Husqvarna 55 & 55 Rancher conversion.

[Den]

I picked up a 55 Rancher, open port, no decomp.
I'd like to convert it to a 55, closed port, with decomp.
Can I simply swap out the piston and cylinder and I'm done?
Is it possible to keep the existing piston, which I believe is a 44mm piston, and change only the cylinder?


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[Vintage Engine Repairs]

I picked up a 55 Rancher, open port, no decomp.
I'd like to convert it to a 55, closed port, with decomp.
Can I simply swap out the piston and cylinder and I'm done?
Is it possible to keep the existing piston, which I believe is a 44mm piston, and change only the cylinder?


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Hey Den, out of curiosity - what’s the reason for wanting to mod it?

 

[grizz55chev]

Hey Den, out of curiosity - what’s the reason for wanting to mod it?

More power, obviously.

 

[Vintage Engine Repairs]

More power, obviously.

a closed port compared to an open port really makes a noticeable difference in power in the cut?

 

[grizz55chev]

a closed port compared to an open port really makes a noticeable difference in power in the cut?

So they say.

 

[Vintage Engine Repairs]

So they say.

I didn’t expect it to make any noticeable difference, but I’ll do some digging for my own curiosity.

 

[5155]

On mine the CP has a windowed piston and the OP does not.
I have not tried swapping just the piston.

Yes top ends are interchanged easily.
AM top ends I've tried are junk.

The plastic intake partition female drywall threads, impulse tube, and fuel line are the common things to cause lean out leaks.

If mine and tested and ran good I'd leave it alone.
The best runner I've had (of five) was a 51 CP.

That's my experience from working with saws more than on them.

Not a pro saw, but they "think" they are lol.
Been great for firewood and log house building.

 

[mountainlake]

Open port ones are dogs. Steve

 

[Vintage Engine Repairs]

Open port ones are dogs. Steve

I presume the numbers are different too?

 

[ammoaddict]

Open port ones are dogs. Steve

I have an open port one with decomp. Base gasket delete, meteor piston, muffler mod and small timing advance. I promise you it is NOT a dog.

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[5155]

Been kinda watching for a new dog, you know, surprise the wife.
AA is that from my parts? If so how did the squish turn out?

 

[ammoaddict]

Been kinda watching for a new dog, you know, surprise the wife.
AA is that from my parts? If so how did the squish turn out?

No, this cylinder I put on an old 50. Your top end is on the 55 that I took this cylinder off of. The piston was toast, I cleaned up the cylinder as best I could, it still has some light scoring but it runs very well. Your aftermarket top end without base gasket was about .035 squish. The compression was 155. It runs good enough. I loan it out to the church volunteers, that's why I built it. I put a 20" bar on it. I haven't heard any complaints about it. I've ran it a few times, it does ok. Thanks again for sending it to me.

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[Huskvarna hotellgäst]

I picked up a 55 Rancher, open port, no decomp.
I'd like to convert it to a 55, closed port, with decomp.
Can I simply swap out the piston and cylinder and I'm done?
Is it possible to keep the existing piston, which I believe is a 44mm piston, and change only the cylinder?


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You can't use an open-port piston in a closed-port cylinder.
If your existing piston is 44mm diameter then it's a 49cc "50" you've got there, not a "55".
"50" piston is 44mm (49cc)
"51" piston is 45mm (51cc)
"55" piston is 46mm (53cc)

I'm curious as to what closed-port c&p are you thinking of. Partner 5000+ ones must be in short supply.

 

[Den]

You can't use an open-port piston in a closed-port cylinder.
If your existing piston is 44mm diameter then it's a 49cc "50" you've got there, not a "55".
"50" piston is 44mm (49cc)
"51" piston is 45mm (51cc)
"55" piston is 46mm (53cc)

I'm curious as to what closed-port c&p are you thinking of. Partner 5000+ ones must be in short supply.

I want it to be closed port because I want a decomp valve, and also because performance of the CP's are said to be better.
I have three CP Husqvarna 55 saws, one of which is ported, polished, base gasket delete, muffler modded, and timing advanced. All three run great... but I've never run an OP 55 to compare performance.

My thought was to possibly retain the existing OP piston, and just swap the cylinder for a CP one, but it appears that is not possible.

Thought about just buying a CP piston and cylinder from H&L, and then sell the OP to somebody.

Also, I was guessing off the top of my head/memory about the 44mm piston. I have not measured this one yet. It has whatever OP 55 saws came with from the factory.

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[ammoaddict]

I want it to be closed port because I want a decomp valve, and also because performance of the CP's are said to be better.
I have three CP Husqvarna 55 saws, one of which is ported, polished, base gasket delete, muffler modded, and timing advanced. All three run great... but I've never run an OP 55 to compare performance.

My thought was to possibly retain the existing OP piston, and just swap the cylinder for a CP one, but it appears that is not possible.

Thought about just buying a CP piston and cylinder from H&L, and then sell the OP to somebody.

Also, I was guessing off the top of my head/memory about the 44mm piston. I have not measured this one yet. It has whatever OP 55 saws came with from the factory.

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If it were mine and the p/c are OEM and in good shape, I would run it. I have a hyway closed port cylinder I might consider trading, maybe. I used an 028 super pop-up piston with it. It had 250 psi compression and ran like a demon. I actually have 2 of those pistons. I had to grind a spot on it for spark plug clearance. The other is new in the box. I ran about a tank through the saw with that topend on it. I took it off because it was my volunteer loaner saw and I didn't want them breaking the starter by not using the decomp.

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[wcorey]

It is doable to run a slab sided type piston in the cp cylinders but the bottoms of the lower transfers should to be ground out and matched to the case, allowing them to function as bottom fed.

 

[ammoaddict]

It is doable to run a slab sided type piston in the cp cylinders but the bottoms of the lower transfers should to be ground out and matched to the case, allowing them to function as bottom fed.

Wouldn't that more or less turn it into an open port cylinder or am I picturing it wrong?

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[wcorey]

I guess that depends on how you define op vs cp...

You don't remove the bridge between upper and lower, you grind the bottom of the lower straight down though a portion of the cylinder base and 'skirt' and match to the crank case (the cc has a cutaway/pocket that's utilized by the op transfers but not the cp). This bypasses the full piston skirt from blocking the lower transfer.

For the record, I consider the horizontal bridge that forms a divider on the cylinder wall between the upper and lower transfers to define the basic definition of closed port vs open port. Though this one feature isn't entirely where lies the performance advantage but allows for a more efficient 'loop' path through the transfers. There are of course a number of sort of hybridized exceptions to this which tend to confuse things a bit.

If you look at a many side fed cp cylinders vs bottom fed, you'll see that the side fed cylinder is often noticeably taller. Where with the shorter bottom fed cylinder, what is effectively the bottom of the transfer 'loop' is completed in/by a pocket in the correspondingly taller crank case.
Again many exceptions and many ways to skin a cat...

 

[auskip07]

my first rehab was a 50 that has a 55 big bore kit on it. i dont know the power difference between open and closed but bigbore kit will get you what you want.

 

[Den]

I'd like to simply drill and tap the existing OP cylinder for a decomp button/valve, and run it that way. Not looking to spend a lot of money.