Trials Australia Forums

Hi Everyone,

Can someone please identify this for me?

I think that was the OHV Bultaco prototype?

I took an eBay punt with this recently. Hasn't arrived from the US yet. Seller described it as a TY250 head which it obviously is not.

My guess is an early DT250 head with a decompressor fitted into a second plug hole. The decompressor may be the answer when my legs say no.

We do have an XL250 manual valve lifter fitted to the TL250 and it makes for much easier starting.

I reckon David will know what it is.

Now I'm thinking an early to mid 70s MX250.

Hahaha It's a trick question eh.
3-plug head? What WILL they think of next? Those 3 things are decompressors? Seems more than a bit much. Naaah... My 450 Desmo R/T Ducati had a decompressor in the head, tiny thing like a second plug hole - had a little valve in there which was held against it's seat mainly by the compression, you don't need much of a hole for compression to sneak out do you?

I seem to remember Sammy Millers Bultaco having Twin Plugs, one as a Spare and a Decompressor in the Third.
My Looks the same Decompressors as fitted to my Old James and Greeves which had Villiers engines.

whitehillbilly

Looks like some one has added a central hole to your 360 Pursang head Greg

tat ty wrote:I took an eBay punt with this recently. Hasn't arrived from the US yet. Seller described it as a TY250 head which it obviously is not.

My guess is an early DT250 head with a decompressor fitted into a second plug hole. The decompressor may be the answer when my legs say no.

We do have an XL250 manual valve lifter fitted to the TL250 and it makes for much easier starting.

I reckon David will know what it is.

I reckon it's DT250 B(1975)/C/D/E/F/G with a hole added. I've seen a few of these fitted to TY250 twinshocks. They look very groovy.
DT400 heads of the same models look similar.

About that Pursang head Greg, I only have limited Pursang knowledge but here is some info in case it helps with whatever you are doing with the head.
The first big bore Pursang was 325cc (MK6).
The MK7 360 and later 360s has a longer stroke than the Mk6 325 but are the same bore as the MK 6, so the head will probably fit all 325 and 360 Pursangs and 360 Fronteras.
At some point the ignition was changed to twin plugs, firing at the same time and that head is your head.
I don't know if the Mk6 had twin plug ignition, but the Mk7 did have twin plug ignitions.
I don't know which models after the MK7 had twin plug ignitions.
At some point the open class Pursang became a 370cc. I think it was Mk10 but I'm not sure.
All Pursangs and Fronteras before MK11 had low pipes.
I suspect that they changed the head design to 8 holes and a soft gasket when they changed from 360cc to 370cc.
The 370 has the same stroke as the 360.
If I'm right about the timing of the change to the head sealing, this means that your head would fit MK6,7,8,9 325/360 Pursang and MK8,9 360 Frontera and would not fit MK10, 11,12 370 Pursang/Frontera.
It would also fit 325/350 Alpina engines of the models before the change to 8 holes and soft gasket but would need the fins trimmed.
One possibility for why your head has three plug holes is that the original twin plug ignition on a Pursang had died and the head was then modified to take a central sparkplug and run with with a single plug ignition.
Another possibility for the central plug hole was that someone wanted to run a higher compression head on a 350 Alpina engine (in a low pipe bike).
Another possibility is that someone wanted to fit decompressor while the twin sparkplug ignition was still functional.