Cylinder bore question

[steveL]

Hi all,

I have a Douglas Mk V (in bits I would stress). Im looking to get the cylinders rebored and appropriate pistons. Does anyone know what the maximum amount is I can take the cylinders out to (I have seen a pair of +60 thou pistons on ebay) ?

Many thanks

Steve

[eddie]

Steve,
           My advice is that you only get your barrels bored the minimum amount to clean up. LDMCC post war spares can supply pistons of most oversizes up to +.060. The +.060's are stocked mainly for the Dragonfly (which has through studs to hold the heads). Mr Douglas only listed pistons up to +.030" for the Mk1 and 3 machines due to the light construction of the barrels - it is not unheard of for these barrels to crack just above the base flange when bored to the larger sizes. Mk4 and 5 machines had modified barrels that were thicker  - these can be distiguished by the base flange being much stouter - the earlier barrels had the casting relieved between the base studs.
        Even though your bike is a Mark 5, it is possible the barrels could have been changed for the earlier type sometime during it's life!
         Hope this helps,
                            Regards,
                                      Eddie.

[richson]

Hi Steve,

What is the bore at now? Is the compression low? Best to check before you do anything because +60 is the last throw of the dice. My comp which has Mk 5 engine is at +60 and it has quite a wear lip in the bore. I packed it out with an extra base gasket to keep a clearance but let it be.  It does smoke  some when cold, but I reduced that a lot by fitting new valves & guides - you can get those from club spares. I'm putting up with the smoke because there is no easy option after getting beyond +60. Some platings can add a few thou, or I think some have tried sleeves with mixed results.
My advice is to do everything else that may cure the problem you have first. My other Mk5 motor is standard and is being rebored right now under strict instructions to take it out to the absolute minimum. Pat tells me that +10, 20, 30, 40 & 60 are available from club spares with immediate shipping!

Cheers,

Neil

[steveL]

Thanks both. Very sound advice.

The current bores are +20 thou, and I can feel a very slight ridge towards the top (which was my original concern). I intend to replace the pistons, as my box of bits only included one  , so I thought that I would bore it out to the next size (+30) to get rid of the lip and use the next size pistons up.

When I saw the +60 thou pistons on e-bay, I thought they would be a lot cheaper than buying a new pair (I dont know what they went for in the end, they were at £75 with about 4 hours to go). I would guess £100 give or take. In the end both yours and Eddies comments made me think otherwise. As you say, once at 60 thou, its game over.

As always the advice is greatly appreciated.

Steve.