How to run a 12mm piston on a 10mm pin crank (w/ photos)

francisjohn

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Running a 12mm pin piston is possible on a 10mm pin crank, now there are two ways to go about this.

1. Run the 12mm wrist pin on the 12mm piston, but instead of a needle bearing on the upper conrod, you use a bushing.
2. Put bushings inside of the piston, which allow you to run a 10mm wrist pin, and use a 10mm needle bearing as normal.

What do I suggest?

I would rather run bushings in the piston so you can still use a needle bearing on the conrod. Even though TECHNICALLY a bushing is a form of a bearing, I wouldn't suggest it. China doll bearings for example have a 0.3mm difference for a few reasons, 1) Heat expansion 2) Lubrication and 3) If it were 14mm just like the conrod, it would be a press fit every time you wanted to install or remove it.

I'm currently working on a 372XP build, but have this MS460 piston laying around. You will want to have the bushings be the length you need, this is just an example.

Now the wrist pin in the MS460 piston is from a MS440 (10mm pin) which is the same length as a MS460 12mm wrist pin. If you try to use a china doll wrist pin you would need to not only shorten it, but add washers to each end so there is a chamfered side hitting the circlip. I do not suggest this but it is how you would do it if you do it.

Using a bench vice (I keep an unmounted one for small things like this so I don't have to go in my cold garage) is the easiest way as these are a pretty tight fit. Using a mallet to tap them in is a great way to have them go in sideways or cause damage. You want to press them in right past where the circlips go.

Note : The needle bearing used in this post is the same bearing that came in a MS440 50mm 10mm pin top end kit. It should be noted that china doll bearings tend to be longer by nearly 1mm. If you use a needle bearing from a china doll, or of the same dimensions as a china doll then you will need either shorter bushings, or you will want to shave them down. If you do shave down the bushings, you want to do it on the side that faces inside the piston, and not the side where your circlips go.

MS460 12mm piston on an AF80 10mm (14mm) rod. Enjoy.


Link to bushings (click me) I have added a photo as well for eventually when the eBay link is dead, at least people will be able to find active listings when they find this post.
 

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372 is a 50mm, the XP is the big bore 372 52mm
If your jug is still 52mm you might be able to use an ms380 piston. Then you can use the 10mm pin and a needle bearing. The bushing is the death of high output engines. I also believe someone makes adapter wrist pins that fit in the 12mm piston and step down to 10mm for the needle bearing
 
I am just reconsolidating responses
My 2 cents, use an ms380 piston. It is a 52mm bore with a 10mm wrist pin.

I am unfamiliar with any MS380 piston that uses a 10mm wrist pin. If you can show me were there is a 380 piston with a 10mm I would be really interested in seeing it. Using a 380 piston in my scenario would be useless due to the differences in height which would affect intake timing.
The 372XP is a 50mm bore jug.
370 is a 50mm, the 372XP is the big bore 52mm. Husqvarna uses "XP" as their big bore just as Stihl uses the term "magnum" for their big bores. (MS460 magnum would be 52mm etc)

What he was showing the bushings in is a ms460 piston. I thought he mentioned the 372 as a separate build.
372XP 52mm is a separate build. I have a MS460 piston here as an example. I am not running an MS460, I am not using an MS460 piston for anything other than this example.
 
If your jug is still 52mm you might be able to use an ms380 piston. Then you can use the 10mm pin and a needle bearing. The bushing is the death of high output engines. I also believe someone makes adapter wrist pins that fit in the 12mm piston and step down to 10mm for the needle bearing
My friend, you couldn't of read anything I wrote or looked at the photos I posted. This post is literally how to use a 10mm pin and a 10mm needle bearing.


Currently, nobody makes adapter wrist pins. Bushings in the piston..... as shown in this post is your adapter.
 
My friend, you couldn't of read anything I wrote or looked at the photos I posted. This post is literally how to use a 10mm pin and a 10mm needle bearing.


Currently, nobody makes adapter wrist pins. Bushings in the piston..... as shown in this post is your adapter.
Sorry, I just don't like bushings even if they aren't the main bearing.
 
370 is a 50mm, the 372XP is the big bore 52mm. Husqvarna uses "XP" as their big bore just as Stihl uses the term "magnum" for their big bores. (MS460 magnum would be 52mm etc)


372XP 52mm is a separate build. I have a MS460 piston here as an example. I am not running an MS460, I am not using an MS460 piston for anything other than this example.
I've never saw a 370. 371, 372, 375yes but not 370.

 
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