Mk 1 Square 4 piston clearence

[Duncan J Robertson]

Any advise on new piston clearances.
While the engine is stripped down I'm having it re-bored, new +30 pistons going back in.
The Ariel workshop manual states:-
Ring land - 0.020 - 0.023"
Below rings - 0.003 - 0.005"
Skirt - 0.001 - 0.003"
OK to work to these numbers.

[will_curry]

Ideally the piston manufacturer should be able to tell you what the necessary
clearances are as it depends on the alloy used in making the pistons and their
design amongst other things.

[paul.jameson]

Which pistons are you going to use - specifically what compression ratio? The original Mk I pistons are low compression, dished. If you fit the readily available Mk II pistons which have a raised section in the centre of the crown and so are high compression, you are asking for trouble with the cylinder head gasket blowing, especially if the head and/or block has been skimmed to cure leakage in the past.

[Duncan J Robertson]

The seized pistons found were concave and from FW Thornton.
No sign of the head being skimmed in the past.
So where to go from here, thoughts please

[paul.jameson]

We have been here before. Try:

viewtopic.php?t=11675&start=20

[Gui.dorey]

The thread Paul posted was mine. Here’s a short summary of my experience:

I used IMD pistons, which are actually for a MKII and have a raised top. My head and barrel had been skimmed more than they should have, so I ended up with a very high compression. This caused the head gasket to blow after a short period of time, no matter what I did. I did some calculations at the time and had a compression of over 10 I believe.

My solution was to skim down the piston tops to flat and to use a decompression plate at the bottom of the barrel. The plate could have been a bit thinner, but in the end it worked, bringing the compression down to around 6. (I would have to check my old notes to tell you exactly).

Anyway, since then I have had no more problems, the bike runs nicely and just a few miles away from the last torque down of the head.

If your head and barrel haven’t been skimmed, you probably shouldn’t need a decompression plate, but if you do use the MKII pistons I would definitely recommend to skim down the tops to flat.

As for piston clearances, I just followed the manual.

Hope this helps, but any questions feel free to get in touch.

Cheers
Gui

[Roger Gwynn]

There is very little variation in the expansion rates of 'new' and 'old' materials used for pistons, at least since the 1950's. I found the specs for Hepolite and GPM told me theirs, very little difference and I did a test with a domestic oven of an old an new piston, there was no measurable difference in expansion. Measured using a micrometer, not a calliper.

[nevhunter]

Slotted pistons have more "Give" but the crowns run hotter and they are only a non racing piston. A "solid skirt" will need more clearance or it will nip up particularly when pulling a chair. If the Cylinder is sleeved allow more clearance for that.. Freezing to boiling point are 2 good limits to use when comparing expansion rates of Pistons. Forged ones generally expand more. Some cast Japanese 2 stroke ones expand at a low rate as they are high silicon. Slipper type pistons transfer less heat to the cylinder walls. So do ones ground with a lot of Ovality. Nev

[Duncan J Robertson]

I did wonder about the raised piston heads of the IMD (MK2) pistons and comments regarding head gaskets, but I will go ahead anyway.
If it proves a problem and the head gasket's give up, it's easy enough to machine the top off each piston.
Thanks for your thoughts

[nevhunter]

It's not easy to grip it without distorting it.. The thickness must be kept sufficient. Nev