JP piston - pin too tight in small end bush?

Dan, I seem to remember needing to have a new little end bush making to fit my AP / JP piston, but that's not why I'm writing.

These are good quality pistons but I think you will find it heavier than the original. Your bike will run fine with it but you might just find the engine slightly more lumpy. I'm happy with it in my poor man's Manx but if I had bothered to weigh the piston against the original, I would have removed a tiny amount of metal.

Hope this helps.

Hi Dan,

Your old piston is a pre 1948 16H type which is quite often used in ES2 engines.The skirt length has to be reduced and you need vavle cut-aways which I see has been done on the old one, although they look a bit too much to me. If you don't have any spacers under the barrel the compression ratio will be about 8.5 to 1, so you will notice quite a drop in performance if you go down to the original 6.7 to 1 and you will have to advance your ignition as well. There should be plenty of 16H pistons around if you want to stick to what you have. ATB Richard.

hi dan

Interesting topic about jp pistons,The chances are that your replacement piston has a metric gudgeon pin instead of an imperial size it would have had originally ,Companies are phasing out imperial sized parts and substituting easier to make metric parts as they call the imperial system ( non prefered size) this happens often in engineering and can be a pain in the ar-e

eg .6869/.6867 standard pin size on a commando would now be 17.5mm which is roughly .002/.0022 thou larger and thats why you had to ream your s/e bush

I would also like to pass-on that jp pistons need more cylinder/piston clearance to stop them from seizing

check out the M.G owners forum

steve

Previously Dan Field wrote:

Close up of DIY piston pocket!

[IMG]http://i29.photobucket.com/albums/c264/deperatedan/image.jpg1_zpseaxzaibs.jpg[/IMG]

Hi Dan,

Do you know what make of blindfold the PO used ? I did one of these when I was 18 and made a better job !

Hi Dan

You said you found a set of pre f/bed engine plates. Did you buy them?

Were they for a single with Norton AMC box?

Good luck with the piston, personally I think the JP piston was a good route to take and with some weight removed then perfect..

Hi again Dan,

I fitted a pre1948 16H piston to my 1948 model 18 way back in 1972. I was 18 years old then and the old engine was getting tired of all the thrashing it was getting and struggled to hit 75 mph.I cut the bottom of the skirt off and made valve pockets, all with hand tools and also streamlined the inlet port and double ground the head joint. This pushed the compession up to about 8.75 to 1 which was right on the limit for a cast iron head but leaded 4 star just about coped with it. When used with a Brooklands can and a slightly larger main jet and a belmouth this put 10 mph on the top speed with acceleration to match. If I was doing the same today I would fit a 1/16th inch spacer under the barrel to reduce the compression, although you will be O.K. with an alloy head all usingsuper unleaded. The best piston for your situation would be a 1948-54 16H which has a lower dome than the earlier type which gives about 7.5 to 1. This is the split skirt type and will need shortening like the earlier type.

ATB, Richard.

Dan, Regards the pre/fbed plates, an overseas Norton man is helping me. Only the postage that might be an issue. I am very grateful for this after several leads that came to nothing .I'm confident and so going to start the project once I have successfully had my trial Rattle run to the coast on my 650. I'll tell you about that on another thread. It might be tomorrow if we keep the high pressure.

Do tell how you got on with that piston, my money is still on the JP, they are well made but slightly heavy.

Previously Dan Field wrote:

Well I thought I'd be getting my bike back together this afternoon, having bought a new + 40th JP piston from RGM, .... the old piston pin slid out with a bit of gentle heat as you would expect. I cleaned up the case faces and made sure the oil ways were clear, fitted a new gasket (with grease not loss of blue gloop!) I fitted the new rings and one circlip and set about fitting the new piston, sadly I couldn't get it to fit, the pin is a nice fit in the piston, but impossible to fit into the rod. I tried some serious heat and that didn't help either. Having taken if off again I measured the two pins ( the piston I was replacing was a heptolite) the JP pin is at least 0.003/4 mm bigger than the original. So I seem to be left with trying to get my money back or trailering the bike to an engineer and having the bush reamed. Is this a common problem with JP Pistons? Are GPM any better? I had no such problem with the Jubilee! Dan

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G'day Dan,

A suitable adjustable reamer should assist you and a really must have in the tool cupboard, plus you will be able to achieve a perfect fit!

Rgds Steve

I have been following this thread with interest and would like to say a couple of things that I have done successfully in the same situation, if the piston assemblyis too heavy, it can be lightened by boreing out the gudgeon pin to rapidly lose some weight, though, a littlecomparisonof the wall thickness of same outside diameter of lighter pins is advisable. This is on top of reducing piston skirt thickness.

To ream out the bronze gudgeon pin bush, I use an adjustable reamer taking manylight cuts with no problem. Insert the reamer through bush, and gradually expand in positionafter a couple of turns.You will need a pin in the bushclearance of .001" to .0015", or an easy slide fit with the slightest percievable rock. Too tight and it will starve of oil when running.

Sharp tools are required for brass/bronze, as a dull edge will suddenly dig in as excess pressure is exerted to get it to start to cut, in my experiance.

Regards,

Paul

Must agree with Paul - using a reamer is not too difficult, only mildly terrifying the very first time. If I can ream a small end successfully, anyone can...