Douglas 2 3/4hp (and EW) - Stainless Steel Valve Caps + Washers (Various Types)

Product no.: 0897 DG234_010s

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This item listing is for Douglas 2 3/4hp Valve Caps, CNC manufactured from stainless steel - in 3 styles - the non spark plug/hollow type, 18mm spark plug type and 14mm spark plug.  This is a multiple item listing and it also offers Copper/Abestos type crushable valve cap washers to fit these Douglas valve caps.

These valve caps have been CNC machined by us from stainless steel billets, using original 100 year old (and very rusty!) Douglas 2 3/4hp valve caps as the pattern.  If we say so ourselves - they look very pretty and will definitely transform the look of your early Douglas twin, particularly if you are currently using corroded or rusty original items.

We took some time making jigs and writing programs to get these items correct - and as per the originals, the hex is milled to the original 'Imperial' size hex spanner size.  Being CNC manufactured, the machined finish is very good - but they are then lightly polished on a rotary polishing mop with stainless soap, which helps give them the look of original nickel-plated items.

We can also supply the original style copper/asbestos type crushable washers (although, in these modern times - the heat resistant material inside is no longer asbestos).  These rings can be supplied individually, or in packs of 6 - as they should be replaced each time the cap is removed.

There are 3 types of Douglas 2 3/4hp valve caps available as well as the crushable washers, as follows:

- 0897_1: 'Plain/Hollow' type Valve Cap - these are fitted to the lower valve of each cylinder, two required per engine.  The accompanying photographs show two original Douglas valve caps alongside ours, for comparison

- 0897_2: 18mm Long Reach Spark Plug Type:  I am not sure if 18mm plugs and 14mm plugs were fitted to Douglas and 2 3/4hp models concurrently (as this model was manufactured for a long period from 1910 to the mid 1920's but I have original valve cap examples of both sizes - but 18mm plugs are often associated with Veteran motorcycles, and I have decided to fit this size of spark plug valve cap to my own 1919 TT Rep, which was used for the photographs in this listing. 

Again, an original valve cap was used as a template. 

Note: if you are going to fit 18mm plugs, you will need to find the original type 'Long Reach’ 0.750” length thread plugs – which are now quite rare (0.5” is the more common length).  We can also supply modern reproduced KLG ML50 18mm plugs which have this 0750” length – see Item listing 0896.

As a final point on the length of spark plug threads – If you look at the original Douglas 2 3/4hp workshop manual, there is a cutaway view of the entire engine – which includes a cutaway of a valve cap.  This does show that the original spark plug was slightly recessed into the valve cap – but I am sure that fitting a more readily available 0.5” length 18mm plug would make the spark too recessed, and possibly affect valve timing and combustion.

These ‘spark plug’ type caps were fitted on the top of each barrel, two required per engine

- 0897_3: 14mm Long Reach Spark Plug Type:  Almost identical to the 18mm type, accept this valve cap will accept readily available 14mm 0.750” thread length spark plugs.  Again, an original valve cap was used as a template.  Using this type of valve cap will allow you to use most type of modern 14mm long reach plugs . . . as well as many readily available ‘New Old Stock’ vintage spark plugs that come up on ebay.  We can also offer a variety of spark plug accessories (Item 0895) that will make your modern spark plug look more veteran!

- 0897_4: Copper/Asbestos style Crushable Valve Cap Washer: Good quality ‘crushable’ copper/asbestos type washer for sealing valve cap.  Modern filling, but of type fitted originally and looks good when fitted with this valve cap.  This item is for single washer

- 0895_5: Valve Cap Crushable Washer – pack of six:  As above, but more economical as pack of 6.

- 0895_6: EW Valve Cap Query:  I am not an expert on the Douglas models that followed the (first) 2 ¾ hp 350cc models, but am aware the 350cc sidevalve EW model that immediately followed this model in the mid 1920’s had a valve cap that visually looks similar to the type we have made.  When I tried to fit an original 2 3/4hp cap to a EW cylinder head I have, I found that actually the valve cap holes on the EW head are fractionally closer together than the 2 3/4hp models!!

I have no idea why Douglas did this, but it does mean that our valve caps (as manufactured) do not fit in the EW head without a slight skim to the outer diameter. 

If you wish to upgrade your EW model to these stainless valve caps, then email us on and I will consider if we can do this.   If enough interest from EW owners, I will also consider a new batch in the future with the fractionally reduced outer diameter

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