Weighing and matching Pistons/con rods

[Geoff Lebowski]

When I cleaned/decreased the Pistons from my 2.1 the ink printed on the big end washed off!
I know that each piston is with it's original conrod.
I know that each big end cap is still with it's original conrod.
But, I have no clue as to which position each of my piston/conrods came from. I can't really see a problem as they all measured pretty much identically, the engine runs excellently (apart from my oil in coolant problem) but my question relates to weight and if those piston/conrod assemblies are matched in 'moving pairs', that is that pistons 1 & 4 are matched as a pair weight wise to pistons 2 & 3.
If they are, what sort of weight difference are we talking between 1+4 and 2+3?
I didn't think to weight them before I assembled the engine. There is a minor engine vibration around 1500 rpm that quickly disappears but I don't recall this vibration being on the donor engine. Thoughts?

[Peter.N.]

Never thought about this, I have always assembled them randomly and never had any trouble. I think they would have to be a long way out to give a problem, its not exactly a high revving engine.

Peter

[Bick]

Not ideal to mix them up but in you defence moden machining practises mean all the pistons should be very close. They will have bedded in to their respective boars so i would give each boar a good glaze busting and you should be okay.

[Peter.N.]

Ah, I was referring to a replacement of a set of pistons, not replacing used ones, don't think I have ever had occasion to refit the original pistons, not all of them anyway.

Peter

[Stickyfinger]

I cannot see the problem, the "throw weight" is still matched.

[lexi]

Considering that the piston part numbers, con rods are the same? (check it) then I think that is the tolerance that the engine is built to.
The flywheel /crank and possibly harmonic balancer would take care of the rest. Did you hone the bores or fit piston rings?

Are your engine mounts all good and OE?

[Geoff Lebowski]

I cannot see the problem, the "throw weight" is still matched.

not necessarily...
If piston A & B weighed the same as piston C & D then A & B would be fitted to 1 & 4 positions and C & D would be 2 & 3 positions. This is assuming they are matched in pairs at the factory.
If I mixed them up accidentally to result in a 1 & 4 position set that is off weight to the 2 & 3 position then wouldn't that create a vibration?
A appreciate that from modern manufacturing things are pretty close (negligible?) but I assumed that it was due to weight that the the pistons/conrod sets were numbered position wise. Is there another reason?

Lexi - the bores were re-bored, so nice and clean. Everything else was original.
I have not got a harmonic balancer pulley fitted for a few reasons, but my guess is they were fitted to dampen air con compressor 'shocks' more than anything else, they would have minimal impact on engine vibrations surely?

Regarding engine mounts - interesting as I have been having thoughts about these. The ones fitted are GSF replacements and are noticeably harder then the originals they replaced. So much so that I consider them a waste of money and will be fitting the previous (perhaps slightly tired, but perfectly useable) O.E. ones back in the spring!

[Stickyfinger]

Ahhh....I was thinking they would be matched as a 4x set rather than in pairs.
When rebuilding my Healey engine, I spent hours matching the weights across all six and the chamber volumes.....grind

[Bick]

I cannot see the problem, the "throw weight" is still matched.

''Lexi - the bores were re-bored, so nice and clean. Everything else was original.''

So you did use new pistons then?

[lexi]

Not sure about the harmonic balancer Geoff. My thinking is that it is there as a measure to deal with stuff that is a physical limitation of the design. Rather like the way that a 3 bearing crank will wear things a certain way, no matter what, then the five bearings sorted a lot of that. You may be spot on with air con thing as it came out round about then. In the dim past I faintly remember the balancing procedures. It's gone, along with the piles of Hot Car and C& CC Mags
Modern DMF'S counter the air con "kick" as well I think. Wow......nice one on the rebore.

[wurlycorner]

What car are we talking about here? (can't see any mention in the OP or OP footer?)

A common or garden modern engine won't have faffed about with matching piston/rod weights, they'll all just be within a certain spec. The only reason for a piston being in a specific bore, will be bore/piston diameter, IMO.