Re: Carb cleaning tip, what not to do

[Stan Chernoff <az589@lafn.org>]

Keith,

Dean should be able to give a definitive answer on matched pistons and
domes.  He carries Z-Therapy remanufactured complete carburetors for the
same price as Z-Therapy with an added free gasket bonus.   At least he
should be able to supply the acceptable clearances.

Z-Therapy is one of the recognized authorities on the Hitachi S-U carbs.

Stan
===============
At 06:11 PM 7/29/2004 EDT, Keith0alan@aol.com wrote:
>Well since I'm a testing kind of guy I decided to experiment a little. I 
>pulled 6 dome/piston pairs from carbs I've recently done. The dome to piston 
>clearances were all between 1.5 and 3 thousanths of an inch. Pretty tight 
>tolerances. Then I took the springs out and plugged the holes in the
pistons. Turning 
>them upside down and dropping the pistons in they took between 2 and 6
seconds 
>to leak down. Not as consistant as I was expecting. Then I tried
something, I 
>wet the piston edge with silicone spray. One thing you will notice when you 
>pull your domes is that the piston edge is wet with oil or gas. With the
piston 
>edges wet the pistons did not leak down at all. One thing I noticed was the 
>grooves on the outside of the piston. This is a technique used on some small 
>engines. The grooves hold oil that acts as a piston ring sealing the space 
>between the piston and wall with very little friction. I dragged out the
piston and 
>dome tht hat been sandpapered. It took about a second to leak down dry and 
>about two seconds wet. The piston to dome clearance was about 6 thousanths.
>
>       I can't say for sure that pistons and domes were matched at the 
>factory but the tolerances are awfully tight. I can say for sure that this
piston 
>and dome were ruined by being sanded. 
>
>keith williams

>
>  I heard that they were matched sets, but you know how rumor and inuendo
go. The other interesting point here would be how many times these parts
have been taken off the cars over the past 30 years and cleaned in a
solvent tank and the parts switched by unknowing mechanics and put back
incorrectly no longer preserving the "matched set".
> 
>Dean