Ariel Owners Motorcycle Club

I am looking for pistons for an iron 4G. My bores are currently +0.020" but are in need of a rebore. I am investigating a number of options but would appreciate some advice/experience please. My choices seem to be:

1. Source some decent used (or NOS) standard pistons;
2. Use Draganfly Mk2 +0.040" pistons;
3. Use IMD Mk2 +0.040" pistons.

I have looked at some used hepolite pistons but, by the time I factor in new rings and gudgeon pins, the cost would be more than the new Mk2 pistons. I would however prefer to keep the low compression feel to the 4G engine.

Draganfly says that Mk2 pistons will up the compression of a 4G engine to 6.9:1. Has anybody had experience of machining Mk2 piston crowns flat to keep the compression ratio closer to the standard 4G ratio of 5.8:1?

What is the difference between Draganfly and IMD pistons?

Thanks in advance, Pete

Hello Pete,

Some kindly soul recently mentioned a set of +0.060 NOS low comp pistons for sale on this forum. They will shortly go into my 4G as it smokes somewhat at present. The current pistons are + 0.030 low comp and have done about 2000 miles in this engine, with excellent results on at least 2 cylinders. I know there is pitting on at least one of my bores which probably explains the smoking issue. You are most welcome to come and see whether these pistons might suit your engine and I am quite happy to dismantle my engine whilst you are present so that we can assess whether it is the bores which are a problem, or whether the pistons have had their day. I did take them out of another 4G engine some years ago but my view is that they have life in them yet (as the other 4G engine had valve guide problems).

Paul

Great offer Paul, I'll PM you to arrange a time. Pete

Hi there. Let me state upfront, I am a novice who has probably bitten off more than he can chew in restoring a basket case 1948 4G…..

Nevertheless, after a rather long time leading up to this point and getting everything else done, I am now at the stage of putting my engine back together. Unfortunately (fortunately?), it looks like I will have to purchase new +0.040” pistons and re-bore my barrels accordingly. I am hoping somoeone can point me in the right direction about which type of pistons are best to buy please? I know there are some on Drags but I have no idea if these are the best option.

Many thanks for your help in advance.

Cheers - Ian K

hi , i thought someone in the club was getting them made and was advertising them on the forum awhile ago

I believe that Draganfly are stocking IMD pistons, ask them!

The IMD pistons which are widely available are excellent, but they have a much higher compression ratio that the pistons originally used in an iron 4G. You will probably get away with the higher compression ratio on a 4G since the iron head and cylinder are much less prone to warping than the poor alloy used in the alloy Mk I engine. If you are concerned about the high compression ratio, turning the dome off the top of the IMD pistons to a flat surface will reduce the compression ratio to about half the difference between that of the original pistons and that of the IMD ones. But turning down pistons in a lathe is not a straightforward operation since you need to hold the piston by the gudgeon pin in order to prevent distortion. That involves making a jig to hold the piston.

The original lower compression pistons made for a much less harsh engine than one with the higher compression ratio pistons fitted. Having ridden iron 4Gs with low compression pistons very enjoyably for many miles, the only one I have ridden with high compression pistons was pretty horrible to ride (for me) although the owner seems to like it. But the higher compression will give better fuel economy.

Over to you Ian, to make a decision.

A flat topped piston has the least area for heat to get to it. IF you wish to take metal from the top make sure it doesn't get too thin. Nev

There is one pair of 600cc; +60 NOS pistons on Ebay right now.

In answer to Nev, there is plenty of thickness in the top of IMD pistons to turn down to flat. IMD have assured me of that and measurements confirm it, but anyone proposing to do this can always check again.