pinto piston rock vs deck clearance

[onecamohv]

Hi all,

Something im working on at the moment and i thought i would get other experience on here in specific to pintos when working with such close tolerances.

There are many ways/theories/suspicions/methods to do this, and in my opinion thats all they ever will be as you are only anticipating what might happen inside the cylinder at any given temperature/RPM, so all you can really do is make the best educated guess from experience.

So how do you factor in piston rock when working to very tight squish tolerances? with forged pistons the rock is quite alot, and this obviously reduces when up to operating temp. However, there is still the chance given the taken measurements that the pistons could kiss the head on full rock one way or another.

i know the accepted pinto minimum is .75mm (few tenths under .030") but when a forged piston can rock 8/9 thou things are getting close for comfort. theres many theories floating about: some say take the average of the min max rock, some say on a flat top such as the pinto take the dead centre of the crown and dont worry about rock it will 'likely' be ok if warmed up correctly, some say take the average min/max and add 4 thou.

so what say thee in relation to accralites on cossie rods? in my case at the moment im getting 16 thou of rock which is pretty much bang on for a forged piston in a new bore, so essentially 8 thou of rock above the centreline of the pin. if i was to set the squish at 32 thou, im down to a theoretical 24 until the engine warms up.

would be interesting to hear others opinions on this

[katana]

I've no definitive answer but logic says you have engine block temp and piston temp. The former takes a while whilst the latter should be less than a minute and in my experience very few engines get ragged within a minute of start up! Forged pistons obviously 'grow' significantly hence the clearance but are quite stable once hot. I can't say i've ever considered it a problem and used forged pistons in most things that i've had.

[rallyrob]

I've got no idea about this. But to a certain extent does the rock depend on what tolerances you have machined the bore to for the piston for any given appliance? I mean in that you could machine a piston slightly on the loose side or on the tighter side. An this would give you more or less piston rock.

[onecamohv]

I've got no idea about this. But to a certain extent does the rock depend on what tolerances you have machined the bore to for the piston for any given appliance? I mean in that you could machine a piston slightly on the loose side or on the tighter side. An this would give you more or less piston rock.

Well yeah you're dead right, a sloppy clearance will give you even more rock, but even with the recommended piston/bore clearance there is still the 16 thou that i have which seems to be the norm for an approx 3in bore with a forged piston of a similar length which at least needs to be thought about and considered i think.
Will the piston rock to its max, probably not, and like was said above only for a short time, but i was just wondering other peoples take on it

[rallyrob]

Okay yeah, I see what you mean.
Not even sure the guy that helped me with my engine years ago considered squish or squish clearance either tbh. It was a long time ago now though.

I've got Cast pistons on cossie rods.

So the expansion rate of Forged over Cast pistons is different then is it. I Can't say I knew that either tbh.

[Enea]

If I understand correctly, maybe you are missing the fact that also the engine cylinder bores will expand with heat, not at the same ratio of the piston off course, but you will have those few 0,0xmm of the cylinder expanding.

Here we have rally and racing car guys that request the bore size 0,01mm under the spec, so on the tighter side.

[katana]

Here we have rally and racing car guys that request the bore size 0,01mm under the spec, so on the tighter side.

Can be risky for a cold seizure - you have to be disciplined to allow sufficient warm up time before abuse!

[Graham]

Hi all,

Something im working on at the moment and i thought i would get other experience on here in specific to pintos when working with such close tolerances.

There are many ways/theories/suspicions/methods to do this, and in my opinion thats all they ever will be as you are only anticipating what might happen inside the cylinder at any given temperature/RPM, so all you can really do is make the best educated guess from experience.

So how do you factor in piston rock when working to very tight squish tolerances? with forged pistons the rock is quite alot, and this obviously reduces when up to operating temp. However, there is still the chance given the taken measurements that the pistons could kiss the head on full rock one way or another.

i know the accepted pinto minimum is .75mm (few tenths under .030") but when a forged piston can rock 8/9 thou things are getting close for comfort. theres many theories floating about: some say take the average of the min max rock, some say on a flat top such as the pinto take the dead centre of the crown and dont worry about rock it will 'likely' be ok if warmed up correctly, some say take the average min/max and add 4 thou.

so what say thee in relation to accralites on cossie rods? in my case at the moment im getting 16 thou of rock which is pretty much bang on for a forged piston in a new bore, so essentially 8 thou of rock above the centreline of the pin. if i was to set the squish at 32 thou, im down to a theoretical 24 until the engine warms up.

would be interesting to hear others opinions on this

all i can tell you is, is when i run forged pistons with 3.5-4 thou clearance and .030 squish on an engine which gets reved to 8K, there is no piston to head contact as such but the piston is likely to be clean in the squish band

[onecamohv]

all i can tell you is, is when i run forged pistons with 3.5-4 thou clearance and .030 squish on an engine which gets reved to 8K, there is no piston to head contact as such but the piston is likely to be clean in the squish band

thanks Graham, and you are measuring from the centre spot of the piston and not getting caught up in the rock ?

[Graham]

thanks Graham, and you are measuring from the centre spot of the piston and not getting caught up in the rock ?

correct meassure at centre or front and rear edge line with the pin, in fact thats a good check, you will often find piston height is different front and back

[onecamohv]

yep thanks Graham, odd the funny 70's engineering nuances you get in old ford and BMC engines, crank is unground on the big ends, yet ive a few thou difference across the deck heights. did ALOT of measuring, swapping etc. rods, pistons are predictably identical, turns out the crank is ground off centre here and there.... mad. nothing i cant rectify when skimming the crowns but you woulsnt want to make any assumptions, most notably that all deck heights would be the same during dry build. im at an average of 49 thou deck height at the moment so plenty of safety in the crown thickness