8 replies to this topic

[Dusky]

Hi!

Im rebuilding the engine and while the engine is getting a rebore im trying to match the weights so the engine is even better balanced..

But where should I shave the weight of the pistons? And where should I do that on the conrods? Amy hints?
If someone could show / mark this on the following pics that would be great.
Will conrods need new heat treatment?
Always happy to learn!
https://www.dropbox....204056.jpg?dl=0

https://www.dropbox....204751.jpg?dl=0

[carbon]

Do you have accurate weights for all of the the pistons? Would suggest weighing without the gudgeon pins, then weigh the pins all separately. Then put heaviest piston with lightest pin. If you then still have a slight range of weights then I've ground a little off the end of the gudgeon pin to get them all matched. Aim to get them all within 0.5 grams for a fast road engine.

 

Conrods are a completely different kettle of fish, takes a lot more time and effort. Have you managed to get repeatable end-to-end weight weights? This is non-trivial, suggest you post up the results because this may determine where you can find weight to remove.

Edited by carbon, 20 April 2015 - 09:10 PM.

[Dusky]

Pistons all weight within 1 gram, so do the pins, except for 1 weighting 3 gramms more.

 

I did weight them end to end ( and can repeat the results), the lightest rod being 668 grams total.

 

Wich is a bit of a difference as all the otehrs are  around 672, big difference I guess!

 

I read vizards about this, but can't fidn the A+ rods in my blue edition of the book

thanks!

[Spider]

With the reciprocating components, I wouldn't bother going overboard trying to balance them, The Spinning Parts, Yes, but those that go up and down, no. These parts, in operation can never run 'in balance', if they did, there'd be no forces to make it spin in the first place!

[Dusky]

Yeah, but I mean to just make them weight equal, this is my theory : 

 

it's attached to the crankshaft, so every cycle the rod 'hits' the crank when goign to top/bottom dead centre. So when it all weights the same, it would hit the crankshaft (more) equal ( I know compression etc etc, all variables should be identical too), but it would just add a little balance then in that way?

 

Biggest question if pistons and rods should be individually balanced, or toghether.

Strange that 1 gudgeon pin weights 3 gramms more really

Edited by Dusky, 21 April 2015 - 09:01 AM.

[jaydee]

All parts have to be done individually, apart from the gudgeon pins (that why you swap them in first istance)

When you weigh a rod, you have to weigh the little end and the big separately, all little ends must weigh the same and all big end caps must weigh the same

Its vital how you run the drill bit across the rod or it will cause cracking points, how you grind should be covered in the vizard book (even in the blue one).

[Dusky]

Hmm, can I use an 'old' pin?

 

Was planning to use the pins that came with the pistons , but doesn't matter that I would use 3 new and 1 old ?

Thinking about filign the pin a bit/ borign the centre a bit to lose the 3 gram

 

Will have anothe rlook tonight at the pistons,b ut can't figure where I can safely remove material!

[jaydee]

Measure them and see if theres a big difference in the inside diameter, although i wouldnt bother with boring them

Reusing pins is absolutely fine (unless they're shot) beware if using interference fit pins it will be a pain because you need the correct interference and its likely that certain pin wont fit a piston while fits another one just fine

imoh with interference pins you keep them where they were originally fitted, otherwise you get a bunch of pins and carefully select from them

To remove material form the piston you have to carefully shorten the skirt (not the crown), carefully because you'll notice to remove 1gram you'll need to skim a fair bit, there made of a very light alloy

[carbon]

Dusky,

 

Looks like you are lucky.

 

Stick the heavy gudgeon pin into the lightest rod, job done.