Head clearance measurement with a dished piston

[eekern]

I subtract the depth of the dish from my clearance measurement using my head clearance gauge. Is there a better way? What is your method?

 

[Tom Foley]

Simple caliper depth measurement to dome of piston at tdc . Then another on the button and shims . If sleeve fit is real tight rotate to tdc with head button on tight then remove .

 

[David Murany]

Zero out your gauge with the piston against the button instead of a flat surface. Reassemble engine and check it. Easiest way to get it right for sure.

 

[Glenn Quarles]

I just use Plastigauge (sp?).

GQ

 

[Tom Foley]

This is a great method (David Murany )but requires another piston or your piston assy to be removed . I will try this on my VAC 91's later today .

 

[steve wood]

I just use Plastigauge (sp?).

GQ

ditto

 

[dave roach]

With the liner and piston in the block use two screw with washers to hold the liner down and bring the piston to the top and use depth mic.to check from the top of liner to the Top of piston.Then depth mic.head button and subtract the number and that your head clearance.

Dave

 

[steve wood]

With the liner and piston in the block use two screw with washers to hold the liner down and bring the piston to the top and use depth mic.to check from the top of liner to the Top of piston.Then depth mic.head button and subtract the number and that your head clearance.

Dave

but t his won't work with the tapered/cone/hemi type piston. plastigage does

 

[Tom Foley]

Good point Steve !

 

[moparbarn]

With the liner and piston in the block use two screw with washers to hold the liner down and bring the piston to the top and use depth mic.to check from the top of liner to the Top of piston.Then depth mic.head button and subtract the number and that your head clearance.

Dave

but t his won't work with the tapered/cone/hemi type piston. plastigage does

honest question, steve. why not? squish is the clearance around the outer pereminter of the button & piston. it is used to control/create the rush of cooler mixture to the center of the chamber to help cool/quench (i have heard squish also called quench) detonation (at least to my understanding....). so, that being said, why would a countoured top of the piston & button be an issue? couldn't you just measure at the flat surfaces near the outer edge? most of my knowledge & theory comes from my 40 some years as primarily a 4 stroke mechanic, but a lot of stuff does transfer to our nitro engines. if i'm out in left field, 'slpain it to me. if i need to relearn something here, i want to

. personaly, i do exactly what dave roach does. i like to measure with everything in place, to allow for all the clearances in the bearings, rod & piston to be accounted for.

 

[steve wood]

the piston and button,on some of the newer engines any way,have a tapered piston and a tapered button that matches.these are the surfaces that you want to measure the gap of.measuring the flat of the top of the piston tells you nothing.you have to measure the depth of the button at the o.d. and then measure the depth of the piston at the o.d. at the bottom of the taper,which is about .007 to .008 lower than the flat at the top (measured on an optical comparator).pretty tough for most people.use a piece of plastigage,roll the engine over top dead center once and you have your clearance

 

[Bob Morton]

Great stuff. Make sure you check the head clearance over the wrist pin.

Nails

 

[eekern]

Thanks for the input guys. I now have two makes of motors with the tapered and dished pistons. I will have to get some plastiguage!

 

[moparbarn]

thanx for the info, steve. being a racer of old, worn out engines

, i had not seen any of the newer pistons or buttons. i had assumed that there was enough of a flat surface around the OD to measure at. with no flat surfaces, plastigauge would be the easiest for most of us.

 

[steve wood]

most,if not all novas have the dished ,tapered piston now, picco 45 also. might be more that i'm not aware of

 

[eekern]

new OPS next 21 does as well...