Stihl MS390 versus MS360, Worth Fixing the 390 up?

Line boring the halves would be simpler than your 2pc idea. How would you bolt the head down?

 

I was sort of thinking of the opposite of the quarter ....................... drill and tap a hole in the top of the cylinder (sort of like a decomp valve hole), run an appropriately sized bolt up through it so the head of the bolt "replaces the quarter", lock it down with a nut on top! That ought to work!

 

What about the wrist pin? It would be off center. Is there clearance to the head off center?

 

I was talking about the top of the cylinder, not the piston ...................... but that would work too if it could clear the pin/bearing/rod top. Gee, I should have tried that on this old 460 piston I crushed with a stop tool!

 

A bolt will cause hot spots, which will cause detnation.

 

The 039 I built for the clamshell build off had a welded popup in it..
Not the best way to go.
And only netted about 10 pounds of comp. Not worth the trouble.
Should have used a taller piston and cut it to suit.

 

They add weight to the piston.

 

I was also thinking a taller piston out of something as well and cut to chamber.

 

All in all, piston work on these is not the best bang for buck or time that I've seen.
Getting a good set of port timing #'s and good ole fashion shapes are what they really respond to.
"Make em a better pump"...

 

My $33 Chinese parts kit received as advertised, that's a Zama carb too. It took 15 days to get here, a few more than normal, but I saw that shipping went from Chicago to somewhere in MO, then to Cleveland, then to me, a round-about US route.

 

I got my huztle bran DP muffler I ordered in like 7 days!

 

Hmmmm ...................

HYWAY PISTON / RINGS KIT AFTERMARKET Fits STIHL TS400 TS 400 (H31400)

HUSQVARNA 460 RANCHER CHAINSAW PISTON (49mm)

This looks sort of tall: Wiseco Honda CR80 CR80R CR 80 80R Piston Kit 49mm 1986-2002

Never seen rings like on this one!!! Piston Kit with Pin Rings Clip Set For YAMAHA XC125 XV250 STD Standard Bore 49mm

 

The TS400 would work, but again, not sure the gains in comp would be worth the time and expense. May be ahead to spent the time and effort on carb work and port timing for gains.

 

The save-a-thread I ordered came in today so I proceeded to tap out the plug hole on the cylinder. That went perfect. I had cleaned the exterior of the cylinder yesterday, but was waiting until I tapped it to de-carb the top of the head. I started to clean up the carbon with a brass wire brush on my dremel wand and noticed what appears to be a crack on the curved edge of the "head" opposite the spark plug hole. I "painted" it in red. Does not look like a casting imperfection or defect to me, it looks like a crack!

What do you guys think?

 

Sure kooks like it could be a crack to me? But on the phone and a pic its hard to say. Be odd I think for a casting flaw. But I am no metal expert either.

 

Looks like a casting flaw. The reason I say that is because the edges are rounded/smooth. If it is a crack it's been there for a while. I guess you could have smoothed the edges cleaning up the carbon...but probably not that uniformly.

 

I'd say your correct. The flaw or term is called cold lap.