sq4 MK II piston clearance

[pauldridge]

I am installing a set of new pistons from Draganfly.. they do not have the expansion slots cut in the side. Someone suggested this type of piston requires greater piston-to-cylinder clearance.

Do anyone have the recommended clearance specs for these pistons?

[Simon.Gardiner]

Paul:
The bore size should be the same whatever type of piston is fitted, unless you want to depart from the manufacturer's spec.
If the design of piston requires a different clearance the piston manufacturer machines it to a different size to fit the bore size with the clearance that the piston needs.
(In some cases particular pistons seem to be unreliable and always give problems which are often overcome by changing the clearances but I don't think this is the case with Drags' Square Four pistons, they should be OK 'out of the box' - although I stand to be corrected as I've not used them....)

SG

[pauldridge]

Thanks Simon..
I will beg to differ with you in some respect. My experience, at least with car engines, is that a wise machinist will carefully measure each piston, and then prepare the cylinder bore accordingly. There are almost always some differences in piston diameters, even if bought in a set.

Ask me how I know... I had to remove a vintage Cadillac V8 a second time and disassemble because the first machine shop just used factory specs rather than measuring the pistons and boring the cylinders to match. First time we started it up, it ran for 30 seconds then quit as the pistons expanded. That cost me a whole new set of pistons, and another machine shop!

We always have to learn these things the hard way.

If anyone happens to have the recommended piston-to-cylinder clearances (either original Ariel or Drags) I'd appreciate having those numbers.

[nevhunter]

The PLAIN (no W slots or T slots) always requires more clearance than other types. This type of piston is stronger and the type usually used for racing, and often rattles a little when cold for a short time, and that would be considered normal for a safe running clearance. It's easy to try for too good a fit and run the risk of nipping up for many miles, as it takes quite a while to wear much off. We are also going faster than people did when I was young . When you use it as an outfit it's more likely to nip up also. Unless you enjoy pulling the SQ4 down I wouldn't run it with too small a clearance as the damage is often enough to require a hone and new piston to get the clearance you should have had. The piston runs a lot hotter than the cylinder and it's got an iron liner that expands less than the alloy. Having a good high silicon material can allow closer fits to work. If you don't know what it's made of, heat a known one and your new one from freezing to boiling water and check the expansion with a micrometer and compare the two. Nev

[david.anderson]

Thermal slots and split skirts in pistons were designed to absorb some of the expansion of the piston as it became hot. That way the piston could run a tighter clearance in the bore and the piston slap that can often be heard in a cold motor was eliminated. The slot however does weaken the piston and is often the source of a crack that will develop from the end of the slot. For that reason slots are not used on performance engines. Not long ago someone posted a photo of a VB piston that had separated from the slot, so it can even happen in in low performance engines.
Like Paul I have seen considerable differences between pistons that were supplied as a set. I saw 2 Taiwanese pistons that differed in size by .002”, and I have seen considerable differences in weight between pistons.
All pistons that I have bought for many years have been supplied with paperwork that gives the required clearance for the piston, and most ring sets have quoted the end clearance required per inch of bore. As Australian conditions are hotter I have found that to bore to maximum clearance saves problems.
The information that I have for a VH which was supplied by Drags is in an iron bore .005 - .007” for a solid skirt piston and .002 - .004” for a split skirt piston. But for a VH piston in an alloy cylinder they quoted .002 - .004” for both solid skirt and split skirt. So strangely enough with the alloy cylinder both pistons are the same? which seems strange.
I do not have anything for the 2 types of pistons in a 4 but I can say that my 4 was bored to .0025” clearance (outer range per manufacturers specification, old stock Hepolite) and my pistons are thermal slot. So my guess would be .0025 - .003” for the solid skirt.
Paul, what brand of pistons do you have. If they are Gandini then I have only heard good things about them. Drags should be able to provide the correct clearance.
David