Big bore kits

[Huskybill]

im reading all I can on this subject before I mod any saws. Some are complaining about the added vibration using a big bore kit with a heavier bigger piston? If you skip rebalancing the crankshaft you get less performance and shortened engine life.

 

[Leafy]

I can imagine that. When I put the big bore on my 372 I used a 272BB piston, it's close to the same weight as a 365 piston so I didn't notice that issue.

 

[buzz sawyer]

I put a BB on my Makita 6201 and have no complaints. Haven't noticed any increased vibe and it has LOTS of power. Some say you're better off getting an OEM 7900 P&C and have it modded but IMO it compares very favorably to a stock 7900 and has the power I need.

 

[trappermike]

Chainsaws in my experience are not too sensitive to piston weight increases,a BB piston is only slightly heavier and should not cause any problems at all. I know this from building 064's with a 056 Mag2 piston on them for work and racing,and the Mag2 piston was a lot heavier than the original 064 piston. None of the motors ever failed,even when ran to extreme revs,but I did feel increased vibration in them.
Don't worry about it they will be fine.

 

[ironman_gq]

Single cylinder engines in general don't respond well to balancing, the piston and crank weights are rarely ever opposed to each other and the flywheel weight is somewhere around 90* from the crank and piston depending on your coil location. In reality it's a mostly wasted effort to try to balance a single cylinder, there are gains to be made in lightening the piston but balancing it is almost useless.

 

[Leafy]

So why have such big counterweight on the crank? Is it just to counteract the big end of the rod.

 

[ironman_gq]

It’s a half hearted attemp to balance the reciprocating weight. It doesn’t make a huge difference considering that saws in the same series often use the same crank over a range of saw sizes (024-026, 044-046 for example)

 

[trappermike]

You are right Leafy,the cranks counterweights oppose the weight of the rod and piston.How the factories figure out how to "balance" them I don't know,all I know for sure is you can alter that if you're careful,changing piston weights has not caused me a problem ever. When I built the the Ti crank I was on my own,I studied metals tables and did my best blind stab at it and it was ok,but it really was a mathematical question(weights of metals etc.),so many questions I was happy when it ran ok.

 

[NSEric]

The big counter weights also help fill up the crank case, if there's too much room in there the saw will be sluggish and slow revving and they push the air/fuel mixture into the transfers.

 

[trappermike]

Right,higher crankcase comp. ratio means more power. I do admit that some aftermarket BB kits have some pretty sloppy cylinder port castings,but if you can clean them up they are ok.

 

[Huskybill]

Balancing your crank.

 

[stihldragon]

M-tronic carbs are not fun. I like to have the option of adjusting to the sweet spot myself.

 

[stihldragon]

I want to use a bigger top end on my wood boss 028av. I'm interested in hearing what other saw builders have to say about my vintage performance mod. Help me give it extreme power.

 

[stihldragon]

I want to use a bigger top end on my wood boss 028av. I'm interested in hearing what other saw builders have to say about my vintage performance mod. Help me give it extreme power.

Make me great arboristsite!

 

[stihldragon]

I lightly sanded and polished up that score and it runs like a new one

 

[Huskybill]

I wonder how far away from being balanced even these stock assemblies are? I think the closer there balanced the more rpm we can pull out of the crankcase assembly.