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dommie 99 pistons

Submitted by steve_coleman on Wed, 22/02/2023
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Heavy Twins

I have just had a rebore on my 99 barrels and about to fit a new pair of old stock + 020' Hep 13325 Pistons. these have identical valve cut outs and marked LH and RH but no indication of which way round to fit as a 180 deg turn puts the cut outs in the same position.  Am I correct in assuming they can go either way?

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Submitted by david_cooper on Wed, 22/02/2023

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If the cutouts are the same, you've answered the question. There is no front and back. Only a left and right.

If there's writing on the top, I place it so it's the right way up when you look down on it when sitting astride the bike the right way round.  Then if you ever come to remove and replace them, they'll go back the same way.

 

Submitted by peter_shand on Wed, 22/02/2023

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Are those slotted skirt pistons?  if so, there is a danger that the top of the piston can come off.

Later pistons did away with the slot and had holes behind the oil control ring to stop them parting company.  

Thanks Peter, Yes they are Slotted skirt, as used as standard from 56 to 59 and on 88s beyond that, Ive used them a lot and in my experience there is very little danger of the top breaking off, unless continual high revs are used and I don't do that. 

Submitted by robert_tuck on Wed, 22/02/2023

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As you say , the slots dont have to be an issue with restrained use,  the SD rings they come with can be troubled if a six start pump gear set  is used. I also prefer to use NOS parts .

Submitted by steve_coleman on Wed, 22/02/2023

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Thanks Robert, the only case of breakage Ive heard of was from Dale Middlehurst who told me he had a breakage but only after prolonged high speed motorway use. I like them because they have a long skirt and seem to run quieter than most pistons, but maybe that's just my imagination. If you look inside them the crowns are supported by very large webbs. but getting back to my original question, Its alway sightly bothered me that there are no front or back markings only left and right handed I guess looking for conformation that they can be used either way around.

Submitted by gordon_johnston on Thu, 23/02/2023

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I have had a few of these fail. But then again I wasn't treating the bike gently.

Submitted by alan_osborn on Thu, 23/02/2023

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If you turn the pistons 90deg  then how can they fit the con-rods? Perhaps you should have said 180degs?

If you fit them as marked LH and RH (sitting on the bike) then all is well.

If you think of the valves in the head, the EX is out side the centre line while the INLET is on the centre line.

Thanks Alan. Yes I should have said 180 degs , I now understand that they can be fitted either way around from new and if they are fitted as you say LH and RH correctly, the valve cutouts will be correct either way round. Thanks everyone for your comments

Submitted by david_cooper on Thu, 23/02/2023

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Regarding failures.. I thought this only happened with Commandos?  Did it affect the 500/600/650 also?

Submitted by alan_osborn on Fri, 24/02/2023

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I understood the slotted pistons only failed on Commandos, AS the slotted pistons were only fitted to Commandos. That was in the 70s when I rode a Commando! I never heard of slotted being fitted to other engines.

As regards pistons being marked LH and RH if you follow that then any subsequent removal/ rebuilds should come out correct. Making it up as you go along needs a very good memory to remember on the next rebuild. They will not be correct 'either-way round' follow the valves, that is what the cut outs are for!

I will tell you another story later re pistons. Watch this space.

 



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