Budget GX390 stroker build.

When i called arc i talked to jody & after explaining that i ordered a 90mm stroker piston he told me they don't carry 90mm just the 88mm
 
Sounds like a headache....i would call ARC and get my money back for the piston, then find another 90mm piston somewhere else that is correct for your build, there are other suppliers out there like Vegaskarts, Havoc Racing, and others. Also, there are motorcycle shops that sell 90mm pistons that can and do work in these engines as long as you order the correct compression height, make sure it has the correct wrist pin size that your rod uses, and get the rings to go with the piston
 
Sounds like a headache....i would call ARC and get my money back for the piston, then find another 90mm piston somewhere else that is correct for your build, there are other suppliers out there like Vegaskarts, Havoc Racing, and others. Also, there are motorcycle shops that sell 90mm pistons that can and do work in these engines as long as you order the correct compression height, make sure it has the correct wrist pin size that your rod uses, and get the rings to go with the piston

Weddle this is just another example showing that you know nothing about building actual race engines. No one makes a 90mm pistons that are exact fit. Mainly because there are too may variables to consider and this all part of what goes into building modified engines. This was also a new part we carried and our website dicription was wrong. Me and this gentleman has talked and we are going to do whatever we need to for him.
 
ARC fixed the problem. Got it all sorted out. I will still need to do some calculations for the compression ratio, but now there is a light at the end of the tunnel. Thanks all for the input so far, and i look forward to being able to continue the build. Next up is CC the head, do some math, mill the piston, and buy the right thickness of headgasket. I think the hard part is now behind me and it should be a sinch to assemble everything once I get the piston milled by my machinist friend.
 
Ok, next up is calculating compression ratio. I found out that I need to put the piston .005 in the hole at TDC with everything else to get a compression of 10.2 to 1. I measured the head after having lapped valves from bullfrog racing (stock diameter valves). Head volume came out to 40.6ccs with a class-A burette so we are spot on there, did it 3x to be sure. head gasket will be .043 compressed thickness. bore 90mm stroke 70mm. So after it is all said and done, we are looking at this compression calculator https://speedmaster79.com/tools/engine-compression-ratio/ to get a static compresison ratio of 10.2:1 which is called for by my cam card.

Now the next issue I noticed was that the new valves are 92.67 ish mm end to end, and the old stock valves are 85mm end to end.... so that would mess up my rocker geometry a bit. We are looking at basically needing to get an extra 1/4" as i did not shorten the block, or head at all to achieve this compression ratio, and as far as I can think, the gasket is about the same thickness as stock.

SO, with that said, im thinking about clearance now. Do I need longer push rods? Will the stock rockers still fit on the studs? Will the Rockers clear the stock valve cover? Anything else I need to think of?


Also got the 32mm flat slide mikuni on the way and some stainless pieces to make my exhaust. Im going to tig it up and make it a two stage. The tubing I got was 1 1/4" x 1/16" wall and 1 1/2" x 1/16" wall. I also bought a 90 deg bend in the 1 1/4" tubing so I should be good there. Im thinking about 16" minimum length but not sure about how to calculate where to put the step from 1 1/4" to 1 1/2" target max rpm is going to be 6500, and Im thinking im going to use that comet clutch for transmission eventually. The 780 series I believe, its either that or the bully clutch. I havent decided wether to put this on a mini bike or a mini buggy. Kind of ont he fence, but im leaning toward mini bike
 
Alright, got the piston milled, it looks great! I need to jump through a few more hoops now in order to get the valvetrain geometry correct.

I had to get longer push rods as the valves were about 1/4" longer.
I will also need to space out the rocker arm stud a little bit with washer(s) so the rocker arm will fit
I will also need to measure where the rockers make contact with the valve ends by marking the top of the valve ends with magic marker, and cycling the engine a couple times.
Also, the single rate springs have a spring installed height (you guessed it) a little taller than what they are supposed to. I have done only a tiny bit of research, but it looks like that this may or may not be a big deal. Im wondering if there are some special washers. I also have a tig welder, so I am wondering if i could just tack an aluminum washer there at the appropriate thickness so the spring installed height is at spec, or if i could use like a copper spark plug index washer instead. Just some thoughts.
 
Anybody Got Porting Advice?

Well, its time to start assembling. Finally had enough saved up to continue the work. Had the piston, rod, and crank balanced.

I ended up deciding on a 10.2:1 compression ratio, and I also ended up getting cut to length push rods that ill be taking maybe .010-.007" off of the stock length due to the thinner head gasket (.043" vs the stock .050")

Im sitting here looking at the head and I really want to do a good job porting, but at the same time not loose any bottom end by hogging it out. Does anyone have any suggestions or guides on how to port it right? Intake/exhaust?

I'm looking more for where to remove material rather than how to remove material. Any successful port job pics would be appreciated.
 
Okay, I did a bit more saving up, and have purchased the black venom 1.2 ratio rockers, and practiced porting on another gx390 head.

Porting: im comfortable with it now
Valves: they are longer than stock, so i may need to shim the springs up
spring install height: stock gx390 install height is 1.200-1.250" per kinetic competition racing engine's advice
springs ive got the single HD springs (Approx 50 lbs at 1.125 installed height, 42lbs at 1.2, with 110 lb open pressure (.375 lift).)

so im hoping that i can get things rolling soon on assembly. Now i know that my setup could accomodate a very high lift cam, but I'm just wanting durability more than anything, without sacrificing a lot, which is why I went with the mild cam and 1.2 ratio rockers.
 
Its been a while since i updated this, but just to let you all know. I finished the build!!! The engine has so much torque, and I put it on a mini bike (baja warrior mini bike). Long story short is that it has just sick amounts of torque. Using the 275 cam and 1.2 ratio rockers and wondering if i should go up to a 308 or 310 cam. I'm going to put a wheelie bar on the bike to take advantage of the torque at start.

Also, this is a question on the torque converter (40 series). If feels like the bike revs high fast, then slowly gears up. Im wondering if in a drag setting i would need to do the bully turbo clutch or something similiar instead of the torque converter. Thoughts on cam and torque converter>?
 
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