GX 390 question.

M

mattf76

Member
Not looking for a debate here, just wondering with the stock GX390 head what is the biggest intake and exhaust valves that can be put in the head without using larger seats. (keeping factory seats). Also if Stock piston was to be milled to be a flat top, is there a billet rod out there and what length to bring the piston further up the cylinder closer to deck using stock stroke crank?
Thanks,
Matt
 
Most stock pistons have .080 to .085 depth dishes. Parsons sells a rod that is longer but I think it is .200 longer. Clements has a long rod also but again too long for this. Both of these long rods where intended to be used with shorter compression height pistons they also sell. So I know of no billet rod available to accomplish what you want, You could just mill the piston flat and then have the deck cut to get zero deck height. Another option is to get a 420 crank (66mm stroke instead of 390 64mm) for about $70 and install that. It will give you a millimeter more piston height (.040) you can cut off the piston making it nearly a flat top and also about 408 cc engine. Just some thoughts
As far as the seat deal goes, using a larger valve with the stock seat will hurt your flow not help you. For it to help, you must open up the internal size of the seat. If you do, your seat will be thin. Could warp easily and distort causing leaks or worse, come loose. The most I would open up the stock seat is about 1mm. then you could use a 36mm intake instead of a 35mm. Not sure it would be worth the trouble. Larger valves must have larger seats to have any gain over stock size.
 
There is a new "hemi" style 420cc out at harbor freight that is supposed to have a flat top in it.
Could go with that and change the head out?
 
rkcarguy said:
There is a new "hemi" style 420cc out at harbor freight that is supposed to have a flat top in it.
Could go with that and change the head out?
Click to expand...
Yes, it does have a flattop but is in the hole about .030, or mine is. cam bosses are 390 size. Head cced out to 41cc so no bigger than the regular head but should flow a lot better but valve spring options and rockers a unknown. I haven't even messed with mine. The head is in a box somewhere.
 
mattf76 said:
Not looking for a debate here, just wondering with the stock GX390 head what is the biggest intake and exhaust valves that can be put in the head without using larger seats. (keeping factory seats). Also if Stock piston was to be milled to be a flat top, is there a billet rod out there and what length to bring the piston further up the cylinder closer to deck using stock stroke crank?
Thanks,
Matt
Click to expand...
Most people use 38mm intake and 33 mm exhaust on the stock seats because they are much wider than most seats but you will need to bore them to leave about a .060 exhaust seat and about a .040 intake seat. The only stock piston you would want to use is the one with the millimeter top ring,mill it flat and zero deck the block with a .032 head gasket. you can mill the head up to .140
 
The thin ring light weight Honda piston is thick enough to put about 1/8" deep valve relifs without hurting anything. I've milled those pistons before to a 2 ring piston. That's how thick they are. When you mill it that thin it don't last long! I know. Been there and done that. Blown 3 pistons. All it does is blow holes in it. They are nice pistons.
 
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