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JP Twin Pistons too heavy #767438 03/07/19 9:48 am
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Matchless Offline OP
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I am in the process of rebuilding a 650 CSR engine for my 1961 AJS. The JP pistons arrived yesterday & as soon as I took one out of the box I could tell it was much heavier the the original factory piston. The AMC version (std 72mm size) weighs 355g & the JP (+0.020") is 427g. As these are currently the only pistons in this oversize available, does anyone know if this will cause vibration problems higher up, lower down or just more? 72g is rather a lot of extra weight but as AMC dynamically balanced their cranks it would seem a shame to have to have it rebalanced.

Martyn.

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Re: JP Twin Pistons too heavy [Re: Matchless] #767439 03/07/19 10:03 am
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Even with rebalancing it will vibrate more as there is more reciprocating weight and it's all at the furthest away point on the end of the rod. If you leave it unbalanced it will be even worse as the balance factor is linked to the frame, at what revs it will be worse is a lottery as its linked to the harmonics of the frame.

If you are going to use the JP's then check them over very carefully as they often miss operations out like machining the relief area for the rings, the rings themselves can be an issue too as they take a long time to bed in and so risk of bore glazing is higher.

Re: JP Twin Pistons too heavy [Re: Matchless] #767490 03/07/19 11:11 pm
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Rohan Online Content
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Originally Posted by Matchless
but as AMC dynamically balanced their cranks
.


I'm not entirely sure that this is correct.
As far as I've seen quoted, Enfields were the only (big ?) maker who dynamically balanced their cranks
- and maybe thats because they were terrible before they started doing this !
In fact, there is much chat about that AMC's approach to even static crank balancing was a bit haphazard,
to say the least....

A lot of folks in this neck of the woods use JP pistons, for all sorts of applications.
While they all mostly are heavier, and you need to inspect them carefully as kommando sez,
unless you regularly use all the performance, it doesn't seem to make much difference to most folks. ?

(Pottering around sedately is what most classics are used for these days, but if you intend to use it as a motorway burner then your mileage may vary...)
hopethishelps

Re: JP Twin Pistons too heavy [Re: Rohan] #767522 03/08/19 9:49 am
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Matchless Offline OP
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I'm afraid I don't do pottering. All my bikes get a damn good thrashing.
On the subject of dynamic balancing, the flywheels on AMC cranks are seperated by the centre main bearing & each flywheel has a different set of balance drillings, as in different position, depth, & number. If the crank had been statically balanced this would not be the case.
As for the pistons, I have started to machine the insides to shed some weight. The crown thickness of the original pistons is less than 7mm whereas the JPs are a ridiculous 11.5mm. I imagine they use a common casting for lots of different pistons. I have also taper bored the insides of the gudgeon pins & shed 8g in the process. They looked more suited to a diesel engine as supplied.

Re: JP Twin Pistons too heavy [Re: Matchless] #767523 03/08/19 9:58 am
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kommando Online Content
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Quote
I imagine they use a common casting for lots of different pistons.


Correct, they hold in stock the master castings which they then use to cover a wider range than normal with small batch production, as a concept of last resort its good but risks most in the range being heavier than original but if there are no other choices other than making you own it is the best option.

Look at the pin again as well, there may be a lighter alternative with a tapered bore that saves more weight than your DIY machined ones. It all adds up.

Re: JP Twin Pistons too heavy [Re: Matchless] #767526 03/08/19 12:10 pm
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I'm no expert on anything...but... On eBay right now are several sets of oversize 650 Matchless pistons claimed to be new old factory stock...
Even a careful balance job can not mask excessively pistons as Kommando said..Depending on the actual aluminum used for the pistons, you may be able to shave off some weight but not 72 grams, I had a 650 Triumph with .080 pistons that ran well but vibrated more than usual at normal road speeds....Found another cylinder and used .040 pistons that were about 20 grams lighter..The engine vibrated less...I believe you will not be happy with the heavy pistons..


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: JP Twin Pistons too heavy [Re: Matchless] #767593 03/08/19 10:09 pm
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Originally Posted by Matchless

On the subject of dynamic balancing, the flywheels on AMC cranks are seperated by the centre main bearing & each flywheel has a different set of balance drillings, as in different position, depth, & number. If the crank had been statically balanced this would not be the case.



That might be worth investigating, historically speaking - but could a previous owner have done that ?
Enfields claimed that they alone were doing dynamic balancing of their twin cranks, and it was considered sorta new technology at the time, for brit twins anyway.

AMC in later years reportedly had a less than perfect record with Norton engines, it turned out the chap doing it was just drilling the same pattern of holes on everything,
without actually individually testing the new cranks. Or so the story goes.....

Piston weights are not some holy grail, I've got some engines with IRON pistons in them (although not parallel twins), although 3 extra ounces per piston does sound like
a bit, it will just shift your balance factor to a few % lower.
A few new holes in your crank would counteract the new piston weights, if you want to keep the same % factor - if there is room for them ??
Or some mallory weights.

Re: JP Twin Pistons too heavy [Re: Matchless] #767637 03/09/19 11:43 am
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Sadly the pistons on Fleabay are for the earlier engines. The combustion chamber & piston crown from 1960 on are very different. The pistons are also rather odd as they are much longer from crown to pin centre than anything I have seen in a motorcycle engine. This makes it difficult to adapt other pistons without shortening the barrel. As this engine is as it left the factory (one owner) I am loathe to do that. It was very nice stripping the bottom end to find it had never been apart. This is also why I think the crank has been dynamically balanced. I am also building another of these engines, the very rare 750 version & the crank appears to have been balanced in the same way. Thankfully I already have pistons for this one.

Re: JP Twin Pistons too heavy [Re: Matchless] #767661 03/09/19 4:14 pm
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If you still have the original piston and it is good shape, Maybe a service like this near you could help.
Link
They state they can add .020" to a cylinder wall.

Re: JP Twin Pistons too heavy [Re: Matchless] #767722 03/10/19 12:02 pm
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That's a good idea if rather expensive. The original pistons would need the ring grooves re machining for some new thicker rings as they are worn, but the diameter of the skirts is still within tolerance. I will look for a company in the UK doing this type of coating.

Re: JP Twin Pistons too heavy [Re: Matchless] #767877 03/12/19 12:20 am
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the coating does not work!Expensive mistake for me.Your best way is to sleeve back to original & use the standard pistons again with new rings

Re: JP Twin Pistons too heavy [Re: Matchless] #767901 03/12/19 1:50 pm
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Originally Posted by Matchless
... The original pistons would need the ring grooves re machining for some new thicker rings as they are worn, but the diameter of the skirts is still within tolerance.

The original pistons in my 31 CSR had been modified for metric rings (2mm and 4mm thick) and I found that the same era BMW 50/S is also 72mm bore, so I use them.

Rob C

Re: JP Twin Pistons too heavy [Re: Matchless] #767970 03/13/19 2:25 am
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Some Matchless folks have ventured that AMC didn't have any dynamic crank balancing equipment, and they certainly weren't advertised as such,
so its likely that your crank holes were simply from a static balance.

It could be interesting to experiment with some bobweights, and see what the % balance factor applied was ?

Re: JP Twin Pistons too heavy [Re: Rohan] #768166 03/15/19 9:44 am
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Rob, thanks for the BMW ring info. Matchlessman, I spoke to Langcourt the other day & they think that my barrels will be okay for their version of Nikasil but will confirm upon inspection of the barrels. Which company did you use & what was the problem?
Rohan, the 650 cranks were balanced to 65% but the weights used would have been the theoretical weight of all the rotating parts & also the same for 65% of the reciprocating parts, not the actual weight.

Last edited by Matchless; 03/15/19 10:33 pm.

Moderated by  Matchman62 

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