Heplex or JP pistons?

Slotted pistons got a bad name, but I ran with these in a hotted up and heavily thrashed 88 for years with no problem from the slots. What is the reputation of the JP's?.

JP's have a bad reputation, they use a single blank to cover more appications than any other piston maker hence they are heavier in most applications, their rings are far too stiff and they also do not always complete all the maching operations. The Matchless one I saw drew blood from fingertips when you tried to expand the rings to remove them and the ring relief area had not been machined. The bike seized in 20m, new hepolite rings and the ring relief area was machined and then it ran ok.

The bad slotted pistons I know of are the Combat era ones, the slot in the oil ring area was continuous, if there are supporting lugs they do not lose their heads

Stick the JP one in a lathe and take 30g of aluminium off inside the shirt and the underside of the top of the piston. Check it over for missed machining operations, the ring area should be a few thou less in dia than the skirt, check the Hepolite for the exact dimensions. Mismatch from side to side weight wise will be as bad as a badly static balanced crank, the 30g is not needed to take it off if there are obvious differences to the Hepolite and then remove the rest as I suggested on a lathe.

I have had a number of slotted Hepolite pistons lose their tops over the years, the most recent (i.e. 10 or so years ago) being in my Atlas. I am a bit wary of them. The JP pistons I have subsequently used are all still intact. The least worse option?

Don't like to advertise Norvil but they have a good page on the different types of pistons and how to tell the difference.

http://www.norvilmotorcycle.co.uk/techtalk34.htm

Also Joe of Norton Motors says

The "dangerous" Hepolites were for 750s, and the downward slots on these are joined by a horizontal slot each side that runs behind a piston ring (easy to see looking into the piston). these are fit only as ashtrays, but they haven't been made since the early 1970s. If you get them they are still there, new, because the experienced dealers and owners declined to buy and use them!

If swapping pins check length not short and fit in piston. Should float when hot .Also end of pins match clip design.

If one gudgeon pin is heavier than another,it can be bored out easily to reduce weight byusing a sharpened up masonry drill in a lathe. Also, as mentioned above, pistons can be machined inside the skirt to relieve weight and also, if still heavy, metal can be removed from around the pin boss with a rotary burr.

J.P. will also relieve the internal radial diameter of their rings to decrease the wall pressure, if required.

Is there a preference? Both seem to be available. JP have solid skirts. Heplex (n.o.s.) are slotted. (To reduce piston slap?). They weigh less - about 290g with pins and JP weigh 320g.Are the Heplex ones the same as those reported to lose their tops? The bike is 88ss - 500cc, not an Atlas...Thanks for any advice.

JP piston are the AUSTRALIAN Copy of the orginal BHB or Hepilex there all apart of British heavy industry C.o so are the Italian ones Gpm but there both heavy to the orginal pistons the slotted shirt its to control piston wobble but they did have re-enforced boss for the gudgen pin so all this about piston top coming off its all scare tacktics Hepilex piston can take some real hammer but do not

mix up the piston rings with JP or GPM piston rings there have a different raidus to one another! for more information Ask Dale Middelhurst 01422 882951

"so all this about piston top coming off its all scare tacktics". Well, yes, it is quite scary when the top comes off the piston. Take it from me.

Since the slot is not in the groove think I'll keep it. The ones on the Norvil site have half the cross section area removed at the bottom of the groove. No wonder they gave trouble...It turns out that the Hepolite at +60 weighs 325g and the JP at +60 weighs 327g.So they are almost identical weight to the accuracy of our electronic kitchen scales. Next task is honing cross hatch in the bores. Ball hones seem to be in fashion but cost a lot of cash for a couple of minutes use.

Yours Anna J

Previously David Cooper wrote:

â?But neither RGM nor Norvil currently have +30 pistons for 500 Dommieâ?

Possibly they can help: http://www.vintagemotorcyclepistons.co.uk/shop/catalog/browse?sessid=KfXyV2NXQBGML28sf29pQ1IiV1B0JonxwbQzV5tU78c4uJk5tO4cYCbkWvTFXLE3&shop_param=shop_overview_pager%3D1%26cid%3D39%26

Fritz