Race Engine Forum UK

I am building this UT 1.6 engine, in which i will install:
new forged pistons (protoxide 87mm)
stock crank and rods (from the same 1.4 UT engine)
stock UT 1.4 Flywheel

Since the crank and rods will only be polished and the pistons are new, and the FW will remain untouched, do you think that it's worth it to balance the components, or just a waste of money, considering no weight will be taken from the parts?
The engine will do max. 7200RPM.

Well worth the money. Not a place I would consider cutting corners to save a few dollars.

Always and I stress always balance the rotating assembly with all attached hardware, flywheel, clutch, front hub. Also have them verify the pistons for match and the critical rod weights. This includes both ends of the rods.
If possible have somebody recommend a balance shop thay have used in the past.

I hope this helps.
Robert

Thanks for you reply, although for a number of reasons i am still not sure if i will do it or not. Hope i won't run into trouble, if i opt not to...

Hello, I am the owner of the car that Robert redesigned. If there is one thing that I would not skimp on, it would be balancing. I have owned the my twincam since 86 and the thing I always said is that my car felt like it would blow up at over 4000 rpm. Now with everything balanced, My car quiets down at 4000 rpm and just keeps purring. everything is magnified (weight related rotational imbalances) at high rpms. I am sincerely that my twincam was built right. I don't know if that helps but I would balance!!

I'm thinking of doing the same. This is a home made rig I saw. Maybe we can replicate it easily.

the rig is OK in principle, the great thing about digital scales is there is very little vertical deflection under the weight of the rod. You find out how good your rig is by the weights and how 'repeatable' the measurements are, in other words put the rod on and take it off and put it on again, should be virtually the same.

If anyone wants to know how rods should be balanced (which a lot of folk get wrong) I am happy to explain.

G

hi Guy,

Please do, if you have the time.
Overall weight and big end over small end can be tricky to get right for a full set of rods. Once you start removing material you can end up like the bloke trying to get the legs of his chair all the same length... removing a bit from each again and again until he was sitting on the floor...
Also: where to take material off, and where certainly not?

Related to this: how would you go about balancing pistons? What sort of tolerance is OK, where can you take material off without hurting reliability?

regards
Tom

Guy Croft wrote:the rig is OK in principle, the great thing about digital scales is there is very little vertical deflection under the weight of the rod. You find out how good your rig is by the weights and how 'repeatable' the measurements are, in other words put the rod on and take it off and put it on again, should be virtually the same.

If anyone wants to know how rods should be balanced (which a lot of folk get wrong) I am happy to explain.

G

I doubt people would complain about your generous offer Guy. Maybe a good idea for your new Youtube channel?

The pistons with rings and pin vary from 508 - 504 grams.

I tried for 20 mins grinding the inside of the pin wiht sparks flying off everywhere.

Nothing came off - maybe 1 miligram!!!

Think you need a lathe.

Don't think it's worth it for 4 grams difference no? Unless I'm racing at 7000 rpm.

I will do an article on this but let me say straightaway 4gm variation on a piston is nothing. The material to balance pistons should be removed in a lathe from the region shown below. Alloy is so light that the amount of weight that can be removed is very limited indeed.

OE rod sets (ie: the set dedicated to the engine by Alfa/Fiat/whoever) will certainly be adequately balanced for the job of but I cannot quote weight data as I have never seen it set down in print. How much difference it makes that you are going to run the engine higher rpm than standard recommendation (tacho red line?) I really cannot say. Imbalance on the rods has no direct influence on crank flywheel balance if the crank has 'internal balance' like a straight four cylinder unit - unlike a Vee engine.

It is possible I suppose that extreme out-of-balance rods (undue variations in rod overall weights and thus associated end-over-end weights affect the center of gravity of the rods which ideally should all be in the same place) might exert a noticeable influence on smooth running - but I've never done it - and why should one? I do have direct personal exp of engines running perfectly well with rods as far out of overall balance as 15gm. I would not consider 15 gm on a rod weighing the best part of a kilogramme to be 'extreme' by any mannner of means.

Manufacturers of good race rods will generally achieve +/- 1gm overall and end-over-end but then it is far more straightforward to achieve than re-working OE rods, esp if those rods are not a matched factory set. I don't know if any motor manufacturer sells 'balanced sets'. I think it's an op only undertaken at point of build at the factory. That kinda makes sense doesn't it? How would you feel if you were told you could, 'only buy rods in balanced sets'? In the case of replacement rods (in the days when I bought them which I do not now) I have seen 33 gm variation in 4 new rods.

G

Great information Guy. I'll keep that for records later on.

Did a final weight with pistons and pins, 3 grams difference. leaving it at that, And as people say it won't make that much difference unless you're pushing your revs to the limit.