "technical tips" section is a great resource.
Good luck.
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dave M said:
Dave:
Thanks for your suggestion.
I gave that some thought but during rotation the pistons are alternatively going through compression, combustion, exhaust pressures that simulate weight differences that are many orders of magnitude greater than the very tiny weight difference of a piston groove .020" less deep.
Since one piston is going through these stresses while the opposite one is going through the sames stresses but out of sequence with the other one that would create a very serious apparent imbalance effect which is what the engine is designed to withstand.
I agree in a static situation confirming weight equality would be the thing to do but I don't think in this cases its necessary.
Bob
RX7171, I wasn't suggesting that the removal of a tiny amount of material from a ring groove would really affect the balance factor. I was suggesting this so that you could use the new rings that didn't seem to fit and caused the initial problem.
I was however suggesting that as you had the engine apart anyway a quick check on the wife's baking scales (*Important* only when she is out of the house) would indicate whether the pistons were more or less equal in weight, given that they clearly did not go into the engine at the same time. It is the rotational balance of the shaft that any discrepancy affects not the linear movement of the pistons under compression, ignition etc. relatively small differences in weight are amplified by these forces that you describe, which is why tuners take such care in matching the weights of all of the components when balancing.
I was however suggesting that as you had the engine apart anyway a quick check on the wife's baking scales (*Important* only when she is out of the house) would indicate whether the pistons were more or less equal in weight, given that they clearly did not go into the engine at the same time. It is the rotational balance of the shaft that any discrepancy affects not the linear movement of the pistons under compression, ignition etc. relatively small differences in weight are amplified by these forces that you describe, which is why tuners take such care in matching the weights of all of the components when balancing.
Bob, I agree with Dave.It really isn't an issue if one piston does weigh slightly more or less than the other. If this were a 180deg engine it might make a slight difference, but not here. I've had four engines dynamically balanced and every time they had to equalize the stock crank, one side to the other; by a whole lot more than what you're talking about. And by the way, I didn't notice any difference in vibration, power, or anthing else after all that work.
One thing that one should keep in mind with balancing is that the balance factor in some manuals of 52% dry 63%wet is a typo. Adding oil to the sludge trap is going to lower the balance factor, not raise it. They corrected it in the MkIII manual to read "63dry 52 wet". Otherwise one ends up with a 42% balance factor. I've known several people who did, and guess what? They couldn't tell any difference either. Go figure. It aint that Important on a Commando unlike on most other bikes.
One thing that one should keep in mind with balancing is that the balance factor in some manuals of 52% dry 63%wet is a typo. Adding oil to the sludge trap is going to lower the balance factor, not raise it. They corrected it in the MkIII manual to read "63dry 52 wet". Otherwise one ends up with a 42% balance factor. I've known several people who did, and guess what? They couldn't tell any difference either. Go figure. It aint that Important on a Commando unlike on most other bikes.
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