Dual plugging J heads

You always have the choice of running a GPz crank with 18mm pins but your problem might be mass. Check the difference between the two. The question is if you would be able to balance the engine. If you are looking to use J pistons to power a Zephyr, than MTC will punch the wrist pin holes any size you want.

otakar wrote:

You always have the choice of running a GPz crank with 18mm pins but your problem might be mass. Check the difference between the two. The question is if you would be able to balance the engine. If you are looking to use J pistons to power a Zephyr, than MTC will punch the wrist pin holes any size you want.

Tell me about balance. If the pistons were significantly lighter, say 10% lighter, would the engine be out of balance, or would the lighter pistons actually be better and allow me to turn more rpm?

I guess I thought that lighter was better.

lighter is better it will allow the engine to spin up quicker, but the crankshaft is balanced from the factory using the weight of the stock rods wrist pin piston ring and circlip and piston assemblies so it will affect the balance factor of the crank. it will change the harmonics of the crank. it can affect the level a frequency of vibration in your motor with mostpiston kits you wont notice much difference just be aware it could change

otakar wrote:

You always have the choice of running a GPz crank with 18mm pins but your problem might be mass. Check the difference between the two. The question is if you would be able to balance the engine. If you are looking to use J pistons to power a Zephyr, than MTC will punch the wrist pin holes any size you want.

How about having a machine shop bore the rod small end for 18 mm pins? Does anyone know if that is possible and if the result would be reliable?

Do you realize how much you would weaken the wrist pin loop? Plus those are honed to a PRECISE size, within .00005"

These factors must be taken into consideration. Piston weight, distance from wrist pin to crown edge, wrist pin size and weight, Valve pocket angle, and last but not least dome shape in relationship to combustion chamber shape and size.

otakar wrote:

Do you realize how much you would weaken the wrist pin loop? Plus those are honed to a PRECISE size, within .00005"

No. I am in way over my head
:dry:

I thought they were serious when they said "the only stupid question is the one that you don't ask".

I suppose that the piston would be raised by 1/2 the difference in the diameter of the 17 vs 18 mm. That is, the piston would sit 1/2 mm higher than the original 17 mm wrist pin piston sat- right?

It is my (vague) understanding that it is common on some engines to replace the bushings in the rod small end and then hone them to the proper fit for the wrist pin.

From another forum:
Re: after market piston/rod Wrist pin problem..
Quote:
Originally Posted by AddictsRacing View Post
I went to assemble my engine today all brand new parts eagle rods for the K24 part# crs5984k3d and CP pistons pt#sc70451

The wristpin bore is too small, by alot... from my understanding eagle rods are new to the k24 was there a bad batch out there i should know about?

has anyone had this type of problem with either brand?


im going to give them a call tomorrow.
you have to take them to a machine shop, my k24 Eagles were also too tight for the Wiseco pins.

timebomb33 wrote:

lighter is better it will allow the engine to spin up quicker, but the crankshaft is balanced from the factory using the weight of the stock rods wrist pin piston ring and circlip and piston assemblies so it will affect the balance factor of the crank. it will change the harmonics of the crank. it can affect the level a frequency of vibration in your motor with mostpiston kits you wont notice much difference just be aware it could change

Not quite true. Inline Four cylinder crankshafts are "self canceling" It does not matter how much the attached parts weigh as long as they all weigh the same.

Jay,

Thanks for chiming in.

Can you comment on whether or not one could bore a 17 mm rod for 18 mm pins?

I suppose you might need a thicker head gasket to compensate for the 1/2 mm that the piston would be located upwards.

Just thinking out loud due to what is in my parts bin...

Why would the piston move in any direction at all? You are enlarging a hole around its center. The center is still center. If you bore the hole off center than you have problems. In more ways than one.