Author Topic: 68-mm bore OHV head and cylinder - RA, Sports, or something else?  (Read 7566 times)

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Offline cardan

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Hi all,
Can someone positively identify these parts please? Given the threaded inlet and exhaust stubs on the head, we can narrow it down to S1, RA, OB or OC.
So far as I know, the cast frame to support the rockers has curved sides on OB (and OC?) heads, whereas this head has a perfectly rectangular shape when viewed from the top. Also OB (and OC?) heads have three little fins between the sparkplug hole and the bottom of the head, whereas this head is smooth in that region.
The cylinder is quite different from an OB cylinder, which has fins along the entire length (down to the flange), curved at the top. DT cylinders also have full-length fins, but angular (not curved). The fins on this cylinder are angular, but do not run all the way to the flange.
So I reckon Sports (S1) or RA. Any ideas?
Thanks
Leon









« Last Edit: 23 Apr 2012 at 20:17 by Dave »

Offline Doug

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Leon,

The one view of the head required to make a positive i.d. (well, near enough) is missing! Looking at the rocker perches from the side, do the buttresses angle up in a straight line, or do they have a concave curve? A concave curve would indicate a S1, a straight buttress would indicate an RA. I reckon it is one or the other.

Doug

Offline Dirt Track

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G'day Leon, Doug and all
I always thought the S1 heads looked like the one attached.
Howard.

« Last Edit: 23 Apr 2012 at 20:21 by Dave »

Offline Doug

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Howard,


Not all the S1s have the cross bracing on the perch. At first I thought the cross bracing was only used on the OHV genset, but I have since seen a factory photo of a early S1 solo with them too. Most S1s seem not to have the extra brace.

-Doug

Offline cardan

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Thanks Doug and Howard. Checking my photos, I don't have one from the side! The reason is that amongst the cylinder heads I was looking at there were three distinct types, and these were most easily distinguished from the top and around the spark plug hole, pretty much as I described in the original post. I did make a mistake, however. Only one type has the three little fins below the park plug hole, another has a smooth boss, and the "RA/S1" style is smooth. The accompanying photos shows the three types (left to right).

The question is, what is their age order?

It seems that Howard's photo with the cross bracing, in light of Doug's comments, shows the earliest type (early S1). The subject of the post (in the right on the photo) is the next type - either later S1 or RA. Note that both of these have two fins on the port side (both inlet and exhaust) of the hold-down holes.

Presumably the other two styles are OB and OC. Which is which? Note that the one the left (with the 3 little fins) has two fins on the ports (like the RA/S1), but the one in the centre (with the very pronounced curves on the rocker frame when viewed from the top) has only one fin on the ports.

Leon



« Last Edit: 04 May 2012 at 20:39 by Dave »

Offline Ian

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The one in the middle looks the same as my OC

Offline cardan

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Thanks Ian. OB owners - do your cylinder heads have 3 little fins under the spark plug hole?

Leon
« Last Edit: 26 Apr 2012 at 00:09 by cardan »

Offline Chris

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My OB heads have the three little fins below the plug hole like the left hand head of the three in the photo above. I also have another cylinder head on loan that is apparently identical to the OB heads except that it does not have the three little fins. It is just flat at the same height as the top of the fins on the OB heads like the head shown in the central photo of the three above. I had assumed that this was an OC cylinder head. I am sure that in functioning terms it would be interchangeable with an OB head and if appearance was a worry, with a little bit of work with a suitably shaped milling cutter and a file, the three fins could be created. Chris.

Offline cardan

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Thanks Chris. On your two different heads, do the shapes of the rocker supports when viewed from the top of the cylinder match my photo?

Sounds like we have: OB head at left, OC head in centre, and S1/RA head at right.

At the moment, I'm helping a friend sort out some Douglas stuff. I suspect we are one RA head short. The one in my photo looks like a front head, so looking for a rear RA head - with OB or OC items to swap. Once I'm certain, I'll put an ad on the wanted board.

Cheers

Leon

Offline Chris

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Hi Cardan
  In answer to your question above, no they don't. I had not noticed or taken in the different shapes of the rocker supports. All three heads that I have, two OB front and rear and the odd one on loan which is a rear have parallel cast rocker supports like your photos central and right. Close examination has also revealed that I was mistaken in my earlier posting in that the fins over the top of the OB heads maintain their height right up to the plug hole and then continue with the small fins the other side of the plug and leaving the plug hole with a counterbore before the thread starts. The other odd head has the fins tapering down towards the plug hole, which has no counterbore,and then no fins beyond so although the milling operation I mentioned could be used to create small fins this head could not be made into a true replica of the OB heads. The net result is that with the curved inside shape of the rocker supports it does not look as though your left hand photo is of an OB head. Chris.

Offline cardan

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Hi Chris,
Interesting! It sounds like there are a number of subtly different heads within the main "groups". Maybe they changed each year.
I've seen the counterbore before the start of the plug thread amongst the heads I've looked through, so that's obviously another variation.
Perhaps it's enough to say that OB heads have the "rectangular plan" rocker supports (view from above the "top" as if looking down the bore), and OC heads have the curved style.
Leon

Offline Doug

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The rocker perches with the 'joggle' (center example) were introduced in 1924 on the OB/OW models. According to advertising (caveat-emptor), they were still being used on the OC. It might have been intended, or expedient, to use the S1 cylinder head on early models, as such is illustrated in the OB/OW24 handbook; with the cross bracing no less! It must be remembered the OB/OW was originally going to use a gearbox layout like the RA, but that apparently never got any further than early promotional illustrations.

Regardless, factory drawings were made for the 'new' heads. But, even that does not make all clear. The drawing for the OB shows (or 'shews', in keeping with the era) a 1.00" inlet port and a 1.25" exhaust port. The latter can be misleading as there is a taper opens it further to 1.375" at the gland face. The OW drawing was done a full year earlier in March of 1923. It shews a inlet port of 1.125" and an exhaust of 1.313" (no taper). This is obviously a bit large for a 350cc, and someone has penciled in a note "OB24 Inlet". Presumably a drafting mistake. The other thing to note is the OB drawing is at revision four, and the OW at revision one (with any other reversions having been lost.) So other than adjustments for capacity, they both likely used the same casting and evolved hand-in-hand. Other changes noted over the course of a year of revisions are changing from a flat head joint groove to a diamond groove. Also more mass was included around the spark plug boss, essentially filling in between the spark plug and the cylinder joint surface. The perches got slightly thicker in section, and the seating for the exhaust spring was drilled liberally to minimize surface contact area. The fins between the perch became larger, and project well beyond the spark plug face surface on the later drawing.

Douglas has some freedom in altering the shape of the fins between the perch and perch itself. These were cast separately, and then build into the sand mold of the cylinder head and fused when the main casting poured. So changes did not entail the expense of a entire new cylinder head pattern. It could also have enabled them to use the same patterns for the front and rear castings, by allowing the perch, fins, and spark plug boss to be 'flipped'.

One of the things that I have been curious about is what changes, if any, existed between the OB and OC. The OB/OW had full hemispherical heads, and an included valve angle of 90 degrees. By the time of the DT, the valve angle had tightened up to 82 degrees. I have not had the opportunity yet to examine a set of 'definitive' OC heads to see if they have the shallower combustion chamber and 82 degree valves like the DT, or if that change had to wait for the Dirt Track model. It is probable it all happened with the DT, since the casting was completely redesigned in all other respects. An easy way to check externally would be the center distances for the rocker spindles. The OB/OW is 4.125", and on the DT they had to move inward with the valve angle to 3.625" apart on center. 

Doug

Offline cardan

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I've always wanted to have a conversation about bikes in old English! Shews what a class forum this is...

Thanks Doug. Yep "joggle" is a good term to describe the kink in the part of the perch which is vertical when the motor is assembled. So the joggle was an early OB feature. Where does this leave the head on the left in my photo? It has similarities with the earlier (?) head on the right (no joggle, two fins between the hold-down holes and ports). Presumably it is early - 23, 24...

Could it be a 1924-ish RA? One problem is that when mated with the cylinder shown earlier in the post it doesn't look right: the three little fins under the plug hole are slightly closer together than the fins on the head on the right, so they don't line up well with the fins on the cylinder.

I am trying to help a friend put together and RA motor. The cylinders (103mm long, plus a 10mm spigot - shorter of course than the OB/OC item beacsue of the shorter stroke) came with RA pistons, and are thought to be RA items. There is (I think) only one of the head on the right, but several of the "three little fins/no joggle" type on the left. They just don't look right on the cylinders.

Is there a drawing for the RA/S1 cylinders?

Cheers

Leon

Offline Doug

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Leon,

No drawings for an RA cylinder or head survived, that I have ever heard of. And none at all for S1/S2.

The cylinder head on the left could be 1926-28 TT/I.o.M. model, that replaced the RA. I have seen these fitted with OB/OC type heads, head with a straight perch with a reinforcing lip to the outside, and perches with ribs like a DT; all with gland nut type ports!

-Doug
« Last Edit: 01 May 2012 at 03:58 by Doug »

Offline cardan

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Aghh... thanks Doug. Sounds plausible.

I think I'll define the head at the top of the post "RA" (subject to a straight, rather than concave, buttress). It certainly looks the best match to the cylinder, so at worst it's "very close to RA". I think we're looking for a partner for it if anyone has one and likes the look of the other heads discussed above.

Thanks again,

Leon