GX500 build

[Flyinhillbilly]

So I’ve decided that I needed something bigger than the 238cc small block that I’ve been running on my race kart, so I got a hold of a well used GX390 Honda that had driven a water pump on a farm, and then sat for a few months in a buddies yard.

 

[Flyinhillbilly]

I decided that 390cc just wasn’t going to cut it, so I bought a 96mm piston for a zongshen 625cc. Here it is next to a predator piston.

 

[Flyinhillbilly]

Next, I bought a 69mm stroker crank from EC Carburetors.

 

[JTSpeedDemon]

OHHH SNAP this'll be insane!

 

[Flyinhillbilly]

The next problem is that this block doesn’t have a thick enough sleeve to bore to 96mm, so I called up my buddy at the local automotive machine shop and told him the measurements that I needed in a sleeve.

im officially to the point of using car parts at this point.

 

[Flyinhillbilly]

The next issue was to get the sleeve in the block, so I set it up in my mill and put in my boring head.

 

[Flyinhillbilly]

After boring all I’ve the old sleeve out, plus a fair amount of aluminum I was left with a gaping hole. It also has a few holes where the side cover bolts go.
As you can see, I left a shelf on the bottom for the new sleeve to sit on.

 

[Flyinhillbilly]

Next, I used a dremel with a carbide burr to clean up the thin spots on the break throughs.

 

[Flyinhillbilly]

The next hurdle was how to get the new sleeve inserted into the block since it’s .004” larger than the hole that it’s supposed to go into.
My solution is to pack the sleeve in a container with dry ice, and then add 91% isopropyl alcohol to create cryogenic alcohol that brings the temperature down to -130F.

 

[Flyinhillbilly]

Then you just cover it with the alcohol.
A side benefit is that the fog make it feel like halloween.

 

[Flyinhillbilly]

Then I fired up my pellet grill and set the temperature to 500F.

 

[Flyinhillbilly]

After it got up to temp, I chunked the block in there and let it roast for about an hour to make it expand while the sleeve was chilling out in the garage.

 

[Flyinhillbilly]

After the hour was up, I dropped the sleeve into the block, and it fell right in with a solid “thunk” on the shelf while the alcohol sizzled and spit. I gave it a couple of good whacks with my 3lb deadblow hammer to make sure it was good and seated.

 

[Flyinhillbilly]

Next, I mounted it back up in my mill and inserted my 6” face mill.

 

[Flyinhillbilly]

After about 500,000,000,000,000 passes taking off .010” per pass, it was all nice and flush.

 

[Flyinhillbilly]

Next, I bored it to .008” under where it needs to be for the finished product.
I’ll get my big drill from work tomorrow and chuck up the micrometer hone to finish it up. After that, I’m waiting on a set of adjustable reamers to come in so that I can increase the bore of the small end of the rod by .005” to fit the zongshen wristpin.

 

[FlyFrog]

holy crap this will be a fun one to watch

 

[Denny]

Is there going to be enough meat left to keep from lifting the cylinder? Will you be able to flow enough to keep the cylinder filled and emptied?
All these questions will be answered! Tune in next week! Same Bat time! Same Bat Chanel!

 

[Flyinhillbilly]

Went to the shop and grabbed my 1/2” drill this morning and chucked up my micrometer hone.

 

[Flyinhillbilly]

And just like that I have a 96mm bore GX390.
With the 69mm stroke crank it works out to be 500.13cc.
Now I’m just waiting on my new reamers to show up so that I can mock it all up with a stock rod so I can get an idea of what to order for rod length.