Using A Big Bore Or Bore-up Kit

[Richard-BKK]

Changing an engine’s displacement by altering the bore, with replacing the stock cylinder and head with a bore-up kit, we also need to re-balance the engine. Failing to re-balance the engine will almost nullify the performance gain and will generate extra wear on bearings and the vibration will degrease the driving pleasure.

So how can we (re-)balance our engine, for that you say read this story…

http://www.motorcycle.in.th/article.php/Bi...andBlueprinting

[VocalNeal]

Changing an engine's displacement by altering the bore, with replacing the stock cylinder and head with a bore-up kit, we also need to re-balance the engine.

On a small single it is easy to just weight both pistons, new and old and machine the new one's skirt to the same weight. Edited November 1, 2007 by VocalNeal

[Richard-BKK]

"On a small single it is easy to just weight both pistons" single with both pistons, this is confusing....do you mean it is easy to balance a single? or its easy to balance a two? Did you ever balance an engine?

[Richard-BKK]

If I calculate a simple bore-up set, let say we go from a Honda CBR-150 regular 150cc displacement to a 170cc displacement, nothing so bad. The piston is only a few millimeter bigger, some even make the mistake to put the old piston back as it is so little difference. In weight the piston and his friends is not that much of a change, but doing 10,000rpm we can talk about kilo's of extra stress you put on the bearings and other crankshaft assembly internals.

Maybe if it is so simple, maybe you want to enlight us with the formula to translate the extra weight into the weight to balance things out.

[highonthai]

Porting, polishing, matching, crank balancing, flywheel cutting, cam grinding and jetting.

I just Love this kind of talk...

[VocalNeal]

"On a small single it is easy to just weight both pistons" single with both pistons, this is confusing....do you mean it is easy to balance a single? or its easy to balance a two? Did you ever balance an engine?

As the article says there are two components the reciprocating and the rotating. There are two types of balancing static and dynamic. The piston is only part of one. The reciprocating. So if the new and the old piston and gudgeon/wrist pin assy.weight the same (statically balanced) there is no change. If changing the connectiing rod the same logic applies. So both should be the same. Most kit suppliers know this but I cannot say they do it. In truth the increased weight of the new piston does not effect the rotational balance of the engine but any extra weight would put large stresses on the connecting road assy. The effective weight of a piston at 10,000 RPM is measured in tons! My 115cc Wave "kit" came with a larger gudgeon/wrist pin and a beafed up connecting rod.

To answer your question yes and no. As we were a small team (car racing) and didn't have a dynamic crank balancer we did the static balancing ourselves and then got the crank crack tested , nitrided and then balanced by someone who did.

[VocalNeal]

Porting, polishing, matching, crank balancing, flywheel cutting, cam grinding and jetting.

I just Love this kind of talk...

Modern road engines are pretty well sorted. With advances in casting technology etc.

I'm not sure I have the energy anymore but yes it is satisfying to optimize everything. I.e I spent some hours porting a standard Wave head with a die-grinder and had an egg cup full of aluminium fillings all over the spare bedroom. Then a friend said he knew where to get an inexpensive "big valve" head. When it arrived the as cast size was bigger than my gouged out one. Sounds logical but when one is living in the jungle

[highonthai]

Porting, polishing, matching, crank balancing, flywheel cutting, cam grinding and jetting.

I just Love this kind of talk...

Modern road engines are pretty well sorted. With advances in casting technology etc.

I'm not sure I have the energy anymore but yes it is satisfying to optimize everything. I.e I spent some hours porting a standard Wave head with a die-grinder and had an egg cup full of aluminium fillings all over the spare bedroom. Then a friend said he knew where to get an inexpensive "big valve" head. When it arrived the as cast size was bigger than my gouged out one. Sounds logical but when one is living in the jungle

I agree,

With the advent of moderen cnc machines tolerances have been held very close on case matching and such.

But still love the smell of dicum and enjoy messing around a bit with the motors.

I'm in Los Angeles at the moment and pre running and getting ready for the 1000 (Baja race)

I'm a old bike racer now aging and racing cars. They say, when you age you need a cage....

Can't wait to get back home in Chiang Mai though.