Backwards Piston?

[tallfarmboy]

I'm working on porting a closed port Husqvarna 350 that is getting a 353 piston... Anyway, the 353 piston that I got from Farmertec has 2 rings with pins spaced on the intake side... The 350 piston had a single ring with the pin directly in the center on the intake side... I plan on really blending these transfers toward the intake side, but it will expose the ring ends... If I reverse this 2 ring piston, it should work just fine on the exhaust side. Any input from y'all? Am I crazy? Can the piston locating pins be on the exhaust side?
Thanks!
TFB

 

[huskihl]

I'm working on porting a closed port Husqvarna 350 that is getting a 353 piston... Anyway, the 353 piston that I got from Farmertec has 2 rings with pins spaced on the intake side... The 350 piston had a single ring with the pin directly in the center on the intake side... I plan on really blending these transfers toward the intake side, but it will expose the ring ends... If I reverse this 2 ring piston, it should work just fine on the exhaust side. Any input from y'all? Am I crazy? Can the piston locating pins be on the exhaust side?
Thanks!
TFB

It can be backwards, but the ring ends need to be away from the edges of the exhaust port a ways. If not, as the rings wear out , the gap opens up and may hit the port

 

[huskihl]

It all depends on, with the piston backwards, how far away from the exhaust they are. I'd rather widen the intake and exhaust to 65% than have wide uppers. Can't get more in if you can't get it out

 

[tallfarmboy]

I'm going to have to double check measurements tomorrow. This piston was cheap, so worst case scenario is I'll buy a new piston for a 45mm Husqvarna 51...I think I read where that piston is compatible.

Sent from my SAMSUNG-SGH-I337 using Tapatalk

 

[Deets066]

Relocating the pins for ring ends is an option also, I have not tried it yet though. But others have with success

 

[exSW]

Last saw I tore apart that had the piston in backwards

 

[Deets066]

Last saw I tore apart that had the piston in backwards

View attachment 53089

That doesn't look like any affect from the piston being backwards

 

[exSW]

That doesn't look like any affect from the piston being backwards

Don't really know. Sure struck me as odd. The backside of that piston is like new and the jug cleaned up. As soon as I get it back together I'll do a P & V test. All the lines were good.

 

[Deets066]

Don't really know. Sure struck me as odd. The backside of that piston is like new and the jug cleaned up.

Only problem a backwards piston could cause is a hung ring

 

[Terry Syd]

The pin may also be offset in the piston. I'm trying to remember why wrist pins are usually offset, I think it had to do with piston slap. Anybody else remember?

 

[Stihl working Hard]

Last saw I tore apart that had the piston in backwards

View attachment 53089

Wow that's a hard seizure

 

[David Young]

buy a new piston and the 51 piston will work.

 

[Mastermind]

Wide transfers are a waste of time in my humble opinion.

 

[tallfarmboy]

Wide transfers are a waste of time in my humble opinion.

Now we're getting somewhere... do you feel that the charge loses velocity with a wide transfer?

 

[TJ the Chainsaw Mechanic]

Try it upside down next time.

 

[czar800]

Last saw I tore apart that had the piston in backwards

View attachment 53089

Ouch! That hurt...

 

[exSW]

Ouch! That hurt...

Came to me like that. Piston unavailable. We're working on it.

 

[Mastermind]

Now we're getting somewhere... do you feel that the charge loses velocity with a wide transfer?

It's never gonna flow more than the smallest part of the passage, so why make just the end bigger?

 

[Deets066]

It's never gonna flow more than the smallest part of the passage, so why make just the end bigger?

It has to work like a funnel, but by the time you get everything opened up for the funnel affect. You have huge trans. This works very well for some people but shaping trans is a time consuming art form

 

[Terry Syd]

shaping trans is a time consuming art form

On some jugs it is a nightmare, it may even require building up the tunnel on the outside with GB Weld so that the proper size and shape can be achieved. Other jugs may only require a bit of 'blending' - if you have the right tools to do it.

If done properly, it is a superior way to get more time/area for the transfers. A two-stroke can only begin to 'inhale' a fresh charge when the transfers close. If you raise the transfers 1mm, you shortened the 'stroke' by 1mm.

Measure the distance from the transfers to TDC - that is the working stroke of the engine. Say that distance is 23mm, now raise the transfers 1mm and that distance decreases to 22mm. - The 'displacement' of the engine has been reduced by 4.5%.