My pistons have different weights!

[t20racerman]

I'm building up the A10 and the last set of barrels had +60" pistons and a oversize bore that meant a lot of oil burning and pressure (rebore shop did it wrong - "sorry mate"). Due to lack of funds I was resigned to using them again until I could afford to get them relined.

Anyway, searching for all my A10 bits in the garage (has been in bits for 10 years) I found a really nice set of barrels I'd forgotten I had (I really can't remember getting them), and not only was it a nice find, I'd had them epoxy coated black and they were a tight +20" oversize, with two pistons inside! Yay!

The pistons are different makes though - one is quite a rough casting inside compared to the other one which has AE inside it. They are though both High comp pistons, and had been used in the past as a pair as they have L and R and direction arrows scraped inside them. Thing is, I weighed them on the wife's lovely new kitchen scales and one has a weight of 258g and the rougher one is 272g - 14g difference! I'v tidied up the rougher inside on a bit with the dremel and it is now 271g, but is that enough?
So do I:
- Spend ages with a dremel lightening it (11g is 2cm^3 of ally!)
- Buy new pistons that I really can't afford (and I've just ordered new rings for these pistons only yesterday....)
- Just fit them and forget that I weighed them in the first place

Any other choices?
Anyone got a good nick second hand +20" HC piston of mass 258g or 271g? (includes rings)

I'm tending towards grind and fit...... 

[bsa-bill]

Hi there t20racerman

Seems like a fair difference on the pistons
Now I don't want to complicate things but  ----- does the weight of the rods come into this also, you might be able to gain a tad to even things up a bit

[muskrat]

G'day t20',
              you might find one is cast the other forged. You could take a bit off the skirt length, say 4mm, but that still might not be enough.
You could use as is BUT (disclaimer) I don't recommend, and only till a new set is found and revs kept down. It will probably vibe.
Ready to duck for cover!!
Cheers

[MG]

You could save some weight on one gudgeon pin by tapering the inside both sides if you are so equipped (lathe and carbide tools, cooling!!!)
Then there are the usual areas to take material off, piston dome (inside only  ), pin bosses, skirts (like Musky mentioned already), etc., but be careful and don't overdo it in the wrong spots!

Are the piston crowns the same shape? It is not a good idea to use a flat top and a domed piston together, even if they have the same CR (different combustion chamber geometry, rate of combustion, etc...)

Cheers, Markus

[t20racerman]

It is a problem isn't it?
I must admit, I didn't weight the rods (wish I hadn't weighed the pistons either!) and the bottom end is assembled and in the bike now, so I don't want to go down that road. I've done a little more grinding but that has made very little difference, and like you muskrat I was thinking of shortening the skirt a bit. Also, I will widen the gap a tad where the little end goes - a fair bit of metal there! I need to remove over 2cm3 of ally to get it about right though!

The gudgeon pin idea is a good one MG - think I'll give that a try also. I have a small lathe and there is quite a bit of heavy metal on that. Pistons look identical from the outside - a matched pair in all but weight...

I'm hoping to get the bike up and running in time for a bike rally on the weekend of July 17th - a reunion with old College buddies - and so am pushed for both time and money. I'll have to build it up with these pistons fitted I think, with suggested mods, and see how it goes. If it vibrates, I can then rectify it later when I have the cash/time.

Wonder what new problems tomorrow will bring...?  

[wilko]

Onto the lathe and up with the skirt!!

[Rich]

You could do all that work and find that it has made only a small difference as the conrods should have been weighed as well to do a good job, the big end on both should be the same as should the small end not just the overall weight of the rods, as has been suggested inside the dome is a good place to lose weight, I wish I could lose some of my dome that sticks out in front,
Would it takelog to remove the crank and rods as assembly would not take as long as I presume you have worked out the crank end float shimming?

[t20racerman]

You could do all that work and find that it has made only a small difference as the conrods should have been weighed as well to do a good job, the big end on both should be the same as should the small end not just the overall weight of the rods, as has been suggested inside the dome is a good place to lose weight, I wish I could lose some of my dome that sticks out in front,
Would it takelog to remove the crank and rods as assembly would not take as long as I presume you have worked out the crank end float shimming?

I had the crank modified by SRM back in 1986(?) and they also polished the rods, line bored the little ends (both rods were very slightly bent which is common apparently) and prepared the bottom end. Rods were always fine since then and I doubt there is much difference in weight between them. I'm not trying to perfectly balance the engine anyway, but the piston imbalance is quite large. I'll leave the bottom end as it is I can always change a piston later, but wouldn't want to make the rods imbalanced to match an odd pair of pistons!

I'll see how much I can reduce it all by tomorrow and get back to you all.
Thanks for all the ideas and input - much appreciated.
 

[Brian]

The long and the short of it all is that both pistons must be the same weight and the crowns identical.

The one with the AE on it is a Hepolite. If the crowns are identical and you can get them both to the same weight then you could use them, but I wouldnt. The trouble is depending on how they are made (forged or cast) and what they are made out of (hundreds of different alloys) they could have different expansion rates. Like a lot of things you might get away with it, but then again you might not.

[MG]

I agree to all Brian said, the best idea definitely is a set of new ones.

But if you HAVE to use the mixed pair you got for whatever reason, it is simple enough to check the thermal expansion. I did that on a set of far-eastern pistons of unknown alloy composition once to determine the required skirt clearance in the bores:
I fitted a temp sensor using a long hose clip around the skirt circumference, approx. in the middle of the piston. The sensor was the thermo couple that came with my multimeter, so basically it's a cable with a connection plug on one end. The whole thing went into the freezer for a few hours, so I could mic the skirt diameter at about -10°C, and then at regular intervals (approx. 10°C) until it reached room temperature. Then I boiled it in water and thus got more measuring points from 90°C down to room temperature again.
If you do that with both your pistons, you will get a good indication of how similar (or not) the expansion coefficient is.

Cheers, Markus

[muskrat]

OK t20,
          looking at all the good advice so far and the time line you need. Don't touch the rods or bottom end.  As I and Wilco said a little off the skirt. A little out of the pin. A little off the little end boss. That's about as good as you will get. I'd be aiming at 5g diff.
Mg is right about expansion and if you have the tools do it but if not go 5 thou clearance to be safe.
This is all just to get you going till a rebore and new pistons are available. And remember to get new pins with the pistons.
I do agree with Brian but some times you have to go with what you've got.
A cast piston will have ridges in the underside forged will not.
 Bl@@dy leters are wearing off the key pad.
Cheers

[t20racerman]

OK t20,
          looking at all the good advice so far and the time line you need. Don't touch the rods or bottom end.  As I and Wilco said a little off the skirt. A little out of the pin. A little off the little end boss. That's about as good as you will get. I'd be aiming at 5g diff.
Mg is right about expansion and if you have the tools do it but if not go 5 thou clearance to be safe.
This is all just to get you going till a rebore and new pistons are available. And remember to get new pins with the pistons.
I do agree with Brian but some times you have to go with what you've got.
A cast piston will have ridges in the underside forged will not.
 Bl@@dy leters are wearing off the key pad.
Cheers

A fair summary. I'm not bothered about the expansion 'problem' though as these pistons ran in these barrels under another owner and did a few miles without a doubt looking at the wear marks. (They are both cast pistons)
With a bit of careful whittling, grinding and cleaning up I've got the combined pin and piston masses to within 7g of each other - which has halved the problem at least! I might be able to reduce this down a little more (on a tea break now) but then that will have to do. New rings and circlips just arrived in the post.

Anyway, needs must. It will be better than it was for the previous owner, and may even be better than some other A10s out there where the owners didn't weigh the bits....the last owner certainly didn't!

Thanks all
Adrian

[t20racerman]

OK, job done! Weight difference down to between 2.0g and 2.5g. That's enough for me. Piston hasn't lost any strength as mostly it was removing casting marks, widening of little end opening and reducing the skirt by about 2mm that did it. Also, gudgeon pin has been increased on internal diameter by a smidgin and reduced in length by a mm too. Pistons and pins were hardly a weak point on these bikes, so should all be OK.

Right, back to building it up. Need to get as much done as possible today as I'm at Lords for the Test match tomorrow, and back to work on Monday...