How to: Modify a Husqvarna 338xpt piston to fit a Husqvarna 238xp

[SkippyKtm]

I'm by no means and expert at this, but I'd figure I'd share if someone ended up in the same predicament as I...
I had a nice Husky 238xp with a fried piston and the pistons have been NLA for quite some time. I do know that a 242xp p/c will fit my saw, but I wanted to keep the saw as original as possible.
I figured a 338xpt piston would be a good candidate, plus I happened to have one layin' around...:msp_smile:

Here's some comparison photos between the two pistons before modification (the 338xpt piston is on the right):

Both pistons are 40mm.
The wrist pin size is the same(10mm).
The space between the wrist pin bosses are slightly wider on the 338xpt piston (.3mm or .012). This is not perfect but it will work out fine, I notice that a lot of AM pistons are way sloppier than this.
The deck height of the 338xpt piston is (.5mm or .020) lower than the 238 piston. I'm running it gasketless and the squish ended up at .032 which is not ideal, but I don't have a lathe (yet). Even though the squish is at .032, the compression is very good at 179psi.
The ring locating pin is slightly more centered on the 338xpt piston. This is not a big deal as I checked through the intake port with the piston all the way down (bdc) and the ring locating pin is not exposed and there is still plenty of space left there.

Here's a few photos of the modifications:
First I flattened the sides of the piston to help with flow up to the transfers and to more closely replicate the original piston:

Then I drilled some starter holes so I could start grinding the windows:

Here is the piston nearly complete with windows and additional bevels ground:

The only thing I didn't get pictures of is the piston skirts on the 338 piston being ground to allow for the crank cheeks.
More photos to come!...

 

[SkippyKtm]

I just ran a tank through it and it runs very well. Its got a lot of cajones for a 40cc saw!...:msp_thumbup:

My New toy!:msp_w00t:

 

[TK]

Nice! I have a similar project to do to a 51 piston to stuff in a 55CP

 

[SkippyKtm]

Nice! I have a similar project to do to a 51 piston to stuff in a 55CP

You know I was just thinking the same exact thing about what to do with all those nice closed port 55 jugs on the way home tonight...
That's a way more common problem, I'm surprised that no AM pistons have ever surfaced for the CP 55.

 

[rmh3481]

Good Job.

 

[TK]

You know I was just thinking the same exact thing about what to do with all those nice closed port 55 jugs on the way home tonight...
That's a way more common problem, I'm surprised that no AM pistons have ever surfaced for the CP 55.

My thoughts exactly, with the popularity of that model and the number of posts on here about needing pistons...... If someone could reliably make a piston that worked in that saw it would be a good little venture for a short term. Those saws are only getting older and less of them are willing to rebuild.

 

[jammin_with_j]

nice work, its always great to see creative ideas to solve problems for parts NLA. Thanks for sharing

 

[SawTroll]

.... I'm surprised that no AM pistons have ever surfaced for the CP 55.

I am sure I ran across exactly that one some web page lately, but failed to make a note of where it was.....:redface:

 

[SawTroll]

I have to admit I still am a little concerned about the configuration of that xpt piston in the 238. Do you have a way to compare the performanse to an original piston?

 

[ChainFinn]

I myself, am pleased to know that someone had this idea, and even if the performance would not be as "high" as originally, its still a good trick to get that museum saw turn in to a runner, ranking you higher among the 238se owners with non-runners :msp_smile: