Porting A 390XP Top End For A Worksaw

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This is just a quick walk thru on how I do a 390XP. In no way I'm I saying that this is the only way to do it. There are guys here that can do much more, and make a faster saw. There are also guys here that can do better looking work. I'd venture to say that this ain't my best work.....but I would be willing to stand behind this work. This top end should run for several years without issue, and will put food on a logger's table. It will be quite a bit stronger than a stock top end, but ain't gonna beat any race saws at the cant. This is what I do. I build saws for work.....

 

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So I started by moving other work out of the way......yeah, I do a lot of saws. And, I like coffee.



Site owner Bruce @Gentleman put a rush on sending this.



Yep....that's an OEM 390XP top end.

 

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Some pictures of the untouched jug....



Oh look....I built a tower.

 

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First things first.....gotta get the jug mounted to cut the squish band.

I use these plates here.....




And these threaded rods here.....




To make this "fixture". The quarter gives the jug something to rock, or pivot on when I'm adjusting it.

 

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It's ready now to mount in the 4 jaw.

 

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Some shots of the unaltered ports and the combustion chamber.

Some roughness in the lowers.....



The band before cutting....

 

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Getting the jug setup with no runout at the top of the plating, and at the base of the bore is a little tricky. Lots of back and forth. Adjust the top, move to the bottom, adjust, back to the top to readjust, etc etc etc. Once its within a thousandth or so at both points, it's time to touch off and set the starting points.



The marks I made are so I can go back to readjust quickly. Brake parts cleaner takes the marks off BTW.....

 

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It was time for a new cutter.....



Dial indicator on the carriage tells me how much I'm taking out...

 

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I cut the cylinder extensions while the jug is mounted this way too. The plating is hard on even these carbide cutters.

I also add a bevel while I'm here as well.

 

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Swap the 4 jaw chuck for a three jaw, and it's time to cut the base.



Close up of the way I grind the cutter. I've found HHS makes a slicker cut for me.



This is how I adjust runout here.....

 

[Mastermind]

I change the spindle speed when I cut the base....just looking for a nice finish.



And...here it is.

 

[Mastermind]

70% of the bore is a number you'll here thrown around in performance porting talk.....on a 55mm bore, that's just over 38mm.

Here's what that looks like.



It would fit without any problems.

On a saw that's gonna go to the woods everyday though...I back off a bit on width.

Keep in mind that on this model, I wanted to actually lower the exhaust port some. Adding width to it after lowering it could defeat the purpose by adding area.....

But, I am gonna open the width just a little.

 

[Mastermind]

This card stock is even with the top of the port, but having it here serves a couple of different purposes.

It protects the plating should I make a miss lick.....and it helps me see the shape of the roof better by giving me a straight line to compare to.

 

[Mastermind]

Setting the height of the uppers.....

I have a file for each saw I do that tells me how much to cut out of the squish, how far from the squish to the top of the port, etc.

On this saw, I have 1.200" for the transfers.


Then I color the edge of the card stock with a pencil, so when I touch it with a burr, I'll see the color change.



Now...you can see exactly how far to take the ports up.

 

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The lowers don't need work on this type of build, but I do like to see them smoothed up, and unshrouded.

 

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Now.....we grind.

 

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Roughed in......

 

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Break time....

 

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This is how much material came out.....the quarter is for scale.

 

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And the finished product.