Tappet clearance MK/90+

[tck]

Stamped on the side of my Mk4 it says " .005" tappet clearance", which is correct according to the book, however I have plus heads and valves on my engine where the clearance is given as the 5 times larger at  .025" my guess its the alloy push rods that dictate those increases, however I had one set of steel and one set of alloy push rods and since alloy ones are not available I now have a full set of steel ones, and the camshafts are standard. Accordingly I think .005" is perhaps the correct measurement to use, but some conformation would be welcome.

[Dads bike]

Afternoon
Yes I would go for that, as a very loose rule of thumb, carbon steel will grow by approx 1 thou per 1 inch of length/ dia when up to temperature, whereas ally will grow many times more depending on its make up.

Steve

[eddie]

Whilst the change of material for the pushrods will affect the required tappet clearance, the alloy pushrods will only need a couple of thou extra. The large increase in clearance on the 'Plus' machines was down to the more radical cams used for racing. Remember when dealing with a 'Plus', there were 3 different cam profiles available - the first 2 were for racing, but the number 3 cam was more of a 'hot' road cam. Standard cams for the Mark series were stamped 'T' (for touring) or 'S' on Mark 3 & 4 Sports models (both these cams ran with .005" tappet clearance). The 'S' cam was ground with a .030" smaller base circle - thus giving another .015" lift and correspondingly longer opening.
  Ironically, when the Dragonfly went into production, it had the same cams (T) and valve gear as the Mark machines - just the pushrod material was changed to aluminium, yet Douglas specified a tappet clearance of just .003" - I think this was done to try to silence the critics that had commented about a rattly engine - I have always run my Dragonfly with .006" clearance and find it runs much sweeter.

  Regards,
                 Eddie.

[tck]

Thanks Eddie
I will start at .006, Your help in this difficult rebuild (after 30 years of sitting in the back of my garage that filled this lock down) has been very much appreciated. As I  reach the end of the build and start the recommissioning I unpeeled the coverings on a set of pipes that have been in my possession for longer than the bike and the chrome is perfect! they knew how to wrap goods in the sixties!

[veikko1]

Hello

I have read this discussion, and like asking, how much tappet clearance would be 90+, with alloy heads and barrels and no 3 cams?

Regards Veikko

[eddie]

I have a copy of what appears to be a supplement to the Mark series handbook titled "The Douglas Plus Series - Provisional". It quotes the tappet clearance (cold) as .005" at the valve for the 80 Plus, and .012" at the valve for the 90 Plus. Unfortunately it doesn't mention which cams these figures relate to, or whether these clearances are for the alloy or cast iron heads. I would think that the previous figure of .025" would be OK for the no.1 or 2 cams, whilst the .005/.012" figure was better suited to the milder no. 3 cam. I doubt there was any difference in the clearance with the change to alloy heads.

    Regards,
                    Eddie.

[Jonathan Hewitt]

Hello all, The Plus series booklet I have from !951 for machines 1950 / 51  says to adjust tappets to .025".
 In  the  'Alternative and extra equipment ' section of the pamphlet they mention Alloy cylinders and barrels.
from this I deduce that the setting of 025 would be for cast iron barrels.  But as Eddie says there is no mention of camshaft  profiles.
                Jonathan
 

[tck]

On setting my tappets I find one side has castellated nuts in the outside position while the other side has the adjuster 'plain' nuts under the rocker also on that side when obtaining the adjustment I find the end of threaded stem is only approximately halfway into the rocker body I think this is not enough am I correct in that? I think I shall have to inspect the pushrod lengths (the 'short side' is the original push rods the other are new)