Douglasmotorcycles.net
General => General Douglas Discussion => Topic started by: Ian on 29 Aug 2010 at 06:06
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Folks - a strange issue - I am sure there is something obvious I have missed !!
A friend is reassembling his 2.75hp motor with new pistons. When he put it together the pistons are offset too much by about 1/4" - rear to the flywheel side and front to the timing side. Is there any trick here ? He has the long section of the little ends facing inwards with the machined section of the piston outwards - and the oil holes on the little ends are both facing up which sounds right. Any other options move them further out of alignment.
The pistons are JP pistons.
I have measured up a couple of old cast pistons and the rods appear to have heaps of sideways play - I don't think they have as much in the JP ones. Is this fixed by machining either the pistons or the rods to give more sideways play ? If so which would be best ? Am uncomfortable with machining too much off the alloy pistons ? The old cast ones I have only measure .3" from the outside of the piston to the inside of the short boss !
Any suggestions or help would be appreciated
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Hi Ian
Here are two photos of the crank with standard piston on but the JP pistons have the same set up.
Regards
Kevin
(https://www.douglasmotorcycles.net/aa-files/images/kmd/2010/2-75-crank-pistons-1-750.jpg)
(https://www.douglasmotorcycles.net/aa-files/images/kmd/2010/2-75-crank-pistons-2-750.jpg)
Larger view (https://www.douglasmotorcycles.net/aa-files/images/kmd/2010/2-75-crank-pistons-2-crop.jpg)
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Hi Ian
Further to Kevin's answer it should be noted that the JP pistons supplied by The LDMCC have a much thicker boss than that of the original cast iron pistons which limits the side play necessary to get the pistons to enter the bores. This was done quite deliberately as it was considered at the time of development of these alloy replacements that a thin boss the same as used originally would be too weak. However. to enable them to be used without interference it is necessary to remove a small amount of metal from the short side of the little end. Due to variation in tolerances between individual engines it is difficult to be precise about the amount to be removed but it should not exceed 1.5mm. In many case 1mm is sufficient. The little end is fairly soft and can be cut with a junior hacksaw. Ensure that no swarf is allowed to enter the engine and the saw cut is kept square and deburred properly. If you are unhappy about this procedure, my personal view is that the piston would not be unduly weakened by removing 1.5mm from the inside of the relevant boss: in fact I know of one member who upon receiving a pair of alloy JP pistons machined the boss internally down to the same thickness as the original cast iron pistons and in spite of dire warnings found no problems. At least he never came back to report any. Chris.
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Kevin/Chris - thanks for the responses. Kevins (yes another one !) crank is set up exactly as per the photos so I think the issue is as described by Chris about the thicker boss not allowing freeplay. Sounds as though both options have been used. I may suggest to him that he removes a small amount off the little end and if more needed a small amount off the piston - trying to avoid weakening each too much. I will have the same issue on my sons bike which we are doing at the moment as well so its good to know.