9 replies to this topic

[Back2mini]

I just had new pistons fitted to my 998 A+ rods.  They are press fit gudgeon pins so I had the machine shop assemble them for me.

Thought I would just check the total weight and take a bit off the big end caps if they were a bit variable.

Shocked to find two assemblies (2 & 4) are 19g heavier than the lightest (3) and one is 5g heavier. (all without rings)

The caps alone are fairly close in weight: 196, 198, 194, 196g in firing order.

 

The pistons are new +30 County brand.  Nothing fancy but I can't see how there would be 19g in a small aluminium piston.

 

I could grind away all the forging flash line along the heavier rods and take lots off the ribs on the BE caps, but that would unbalance the caps and I understand the A+ rods were supposed to be fairly well matched.

 

So, I think I must be missing something and don't want to go ploughing in.

 

And yes, I have removed all the bearing shells.

Can't see anything different between rods.

 

I don't really want to remove the pistons again and start by balancing the small ends and then do the big ends.

This engine was pretty harsh when I got it and I worry about rebuilding with such a variation.

 

What am I missing please?

[Earwax]

unfortunately , with those differences , if rebuilt it could be just one of those engines that are never quite sweet.... not horrid but not wonderful... If i recall correctly 5 grams or therabouts difference is a good aim..., so i am thinking there is more chance of  variance in the rods than in the pistons.... even machining the piston won't necessarily balance them along the rod line...  ..

[Stiggytoo]

Sounds like the con rods need a bit taken off to bring them into balance.

[blacktulip]

hi, not sure about the original questions but, while i was assembling my county pistons to rods last night and i was amazed at how much alloy swarf was left on the piston on the underside. they need a thorough clean all over if you havent already done so. Just a heads up mate

[Back2mini]

Thanks for the tip Blacktulip.  In fact it all looks very clean in there; good workmanship by the shop who assembled them.

 

My error in not weighing the rods beforehand.

 

Any chance I might have a mixed set of rods?

 

And if I want to take off 19g, where do people recommend to take it from please?

Anywhere to avoid?

The oil thrower boss looks an easy target, for example...

[Spider]

It is getting to the upper limits of what they let out the doors of the factory, but honestly, it won't make a jack of difference. No reciprocating engine will ever run with the reciprocating parts in balance, if they were in balance, it could never run in the first place. The bits that spin, yes, they are important to get balanced, but the bits that go up and down, 'strict balance' won't make any difference.

[Cooperman]

Unless you intend revving it beyond 6000 rpm a lot of the time it is of little consequence.

It will almost certainly be the rods, but you can't just take weight off as you don't know where the imbalance is and, say, taking weight off the big end of the rod could make the imbalance worse if it is the small end which is over-weight. It is not the weight, it is the balance which is more important.

Piston weights are much more consistent these days with CAM involved in their manufacture.

[Dusky]

If you balance rods you need to balance them end over end, i made the tooling to do this, just because I Had too much time really.. :)

[carbon]

Thanks for the tip Blacktulip.  In fact it all looks very clean in there; good workmanship by the shop who assembled them.

 

My error in not weighing the rods beforehand.

 

Any chance I might have a mixed set of rods?

 

And if I want to take off 19g, where do people recommend to take it from please?

Anywhere to avoid?

The oil thrower boss looks an easy target, for example...

unless you weigh each of the rods end-to-end you won't know where to take the metal off.

[Back2mini]

Ok, thanks everyone.

I don't have time to take the pistons off and balance end for end, much as I would like to.

Will build it and move on.