Inside the Exploded JMS 064

[srcarr52]

It seems, despite the high RPMs, that chainsaws are just "over built" instead of carefully balanced, so you can get away with doing things like putting a 460 piston on a 440 crank & rod, etc. Things you would never do with a car engine!

I'm sure a correctly balanced engine would last longer, but these things generally hold up very, very well already!

A single cylinder engine can not be 100% balanced without using a balance piston or balance shafts because the rotating counter balance causes an unbalanced force perpendicular to the cylinder (for/aft for a vertical cylinder). Generally they are around 50% balanced to get a happy medium between the two vibration modes.

 

[junkman]

If you want blue pistons use blue oil .

 

[RI Chevy]

Lol. Too funny. [emoji23]

 

[Chainsaw Jim]

I've checked balance on a few crankshafts and they were balanced nicely with only the pin and bearing on the rod. You'd need to weld those heavy metal plugs on the crank to help with balancing a piston better.

 

[blsnelling]

It it were a real issues you wouldn't have families of saws built on the same bottom end like the 6400/7300/7900, 385/390, 365/372, 650/660........

 

[Mastermind]

It it were a real issues you wouldn't have families of saws built on the same bottom end like the 6400/7300/7900, 385/390, 365/372, 650/660........

Exactly

 

[Chainsaw Jim]

Doesn't a 385 run better than a 390?

 

[blsnelling]

Doesn't a 385 run better than a 390?

Port timing.

 

[astnmacgto]

It it were a real issues you wouldn't have families of saws built on the same bottom end like the 6400/7300/7900, 385/390, 365/372, 650/660........

Is it possible that they are picking a middle ground that would make it liveable for both displacements or in makitas case building it for the 7300 and then be liveable in the 6400 and 7900. rather than good for one and then a different crank thats good for the other? It makes sense from a business standpoint.

 

[Mastermind]

Impossible to really balance a single cylinder engine at all speeds.

https://www.google.com/search?q=Imp...inder+engine+at+all+speeds.&ie=utf-8&oe=utf-8

 

[jacob j.]

I've checked balance on a few crankshafts and they were balanced nicely with only the pin and bearing on the rod. You'd need to weld those heavy metal plugs on the crank to help with balancing a piston better.

Gary Hunt was doing that for a while. I have cranks from a 272 and a 372 where he welded "scallops" on to the leading edge of the crank throws on each. It looks pretty slick.

 

[Backtroller]

Do you have a photo? I'd like to see what it looks like.

 

[Chainsaw Jim]

Port timing.

What are your records for the two stock? I have two that seem to be the same.

 

[Adirondackstihl]

Was it Homelite or McCulloch that a BP twin?
Wasn't many of them made.
I think @heimannm might have have one.

Okay, not the motor I was thinking of, but anyway.

 

[angelo c]

In my humble opinion it's impossible to be sure of what really happened. I've never really liked these sort of threads because everyone adds their thoughts and ideas from just seeing a few pictures. People begin to blame the builder, the operator, the parts manufacturer, hurricane Irma, etc.....

It's just a saw that blew up. Mess with these things long enough, and we all will see one come unglued.

It's Trump's fault you big dummy!

Nope. It's because of her emails.

RUSSIANS !!!
WikiLeaks ?
Darn Deplorables....

 

[Adirondackstihl]

@heimannm

The video I'm thinking of.....
Is a twin cylinder (Homelite or Mac).
I believe you were tuning it while Mitch Weber held the tach.
It was turning some crazy R's
Believe the vid was filmed at your place Mark.

P.S.
This may or may not have anything to do with balancing.

 

[blsnelling]

What are your records for the two stock? I have two that seem to be the same.

I don't have them in front of me. IIRC, the 385 has lower exhaust and more intake.

Once I found port timing I liked, I learned to prefer the 390. Until then, I preferred the 385 as well.

 

[Chainsaw Jim]

I don't have them in front of me. IIRC, the 385 has lower exhaust and more intake.

Once I found port timing I liked, I learned to prefer the 390. Until then, I preferred the 385 as well.

If the two I have are the same and you didn't have the same numbers with yours then I'm wondering how many variances there are, and which model if not both.

 

[cus_deluxe]

@heimannm

The video I'm thinking of.....
Is a twin cylinder (Homelite or Mac).
I believe you were tuning it while Mitch Weber held the tach.
It was turning some crazy R's
Believe the vid was filmed at your place Mark.

P.S.
This may or may not have anything to do with balancing.

I had a chance to run marks bp-1 in iowa this spring. For only 40-45 cc (i think? ), it was a screamer for sure, sounded like it was angry at life.

 

[PA Dan]

But.....9 out of those 10 didn't come apart.

So...????

Yet...