Makita 84cc BB install

[bayshorecs]

In the middle of installing the 84cc BB kit from Bailey's. The squish has me concerned though.

Before I stripped it down, the squish on the stock jug measured .025 and 175psi cold. I have the new jug installed with ring and gasket (fuel rated) sealant for no base gasket. The squish is .042 with no base gasket. After about 1 minute of running, the compression was 160psi. The piston arrow is facing the exhaust port.

Not much to mess up so maybe the new jugs from bailey's are a little too tall? Certainly glad I did not put the base gasket in now.

 

[mdavlee]

That sounds about like what everybody else has been getting with that kit. If you still have that much compression with squish that high you should gain pretty good by machining some off the base and lowering it that way.

 

[mtngun]

160/0.042" is quite reasonable, even if it is not what you hoped for.

All P&C's have tolerances, fact of life.

In any event, thanks for the compression data.

 

[AUSSIE1]

My 395 BB is running .034 without the base gasket, but the combustion chamber is 6cc compared to the OEM at 7.5cc.
The trouble with a loose squish is you can't play either way.
Sounds like your kit should have your 175psi, there abouts, back cold and run in.

 

[mtngun]

My 395 BB is running .034 without the base gasket, but the combustion chamber is 6cc compared to the OEM at 7.5cc.

AUSSIE1, I'm curious how you cc your combustion chambers ?

I'm familiar with the method to cc automotive heads but am having a hard time figuring out how to do it on a chainsaw. Pics of your setup would be appreciated.

I guess you could put the piston at tdc, tilt the saw so that the spark plug hole is pointing straight up, and then add water through the spark plug hole ?

 

[AUSSIE1]

I had the cyl's on the bench yesterday, upside down, using a medical syringe with some wetting agent added to some water.

My wife being a nurse gets them from the hospital, but they are cheap to buy.

 

[dk27]

but the combustion chamber is 6cc compared to the OEM at 7.5cc

Al
So would the 6cc combustion chamber produce higher compression being a smaller area or would the 7.5 cc one produce more power due more mix being compressed?

 

[AUSSIE1]

Al
So would the 6cc combustion chamber produce higher compression being a smaller area or would the 7.5 cc one produce more power due more mix being compressed?

Yeah mate.

 

[mtngun]

I had the cyl's on the bench yesterday, upside down, using a medical syringe with some wetting agent added to some water.

I hadn't thought about a medical syringe. Should be cheap. I'll look into that.

So would the 6cc combustion chamber produce higher compression being a smaller area or would the 7.5 cc one produce more power due more mix being compressed?

More compression is a good thing, all other things equal.

Amount of air/fuel mix -- measured by weight -- is determined by how much the engine can suck in, which depends on things like displacement, compression, porting, carb size, etc.. The air/fuel mix then gets compressed to fit into the chamber space whether it is 6cc or 7.5cc.

 

[AUSSIE1]

I hadn't thought about a medical syringe. Should be cheap. I'll look into that.

Yeah only a coupla bucks. Handy little jiggers to have.

 

[dk27]

So is one better than the other?

Edit-didn't see mtngun answer

 

[AUSSIE1]

So is one better than the other?

The tighter the comp chamber the better. (Up to a point of course).
Compression equals horsepower, with 180-190 psi usually being regarded as a rough upper limit.
This is why people remove the base gasket to gain some compression.

 

[jsmith40004]

84cc

I don't think that it is all kits. Installed one one a 6401 Thursday night and had 182 psi dry with no gasket.

James

 

[AUSSIE1]

I don't think that it is all kits. Installed one one a 6401 Thursday night and had 182 psi dry with no gasket.

James

Good comp there. What was your squish?

 

[bayshorecs]

I will try to get out tomorrow and run the saw some to tune it in. Will try to post new compression numbers after the day and the saw cools down.

i would mill off .015 from the bottom but it would void the warranty on the jug.

 

[mtngun]

I had the cyl's on the bench yesterday, upside down, using a medical syringe with some wetting agent added to some water.

My wife being a nurse gets them from the hospital, but they are cheap to buy.

I remembered that I had a couple of 10ml syringes left over from an ink jet printer refill kit. Now I have a new toy to play with.

 

[jsmith40004]

84cc

Didn't measure. Most of the big bore kits have a smaller combustion chamber than the original and squish isn't really is important as it was with nothing but oem for a comparison. PSI is the only thing that matters now. I ran six tanks through it today and it's a beast compression is now at 190.

 

[mtngun]

PSI is the only thing that matters now. I ran six tanks through it today and it's a beast compression is now at 190.

Thank you for the data. Sounds like the BB kit are getting better, despite a few glitches here and there.

I've got a 2008 066 BB with wimpy compression (the 2nd generation kits will have much better compression). Meanwhile, I'm thinking of making a pop-up piston to get my saw running decent. The syringe trick will come in handy to estimate how much pop-up I need.

 

[jsmith40004]

Mine may be a fluke, I don't know.

 

[AUSSIE1]

I remembered that I had a couple of 10ml syringes left over from an ink jet printer refill kit. Now I have a new toy to play with.

Didn't measure. Most of the big bore kits have a smaller combustion chamber than the original and squish isn't really is important as it was with nothing but oem for a comparison. PSI is the only thing that matters now. I ran six tanks through it today and it's a beast compression is now at 190.

Thank you for the data. Sounds like the BB kit are getting better, despite a few glitches here and there.

I've got a 2008 066 BB with wimpy compression (the 2nd generation kits will have much better compression). Meanwhile, I'm thinking of making a pop-up piston to get my saw running decent. The syringe trick will come in handy to estimate how much pop-up I need.

Yeah ten ml syringes are good if they have the measurements.

Loose squish with tight comb chamber allows you to play with durations and room to tighten squish to keep PSI healthy.