372 with 385 top end

[Mattyo]

I dunno Jim, when I looked at the 385 gasket vs a 372 chassis, the bolt holes not only didn't line up, but there wasn't much meat too the chassis to hold them in the places they needed to be. Just use a 385 gasket, and get some pix. I thought I had one around here somewhere, but I don't.

are you sure there is such thing as a 12mm wrist pin 066 piston? 372 is 12mm right? and 385 is 13mm? iirc

 

[mdavlee]

I dunno Jim, when I looked at the 385 gasket vs a 372 chassis, the bolt holes not only didn't line up, but there wasn't much meat too the chassis to hold them in the places they needed to be. Just use a 385 gasket, and get some pix. I thought I had one around here somewhere, but I don't.

are you sure there is such thing as a 12mm wrist pin 066 piston? 372 is 12mm right? and 385 is 13mm? iirc

Some of the stihl pistons use smaller pins and bigger wrist pin bearings. 288 and 660 interchange as long as you use the wrist pin bearing.

 

[Mattyo]

Ah..the pin bearing takes up the extra mm...got it

 

[Chainsaw Jim]

Well so far so good...

 

[Mattyo]

more pix Jim... is that gonna seal? did you drill out the cylinder head bolt hole to accomodate the different spacing?

 

[Chainsaw Jim]

The holes haven't traveled far enough to need relief for the driver. The gasket foot print is pretty much the same. I'll put up another pic here in a bit.

 

[Chainsaw Jim]

Well it looks like if I'm going to do this with the 660 piston it's going to be with a two piece cylinder. I've never done a two piece so it would be a great first time project.
I can use the 385 piston if I have sleeves made from milling a 13mm pin to the inside diameter of 12mm and cutting a couple pieces to length. Clips with big ears should keep the sleeves in place if I go that route. It's a scary thought to go using those sleeves like that but the pin itself is what gets the tension from the bearings so the sleeves might not create any extra force or wear on the clips.

 

[Mattyo]

the 12mm /13mm wrist pin seems to be the real buggar of an issue. Ideally you'd want end caps made... that way they don't migrate towards the center, and the circlips will easily prevent outward movement.

the largest piston I found at 12mm is the AM 046 54mm piston... so I figured the easiest choice to gain more CC was buy putting an 046 BB motor on the chassis, which at this point would take considerable work. lots of design considerations here... not an easy task to get this up and running.

 

[Iron.and.bark]

Well it looks like if I'm going to do this with the 660 piston it's going to be with a two piece cylinder. I've never done a two piece so it would be a great first time project.
I can use the 385 piston if I have sleeves made from milling a 13mm pin to the inside diameter of 12mm and cutting a couple pieces to length. Clips with big ears should keep the sleeves in place if I go that route. It's a scary thought to go using those sleeves like that but the pin itself is what gets the tension from the bearings so the sleeves might not create any extra force or wear on the clips.

Forgive my ignorance, any particular reason you are considering turning out the ID of a 13mm wrist pin as opposed to either

Turning the ID and OD of a 14mm pin to make an interference fit?

Making a bushing out of bronze? (Unsure of having 3 difference materials in contact with difference expansion properties).

*edit*

Or make a bushing with an interference fit out of Ali and press into wrist pin holes in piston. Same expansion properties..

Like I said forgive my ignorance.

 

[hseII]

Complete Hack question here, but if you bored the connecting rod to accommodate the 385 Piston, wouldn't that work?...


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[Mattyo]

That would be an impressive boring job...

 

[LBracing]

I'd just swap the rod altogether really nothing complicated here , 066 rod is same crank pin dia as 372 ( 16mm ) , you can put a 13mm pin piston as the small bore end of the conrod is 17mm , the rod is 4mm longer wich is good but might ask for a small spacer under the jug , Voilà !

 

[Red97]

I'd just swap the rod altogether really nothing complicated here , 066 rod is same crank pin dia as 372 ( 16mm ) , you can put a 13mm pin piston as the small bore end of the conrod is 17mm , the rod is 4mm longer wich is good but might ask for a small spacer under the jug , Voilà !

I like this idea.

By the way how long is a 372 rod?

 

[LBracing]

372 rod is 65mm long or short depends how you see it

 

[Chainsaw Jim]

This type of build was done with a 390 top end which the builder bored out the con rod to fit the 13mm pin bearing but apparently it didn't last very long that way.
The 372 rod end is 15mm, the 066 rod end is 17mm.
A sleeve made from an actual wrist pin should have the same expansion properties if it is the same metal right? Or would it even matter? It cannot travel inwards because the bearing cage is in the way.
It seems like most of the older jonsered saws have some sort of sleeve on every moving part so I really don't fear trying it if it can be made to spec and fit right.

 

[Mattyo]

if you can pull apart a crank and put an 066 con rod in there and get it to fit that'd be brilliant...

i just don't know how to do that

 

[Red97]

372 rod is 65mm long or short depends how you see it

Thank you. Just saved me from buying yet another crank I don't need.

 

[mdavlee]

Thank you. Just saved me from buying yet another crank I don't need.

I have a lot of cranks laying around if you get hair brained ideas. Just scent me a text[emoji12]

 

[hseII]

This type of build was done with a 390 top end which the builder bored out the con rod to fit the 13mm pin bearing but apparently it didn't last very long that way.
The 372 rod end is 15mm, the 066 rod end is 17mm.
A sleeve made from an actual wrist pin should have the same expansion properties if it is the same metal right? Or would it even matter? It cannot travel inwards because the bearing cage is in the way.
It seems like most of the older jonsered saws have some sort of sleeve on every moving part so I really don't fear trying it if it can be made to spec and fit right.

I believe with the appropriate Q.C., this method would work.


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[Mattyo]

Does anyone have a vid of pulling a crank apart?