Tuesday, February 5, 2008

Using a Long Rod in an OHV Kart Engine

This Article is specific to the Honda GX200, but the same clearance principles apply to just about any engine.

Since wiseco is eliminating their kart piston production, we want to focus on the use of J&E pistons in our instructions in order to avoid confusion.

The long rod is 3.707" long (+.404"). With a 3.707" rod, ANY piston that will fit the bore of your engine that can be cut to a .520" compression height will give you the same overall length as the stock length rod (3.303") with the stock piston (.924" comp hgt).

We do not have undersized bearings for our Honda rods at this time. If you are running a babbited bearing, your crank wear will be almost nil. That Honda crank is very good.

Some of you guys are talking about decking your block and/or shaving the head. That all ties together with the required compression height of your piston.

Two rules:
1. The minimum distance between the bottom of the head and the top of the piston should be .030". (I'm not going to get into domed piston theory here because it is not applicable here.)
2. The minimum distance at the closest point between the valve head and the piston during operation .090"

Every block is a little different. Assume for a second that the piston and rod combinations are fixed at 4.227" (stock length). Let's say that your gasket is .010" thick. Your piston would need to be .020" in the hole based on the .030" rule. In order to maximize compression, after decking your block, you want to shave the head until the valve head is as close to .090" from the piston when they are closest using the cam you intend to run. That's the very best you can do without a domed piston with valve cutouts. All these things have to be measured by mocking up each engine.

OK, now throw in the fact that the aftermarket pistons will need to be cut down to get them to a .520" compression height to achieve the 4.227" stock length. There's nothing that says you must cut that much off the piston other than making certain that the .090" valve rule is respected.

Since a JE EXF4500-120 piston is only .004" larger than a stock bore piston, and it has a .610 compression height than can be cut down, lets use it.

note: FYI, the JE EXF4500-140 and wiseco 1982p140 pistons, which are .024" over stock bore, have a .620 comp hgt and can be cut down to .520" but the wiseco 1990p120 piston has a .565" comp hgt and may not be able to be cut down to .520". If you follow that, you will understand why the JE EXF4500-120 is the best first choice.

Back to our example above. We said the piston would have to be .020" in the hole. If you mock up your engine with a stock rod and a stock piston and the piston is actually .030" in the hole, then you have two options with the long rod and the EXF4500-120 piston. You could cut .090" off the top of the piston to take it from a .610" comp hgt down to a .520" comp hgt and also take .010" off the deck of the block, or you could just take .080" off the piston and leave the deck alone. In either case, you are moving the piston .010" up in the hole and .010" closer to the valves, so keep that in mind because of rule 2 above.

This long rod is designed to clear the underside of the JE piston without modification to the piston or the rod.

Hunter or I can help you with all this in order to cut the the piston for you. We will need to know how deep your piston is in the hole with the stock setup AFTER any decking you are going to do. We can only assume that you have checked your valve clearance.

Seriously, if you are building an engine, you need to grasp this stuff in order to build the most performance that won't grenade due to lack of proper attention to known constraints.

One last rule of thumb: The piston dome should not be less than .100" thick in the center.