Piston doesn't clear the crank

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Mozenrath

New Member
Jan 13, 2011
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Recently, my crankshaft broke(my fault), so I bought one from Kings Motor Bikes.

I compared the connecting rods, and while I couldn't get an exact measurement, they seemed close enough. I placed the piston on, but when I turn it by hand, the piston skirt doesn't clear the flywheels! It gets stuck, and I'm thinking of slightly trimming the skirt to see if I can get it to pass. Is this a good idea or a bad one?

Also, since the difference will probably effect the timing, should I modify the thickness of the base gasket to compensate?

EDIT: The original rod says "ZAF 60". The new one is Z-L. In case that helps. There isn't a way to swap the rods, is there?(yeah, I can already tell the answer is probably no)
 
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biknut

Well-Known Member
Sep 28, 2010
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I think there's several different pistons.

One fits Power King style motors.
One fits SkyHawks.
One fits Flying Horse motors.

What kind of motor did yours start out as?
 

GearNut

Active Member
Aug 19, 2009
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San Diego, Kaliforgnia
The main difference is where the wrist pin hole is placed.
If you choose to trim the piston skirt, I would first temporarily install the crank (with piston temporarily installed onto the connecting rod, no need to install the piston rings for this simple check) into at least one half of the cases and slip the base gasket and cylinder on. What you are looking for is how far the piston comes up in the cylinder when it is at the top of the stroke.
If it is too high and you will have an insanely tight squish band at best. At worst it will actually hit the cylinder head.
 

Mozenrath

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Jan 13, 2011
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My motor is a RAW, but they don't make motors anymore and they currently aren't selling cranks. I couldn't find any information on the few cranks I could find for sale, so I took a chance.

I think the rod might be 1/8 to 1/4 inch shorter than the original. The piston I have has the pin hole located near the center.

I doubt that the piston will come up too high, but I'll be installing the crank so I have to check anyway.

If I buy a piston, does this one look correct?
http://www.ebay.com/itm/66cc-80cc-p...426?pt=LH_DefaultDomain_0&hash=item41609e4d32

I don't want to buy one from Kings because it takes a week to ship and costs twice as much with shipping.

EDIT: After more reading, I've come to understand that the flywheels are kept together with a pressed fit on the crank pin. If I were to replace the rod on the new flywheel with my old rod, how would I take apart the two flywheels?
 
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GearNut

Active Member
Aug 19, 2009
5,104
11
38
San Diego, Kaliforgnia
Pressing apart a set of flywheels requires a hydraulic press and appropriate press plates or a special jig.
Pressing them back together requires the same thing with the additional need of a flywheel truing stand, and the knowledge of how to properly true a set of flywheels.
You will most likely need to re-balance the flywheels as well considering that the new connecting rod and piston will no longer be the original parts that were mated to flywheel assembly when the flywheels were originally balanced. The new assembly will definitely have a different weight to it and this must be taken into consideration.
 

biknut

Well-Known Member
Sep 28, 2010
6,631
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Dallas
New piston it is!

Thanks guys.
I tried to install a BGF piston in a SkyHawk, and the piston hit the head. You couldn't turn the motor over.

I had to order a SkyHawk piston.

I know your bike is a RAW which I know nothing about.

A BGF piston for a T, X, or Z motor has the wrist pin closer to the bottom of the piston. A SkyHawk wrist pin is slightly closer to the middle of the piston. I thought it was a little less than 1/4". More like 3/16"
 

Mozenrath

New Member
Jan 13, 2011
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California
Well, we'll see what happens. I ordered a piston from luckyearlybird that had the wrist pin closer to the bottom(which is what I need), so hopefully it will fit. I must say, this seems to be the part of the engines that has the most variation.
 

allen standley

Well-Known Member
Oct 22, 2011
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Bangor, Maine
Yeah been there done that. I had very satisfactory results modding a flying horse piston to work in a Grubee Engine. It also involved skimming the top of the Jug to compensate for loss of compression @ TDC. You gotta check my public album... I need to know which engine is J Type and which is DType. Any clear answer on that?.flg.