<VV> TRW Piston Problem

[djtcz at comcast.net]

Is the piston really marked "standard" ? I'm a little surprised you could end up with a standard piston that won't slide into a used standard bore. 


Commercially made oversized pistons generally are made so they >should< have >> acceptable << clearance in a cylinder finished to the nominal size. 
164 Corvair nominal standard bore is 3.4375 inch, so a 0.030 over piston should work OK in a cylinder finished to 3.4675 ( 3.4375 + 0.030) . 
Of course it is best (necessary, in my mind) to measure the actual parts to determine clearance. 


** - The instructions that came with TRW pistons in the 70s said the situation was a little different for standard sized pistons because OEM standard bores naturally varied a little from engine to engine, etc, so replacement standard pistons were on the large size to be useable in the largest "standard" bore. 


If your pistons are L-2206 they appear on this vintage looking chart. http://corvaircenter.com/phorum/file.php?1,file=41584,filename=Forged_Piston_Clearance_1a.jpg 

General piston part nomenclature here - 

http://www.shrirampistons.com/images/pistons-range-big2.jpg 


The diameters up by the rings (the ring "lands") are significantly smaller than the "skirt" . That area needs a ton of clearance because it gets really hot and because it is solid and round, and thus inflexible, unlike the skirt. 
The diameter used to measure clearance is somewhere on the skirt, but always 90 degrees to the wrist pin. Some manufacturers specify clearance inline with the pin, some at the lowest end of the skirt. You have to get the PISTON manufacturer's specification to know what is required. 


Measuring clearance with feeler gages used to be the norm. Mostly back in the days when pistons were iron and had severely split piston skirts. 
"The piston to be fitted should be checked with a micrometer, measuring just below the lower ring groove and at right angles to the piston pin." 
http://chevy.oldcarmanualproject.com/shop/1942_47/4247csm604.htm 


I would always use a micrometer and bore gage today. 


Poor honing technique and lack of stone/shoe truing and maintenance can lead to excessive taper and bellmouth even with a Sunnen hone. Another reason why the cylinder must be measured throughout the process. 
http://www.mscdirect.com/FlyerView?pagelabel=1788&search=06451744&contentPath=/sales-catalogs/big-book 




regards, 


Dan T 



----- Original Message -----

From: "Smitty" <vairologist at cox.net> 
Subject: <VV> TRW Piston Problem 

Smitty Says; I have run into a problem I need some professional advice on. 
Keep in mind I am talking about using a used and somewhat worn cylinder. I 
am using a Sunnen hone which should pretty much guarantee a round cylinder 
bore and no taper. I have measured it and this is proven to be correct. I 
have honed the cylinder to take out a little surface rust from inside. To 
check piston clearance I used a new TRW piston and tried to put it in the 
cylinder. It went into a point just below the ring area, but then started 
to tighten up. I honed it some more and now it goes in but with drag on the 
piston skirts. I am sure the piston is cam ground which would account for 
the skirt being larger than the body of the piston. At this point I got a 
gently used TRW piston and tried it. It also has drag on the skirts when I 
get past the main body of the piston. The piston body to cyl clearance is 
around .004. All components I have mentioned are marked standard. (except 
of course the Cyl). 

I sure as heck don't want to keep honing to get more skirt clearance and 
make more body clearance. What the heck is going on here and what do I do 
about it. I don't feel comfortable with using the pistons with and physical 
drag on the skirts.