adding weight to piston?

[Alan L]

I've got 5 new P&Cs with pistons matched weight group and pin/piston all within 1 gm. I have a 6th new P/C which is 9gm lighter than the rest. Is there any way to add 9gm weight to the one piston/(pin) or do I have to find a way to take 9gm off the other 5.
Thanks
Alan

[Alan L]

Or, can I reduce 1 piston/pin by 9gm and place them at the fan end (1&4).
Alan

[afterburn 549]

Find the lightest and match it .
Thats what most do afaik , and what I always do.
It will not be hard to get rid of 9 gms.
I always shoot for zero, I sleep better.

[Alan L]

I got 5 'heavy' ones and one light one. The light one is the one I just received - at the bottom end of the lightest weight group. So do i have to lighten the other 5, or can I add weight to the light one?
Alan

[0396]

Reread post # 3.

[afterburn 549]

Go lighter is what would do in a heartbeat.
I made a mistake ( Imagine that LOL) -One time and took off weight on the lightest one.
My engine thankfully.
I had to rematch all of them to a EXTREAM light number.
It came out SUPER light .
I shaved from every known spot to man.
This engine has over 30K miles of Spirited driving on it and still purrs.

[Alan L]

Originally Posted by chris_seven

One of the issues of uneven piston inertia is that the individual cylinder will decelerate at different rates within the cycle and this will cause a slight unevenness in the way the engine runs.

If you take a 500cm^3 single cylinder engine at 8000rpm it will change speed by around 80 rpm within a single revolution and this will need nearly 1kgm^2 of inertia as a flywheel to keep it running smoothly. If the friction is consistent then the torque which this generates will change the speed change within a revolution.

I spent some years manufacturing single cylinder engine test rigs used by several F1 engine suppliers and have seen these effects quite clearly demonstrated. At 20,000rpm which was the maximum speed of the test systems we supplied it all became a bit tricky and speed changes increased to around 250rpm within a single revolution.

If you reduce the weight of two pistons then this effect will be increased and could set up some high order torsional vibrations which could result in a reduction on fatigue life.

It would be possible to set up a mathematical model to try to analyse this but it would be much faster to just match the weights.

What I was suggesting was to reduce weight of 1 piston by 9gm to match the other (6th) , then pair them up - at the light end of the crank (since they would be the lightest pair). Each piston in 1-4 position would be balanced as would 2-5 and 3-6. Alternatively, is there no way to add weight to get them all the same? If I have to I guess I'll just have to pull 9 gm off 5, but it seems a cumbersome way to solve the problem. I ended up with a 6th piston at the light end of the lightest group weight.
regards
Alan

[BYprodriver]

Originally Posted by Alan L

What I was suggesting was to reduce weight of 1 piston by 9gm to match the other (6th) , then pair them up - at the light end of the crank (since they would be the lightest pair). Each piston in 1-4 position would be balanced as would 2-5 and 3-6. Alternatively, is there no way to add weight to get them all the same? If I have to I guess I'll just have to pull 9 gm off 5, but it seems a cumbersome way to solve the problem. I ended up with a 6th piston at the light end of the lightest group weight.
regards
Alan

Rebuilding a engine is a "cumbersome" way to stop it from consuming to much oil, but in the long run much better than just pouring in STP oil treatment.

Do it nice or do it twice!

[Alan L]

OK, guess I better get my grinding gear out. Curse. Yes a rebuild is a bit cumbersome to rid an oil consumption issue - but I had just fitted a used set of P/Cs to replace my worn out set - beyond wear limit, but still working OK. But the rings on the 2nd set don't appear to have bedded in. I could have just gone thru the process again, hoping for better results with a new set of rings. But I had a part set (5) of new P/Cs - so decided to 'do it nice or do it twice'.
Regards
Alan

[afterburn 549]

With a 3 beam scale you will have it done inside 2 hours. EZ

[Alan L]

Originally Posted by afterburn 549

With a 3 beam scale you will have it done inside 2 hours. EZ

Not here - haven't been down this track before. Alays a bit nervous before attacking parts. I think within a day and I would consider myself lucky. :-) Have to feel my way with this.
Alan

[afterburn 549]

There is really no magic involved .
Just make sure you leave NO burrs!
I have not ever had a piston comeback because I took too much weight off.
Like I said, I screwed my own up one time really bad.
It came out with pistons that were lighter than RSR stuff.
It would SCREAM!
I talk to the guy that bought it from time to time.
No problems .
We shoot for pirrrfection.......that makes at least hit the target.

[keynsham1]

What weight difference actually matters? There must be a point where it doesn't make any difference as things such as friction etc override any minimal weight differences?

[KTL]

Wait until you start removing material from the piston. It's slow going when removing 9g. You keep milling material away, weigh it, remove some more, weight it again, still not even close, remove more, STILL a long way to go............ etc.

[afterburn 549]

You can take some off the pin too.

[Alan L]

I planned to start on the pins - figured that was where the bulk surplus weight was (and I have a lathe which may/or may not help). Was going to see how far I could get with the pins, then finish on pistons.
Alan

[afterburn 549]

You will do just fine.

[Alan L]

Waynes book says 'the machinist will remove weight from the pin' as per pic. How would they do this? I have spent about an hour with dremel and stone, and have not removed a gm. I still got 45gm to go. It is too hard for my lathe tools.
Regards
Alan

[Ronnie's.930]

Alan, here is how Jim did his by hand - looks pretty sweet (post #4 for pistons/pins) -

Connecting rods balanced

[Alan L]

Hi there Ron. That is mainly about balancing the rods. Jim had another post - which I read, on doing the pistons and pins. He said he used a die grinder bit on the pins. I'm not sure exactly what sort of bit he used - but I have tried a metal fluted bit I used on my intake manifold and I tried a grinding stone. Both suggest i will be here til Xmas before I can reassemble. There must be a more effective way to remove 45 gm of tough metal. Ideally I would like to use the lathe - since I got one.
Thanks
Alan