Attn all Engineers

[Sully60]

I have a question for the engineering types out there, some of you may have done this before but I'd really like a proper engineering viewpoint of the theory behind what will happen when I make the following modification.

First some background:

I am overboring an engine and using piston from another engine to achieve the desired capacity. I have done this particular mod before but the pistons I used were very similar to the pistons I replaced so I only needed a small reduction in cylinder deck height to get the compression ratio somewhere close to where it should be.
I'm now building another similar engine and to my total amazement (not) the pistons I used previously are no longer available.

The pistons I have now have a considerably shorter skirt (no this isn't one of my metaphorical mumblings)

Lower piston pin to piston top height and are considerably lighter than the old ones.

The engine the pistons came from has the same stroke as this engine.

So this time I'm going to take 4mm off the cylinder height to get compression back to where it needs to be.

Now I know about whats going to happen to cam timing (OHC engine) and the chain etc. but what I want to know is what (if anything) will change in terms of the crank/conrod/piston balance and dynamics if I do this without modifing the crankshaft.

I'm keen to hear anyones thoughts on this and will take any advice but no matter what it will go ahead because I have paid for the pistons and was told in no uncertain terms they are non refundable. And I need more (or some) power!

Over to The KB Engineering Dept.

[Ocean1]

I take it that by modify the crankshaft you mean ballance it?

[Sully60]

Lighten, balance, knife edge anything. I'm trying to see what the difference will be because last time the pistons were slightly heavier an that engine worked well without balancing the crank, things won't be the same this time.

[JeremyW]

I might be wrong (probably am!) but is crankshaft balancing done only to see how the crankshaft itself is balanced and if there is any runout in the shaft? Irrelevant of piston and rod weights?

Intuition would however say that with heavier pistons requires heavier crankshaft, with lighter pistons you can have a lighter crankshaft for higher rpm and better throttle response BUT if it is too heavy then it is just stronger than it needs to be and it will be fine in operation.

My opinion is obviously just based on the intuition as mentioned so I would be keen to hear a more qualified opinion.

[Ocean1]

The new piston may be for an engine of similar capacity but if the skirt's shorter and the deck height's lower it it sounds like it was an engine using a shorter rod. Not the end of the world, although it's obviously got less surface area to keep it stable. If it's significantly lighter you'll almost certainly have to ballance the assembly (crank, rod, piston). One way is to make a dummy weight to clamp to the crank pin which weighs 100% of the weight of the rod bigend plus 30% of the rod pin end. It's a rule of thumb that works OK for "normal" applications but if you're running insane revs you need to find someone with a dynamic balancer and do the lot properly.

[jonbuoy]

Couldn't you skim the 4mm off the piston crown instead of skimming your head/barrels?

[Sully60]

Couldn't you skim the 4mm off the piston crown instead of skimming your head/barrels?

Ha! now that would really fuck the compression ratio, did you mean put 4mm on?

[koba]

What are we talking about here? obviously not bonnie pistons in a beezer because you said OHC?

I really don't know shit but I know people who probably do know.

I think balance probably will change as the balance factor is calculated by piston/rod weight Flywheel relationship or some shit like that, Ocean1 sounds like he understands it.
I dunno shit from chewed up dates but this is what I leared (sorta) from my racebike.

My experiance of this is in a big single (KLR600) re-balanced to not use the balance shaft and to work in a different frame.

It relates to "Balance factor"
Which is: "the proportion of the reciprocating mas that is counterbalanced in the flywheel assembly"
The factor it is balanced to determines the characteristics of the vibrations,
A zero balance factor causes the primary forces to act inline with the cylinder axis, conversley a 100% balance factor causes forces to act 90degrees from inline. the forces decrease in magnitude the further the balance factor is from thse extremes.
50% is the theoretical ideal however not always the practical ideal due to differing frame and engine constructions.
About 66% is quite common in old pommie bikes.

Remember Im talking about a big single here so it may work different for anything but an old british type twin (pistons up and down together)

This may be fark all help to ya, but typing it out helps me understand the concept better, someone correct me if I'm wrong too!

Dunno how relevant this will be in your situation but as I say roll on the details and I can ask some old buggers who play with mototrs lots.


Good luck with ya project too!

[Brian d marge]

Bith Oceans and Koba are on the right track , I have built and balanced my own flywheels and it was lets just say an interesting experience

With out knowing what type of motor is being built ,, but a lighter piston an a heaver crank isnt such a bad thing if the other engine was fine this one should be to , though the rpm at which crank dynamic ( vibration/ whirl etc) will change , though the crank recip weight ratio is less so should be better

Side loading isn't as much as it used to be ,,again depends on the application of the engine

Me I have a spacer plate under the barrel and can change the comp ratio easily, in stephs of 1mm 3. 2 , 1 0.5 0.0

So i would machine ,then take the lot to a dynamic balancer ( one who knows bike shafts )

Stephen

[Sully60]

Thanks Stephen
Do you know of anyone near Welly capable of this(the dynamic balancing)? You might be a bit far away at the mo but it sounds like you've probably gone through this before.

[jonbuoy]

Ha! now that would really fuck the compression ratio, did you mean put 4mm on?

Ah yeah

[Paul in NZ]

The balance factor or any counter balancing work on the engine is determined by the engineers designing the vehicle and it reflects the intended use of said vehicle. ie - If you think most users will cruise at X speed and the gearing in top gear is Y then the engine rpm will be Z and thats where we want the sweet spot.

Lightening the piston will change the balance but I suspect that in practise it would not be significant unless the engine is equiped with some kind of active balance device like a rotating shaft etc.

I'm no expert but changine the pin height may affect the load placed on the thrust faces of the piston and obviously you need to ensure the ring material is suitable for the bore...

Sounds pretty exciting though....

[Brian d marge]

Thanks Stephen
Do you know of anyone near Welly capable of this(the dynamic balancing)? You might be a bit far away at the mo but it sounds like you've probably gone through this before.

No I cant help you there , Christchurch yes but wellington no ,,,, and here in Japan even more no hence I did it myself !!!


good luck
Stephen

[koba]

Thanks Stephen
Do you know of anyone near Welly capable of this(the dynamic balancing)? You might be a bit far away at the mo but it sounds like you've probably gone through this before.

I think there is a place in levin called colliers (SP) ??

[Sully60]

I think there is a place in levin called colliers (SP) ??

Yes I've heard of them I might let my fingers walk and give them a call.
Cheers for jogging the memory, harder the suck starting a bus!