gx340 - 390 cylinder heads ?

[thefarmer355]

Are these heads both the same casting? And What size are the stock valves? I have a gx340 thinking of a nice port job and some stainless valves but can't seem to find the stock valve sizes. Thanks T.J.

 

[thefarmer355]

29 views and no reply ? Any advice would be appreciated guys, have tried the search and there are limited topics on the big block hondas/clones

 

[Stangrcr1]

35mm intake, 31mm exhaust. Just measured the stock valves I took out of my 390. I replaced them with stainless stock size valves.

 

[thefarmer355]

Thanks, I think that would be my best bang for my buck being as im just building this as a fun motor. Some port work, might get .100 shaved off of it if my machinist is willing to give me a fair price.

 

[Stangrcr1]

That's about what I shaved off mine. As far as porting, keep it simple. Don't try to hog it out, just clean up the sharp edges and smooth it.

 

[thefarmer355]

Shaving .100 off the head would increase my compression from factory to what ? around 9:5:1 10:1 ? also I can't seem to find an aftermarket 340 flat top piston. Should I have it bored out to 390 or will the factory piston work on a billet rod ?

 

[Stangrcr1]

I just used a new stock Honda piston. Just honed the cylinder a little, new rings, an ARC rod and flywheel. Highest I saw that motor turn was 6200. Bruceo has that motor now.

Never figured compression, but it was definitely harder to pull the pull start...

For yours, unless the original piston has scratches or the rings lands damaged, I would be tempted to reuse as they are good quality.

 

[thefarmer355]

I was thinking the same thing, I haven't taken the bottom end apart yet. Then engine came from the fertilizer industry, off of a nitrogen tanker its a little corroded and rusty on the outside but the cylinder looks new.

 

[65ShelbyClone]

Are these heads both the same casting?

To my understanding, yes. My 340s have pretty big chambers and the stock compression is only ~7.7:1. I have no idea what taking 0.100" off the head would change it to.

I had a thread going on 4cycle about it before the crash and about the only thing different between a 340 and 390 is the bore and carb...and I'm not even sure about the carb.

 

[thefarmer355]

hmm so if i am aiming for around 9:5:1 compression or 10:1 should i take it down to the first cooling fin ? This engine may end up on a mini but will be in a small one seater go kart first. Just building something for fun but I like maximum performance

 

[Stangrcr1]

Only major concern about taking that much off is piston to valve clearance. Should be ok with the stock cam, but could be bad with a custom cam.

 

[thefarmer355]

is the stock piston thick enough to cut some reliefs in ?

 

[Stangrcr1]

Dunno. Never tried.

 

[65ShelbyClone]

Just check clearance with modeling clay using the same hard parts(valve, rockers, cam, etc) you plan to run the engine with, but before milling anything. Then you'll know how much room there is.

My 340 pistons aren't really that thick in the crown. My guesstimate is ~5mm/0.200".

 

[thefarmer355]

sounds about right, always frun trying to get the most out of modifying stock parts. Brings such a smile to ones face when it all comes together