gx390 build

gman3850

well sence i alread have everything lined up on my kart instead of buying a built vc460 im just going to be building my gx390. My buddy had this off one of there work machines because it didnt run well, diag was that it had a bad carb and was low on compression + leaked oil. so i asked if i could have it and he said sure. Ran it for a while after i got a new carb but now it is ready for a rebuild

so this is what i plan on doing
gx390 block
p&p head
1.3 rockers
manton push rods
65lb springs
aluminum retainers
.020 head gasket
new piston
arc rod
69mm stroker crank
246 dur 260 lift cam
billet adjustable flywheel

now my question on this is when doing the 69 stroker crank how much needs to be cut off the top of the piston to be able to work?

BGW

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TheGDFP

Dished slug, you'll only take the outer lip off. In theory, the increase in throw is .100 more, but it's WAY better to mock up the new crank and piston beforehand. Then select gasket and head milling amounts.

Are you boring the block or is the stock bore perfect? 90mm Predator slug is cheap.

Doing ANY work on the stocker, is crowding VC460 buy-in,,,
Block, side cover, and piston are $141

gman3850

yes but the bolt pattern on the block.

for me to rebuild the gx390 ill have around 600 into it, while the vc460 built the same is around 900

TheGDFP

I love the Honda quality, but...

Pattern on block and sidecover match, and are the stronger 10-bolt.

The only difference in price between the builds should be the cost of boring the 390 and a piston for it, versus the 460 bare block, side cover and piston ($141)

http://www.vegascarts.com/product-p/460flatpiston.htm
http://www.vegascarts.com/product-p/460sidecover.htm
http://www.vegascarts.com/product-p/460block.htm

Or the $250 basic kit, that includes a few extra bits.
http://www.vegascarts.com/product-p/vrx46-bkit.htm

gman3850

when i say bolt pattern i mean the foot patern. The bolts that bolt the engine down to my kart.

also im not putting in a 92mm piston ill just be going with a stock 88mm

TheGDFP

Sounds like a nice 419cc beast!
The 1310X-Z5T-800 is the lightweight one, with thinner rings, better for revs.
And, I'm sure you've seen that you have to grind the counterbalance shaft for the billet rod.

gman3850

thx for the piston nnumber, and yes i know i need to clearence, now do u think i should do the 69 crank or just stay with the 64 that is in the gx390

raydhd

For $60 with a new bearing while your putting the engine together I think it's a no brainer! There no substitute for cubic inches or torque.

gman3850

well went and mesured the cart and if i cut a little more of my plate i can mount the vc460 on it. Went on the website to see that the daily special was the 20hp motor assembled for 299. for the crank, new piston and rings it was going to run me another 115 anyway.

also found out the low compression was to lose head. Took off head put .020 head gasket on it and compression psi is where it belongs. So i am going to keep the gx390 for another vehicle

gman3850

oh and this is the toy it is going on