t35 heads and barrels

[flea]

how do you tell the difference between ea of the heads and barrels of the mk1/ 3 / 4 and mk 5 .. are the valves all the same or is the a change over .. thanks

[douglas1947]

MK1 is different to the later MK´s in cylinders and also heads. Also the valves are not interchangeable.

Michael

[flea]

but how do you tell the difference

[eddie]

The Mk 1 heads dont have a dip in the top of the rocker box to clear the spark plug. Also the cylinder head studs are only 5/16" dia - not 3/8" as on later Mk's. Mk 1 barrel fins are more circular.

   Regards,
                 Eddie.

[flea]

thanks eddie, thats a major help ...... so how did they get the major speed improvements on the later models

[Doug]

The change in spark plug location to a more central location did increase speed some, but mainly was to improve running from idle onward. I understand the Mk1 were a little rough on pool petrol, though I have no personal experience. Just have a Mk3.

-Doug

[flea]

thanks doug , have got hold of some t35 barrels and heads that i need to id the models of so all this will help .. s/plug more central ,3/8 studs on later , more rounded fins and studs on mk 15/16 , that exsplains was told of a modification to adapt mk1 heads to fit later barrels by cutting / grinding fins near s/plug so exhaust would fit ... mk 3 upwards are all the valves and pistons the same

[eddie]

The barrel fins only need trimming if you are trying to use generator barrels on a T35 Mk1 engine.   The valve gear and pistons remained the same on all the T35 models from the Mk3 to Mk5.

     Eddie.

[flea]

mmmm .. so what gives the performance improvement from mk1  to 80 plus /90 plus .. the carbs ??

[eddie]

As far as I am aware, there was not a significant difference in power output between the T35's and the later Mks - the redesign for the Mk3 was aimed at eliminating some rough running that had been reported by owners of the early machines. With the subsequent improvement in the fuel available, any power difference between a T35 and a Mk3 was difficult to spot. Most Douglas enthusiasts seem to agree that the Mk3 engines were the sweetest running of the postwar variety - the later mk4 and 5 machines seemed to suffer from poorer quality control (probably attributable to the tooling becoming tired). The power outputs I have seen quoted are:- T35, Mk3, Mk4, Mk5,  18 BHP.  Mk3 Sports, Mk4 Sports  21BHP (this horsepower increase being gained with just a higher lift cam).
    The production 'Plus' models were reputed to produce 25BHP (80 Plus) and 28BHP (90 Plus), but these figures were obtained with a further redesign of heads, cams and valve gear, and larger carbs. The last 'works' development engines were reputed to be giving 31BHP at something like 11,000 RPM!!
    Since the advent of Classic Racing, several Club members have tried their hand at making a postwar Doug competitive - the most successful being George Easton. His Mk3 machine (opened out to 500cc) being a match for a similar age Manx Norton! George later went on to do the same with a Dragonfly engine (which he found to be 'over ported' - so had the ports welded up and reshaped). During the development of his engines, George also found that the higher output Plus/Dragonfly oil pump over-oiled his engines, so he reverted to the standard Mark oil pump.

  Regards,
                 Eddie.

[flea]

thanks eddie , you have answered a lot of questions that have been rattling around in my brain since first getting my mk 1 and now with my mix / matched mk ? project bike and the possible dreams of turning the project motor into a 80/90 plus motor .. while your here tried looking but cant find the info , is there a chart or info on the bolt and stud sizes for the t35 , the project is compleatly devoid of all bolts and the same with motor gear box etc mounts ... it is truely a project made up of parts gathering from everywhere .. a bit here and a bit there .. including a gearbox in compleate pieces and a crank case that is empty and no crank (do have compleate  gearbox and crank housing as well that will fit straight in)