Ariel Owners Motorcycle Club

Yes, Roger, they are indeed round and parallel. At 2.65in they are essentially 0.090 oversize. The oval and taper specs from the hepolite catalogue (1952 version) looks kind of challenging, but then, thinking about it, I guess shaping pistons in this way has been going on for around a century so it should be reasonable to expect a machine shop to be able to undertake the work.

My other thought: is the depth of the ring grooves the same for different sizes of piston, ie., std to 0.060 over?

The thickness and depths of ring do not vary over the the sizes but must suit the piston of course. I should have the dimensions listed if they are Hepolite.

The ring groove depth is to suite near standard bore size. If you go to a large oversize the Piston will be Heavy and the grooves deep. Piston grinding oval and tapered was common once IF someone has the equipment its still a time consuming process I have such a piston grinder But it's considered obsolete these days as are semi finished pistons. The grinder is a REPCO and Made just after WW2 and I don't have the full set of cams I use it just for grinding Cast Iron Pistons which are round. (as are 2 stroke pistons). I also don't believe in having a lot of Cam unless it's a W slot Piston and I don't recommend THEM in any air cooled Motor. Nev

Today I visited the machinist I use to facilitate rebores, sleeving, etc., and he agrees the semi finished piston concept is not only obsolete, but also far too time consuming and tedious to be worthwhile. Which is a pity as it was the only way I could obtain a set of new low compression pistons for the project. So it looks like I will go with the second option of reboring for new 12487 pistons which I can easily get.

Sometimes you may only Have to hone the Bores. Metal removed cannot be replaced. JP make pistons to any specified Oversize.. Nev

Unfortunately the barrels have been left open and damp to accummulate rust over many years. Which is a pity as they are nominally 0.030 over, but much more than honing is required to get a clean bore. Certainly my first choice would have been to keep them as 0.030 over. I think the slightly higher compression later pistons will be OK, especially if I go with the stronger AOMCC con rods.

The Main thing with Alloy rods is fatigue ( High revs.) You don't usually know how your Motor has been treated before you got it.. Clean out the Oil Galleries in the crank , properly. Nev

I intend to buy a set of club connecting rods for my 1948 Sq4 project. Reading the instructions/recommendations booklet that come with them I see that balancing both cranks to a factor of 0% would be beneficial. If I am reading correctly, this would mean adding approx 24 grams of weight to the crankshaft bobweights. My bottom end is still intact, although the original rods have been removed. Whilst rotating the cranks I note there does not seem to be much room around the bobweights to add extra material. Is there a preferred location for extra material (Mallory metal) to be attached?

That part of the instructions is not clear as to whether the additional weight applies to the early cranks or the later cranks with the extra chamfer to the bob weights but I think it applies to the later cranks as the new rods weigh about the same or slightly less than the 1937-47 rods.

Thanks Roger. I will get the new set of rods in October when I visit the UK for vacation. I can then take them, the new pistons and the cranks to the machine shop to see how much fettling they need to achieve 0% balance. Hopefully adding large amounts of extra weight will not be an issue...