Trials Australia Forums

David the open class Bultaco MX bikes all had twin plug heads. Even the El Bandito. They were all 60mm stroke until the MK 9 which had a 64mm stroke to make it a 370. They got 360cc by using an 85mm bore with a 60mm stroke. I know of a MK 10 370 that I am working on at the moment which has what looks like a standard up pipe on it and has a Gem reed valve so I think it is a US one just before the MK 11`s came out. It has a strange left side cover on it which is completely different to the MK 11 ones.

Bully fanatic wrote:David the open class Bultaco MX bikes all had twin plug heads. Even the El Bandito. They were all 60mm stroke until the MK 9 which had a 64mm stroke to make it a 370. They got 360cc by using an 85mm bore with a 60mm stroke. I know of a MK 10 370 that I am working on at the moment which has what looks like a standard up pipe on it and has a Gem reed valve so I think it is a US one just before the MK 11`s came out. It has a strange left side cover on it which is completely different to the MK 11 ones. :Bultaco: :Bultaco: :Bultaco:

Excellent thanks Graham. So I figure that means that the diameter of the sealing surface edge on Greg's head may reveal if it is a MK6 (325) or is a MK7,8,9,10 (360/370) Pursang/Frontera head.
I would love to see a photo of the odd left side cover on that MK10 you are working on

Hi Everyone,

After going through some more parts, I found another head with 3 holes. Here are some comparison photos:


Both heads appear the same except for the number 105 stamped in the underside of the second beadblasted head, maybe it means something? The centre hole looks to me like it was machined at the same time as the others with a similar recess even though they have different depths. They fit neatly on to a 325 cylinder with 83.4 mm bore.

So this head on the left must be a 325 Sherpa T one, is that the right way to think about it?



Also while I am going, I have found this cylinder rebored with new piston and 90Y stamped in the top:


It is 250 size and what would that be for? Would it be good on say a M198 that I have that needs a top end rebuild?

Greg a Model 90 is the first ever Astro 250. These were made for the American 1/2 mile flat track racing. A M105 is the MK 6 250 Astro. A M87 is a MK5 350 Pursang. David I`ll include a photo of that MK 10 370.

Greg Harding wrote:So this head on the left must be a 325 Sherpa T one, is that the right way to think about it?

IMG_20220913_120107.jpg

IMG_20220913_120100.jpg

It looks like model 99 350 Alpina or models 92 and 125 325 Sherpa T.
In the models that followed these, the 350 Alpina and 325 Sherpa T gained a head steady. Your head predates that change because it does not have the fins cut away behind the rear head stud holes.

Bully fanatic wrote:David I`ll include a photo of that MK 10 370.

Thanks Graham. What a fabulous looking bike. I love how Bultaco made improvements during production runs.

DT250 head with the decompressor turned up

Kinda looks original to me.

Decompressor activates quite freely

Hi Everyone,

tat ty wrote:DT250 head with the decompressor turned up....

Alastair, many years ago I was given a DT 250 C and it had a head like a TY 250 and was twinshock. The next model (D) came out in 1977 and was fairly controversial as it did not have shocks on the back, I can remember the conversation like it was yesterday: They look stupid without shocks! Anyway I think your head is off a DT 250 D or onwards and I am not sure when they changed again but they came in Blue, Red, Grey and I think Yellow and Green? Here is a Blue one as I remember them with twin plugs, if you look closely you can see a spark plug without a cap:

Bully fanatic wrote: I know of a MK 10 370 that I am working on at the moment which has what looks like a standard up pipe on it and has a Gem reed valve so I think it is a US one just before the MK 11`s came out. It has a strange left side cover on it which is completely different to the MK 11 ones.

Graham, what is strange about the left side cover? Here is a photo of some of mine:

Bully fanatic wrote: A M105 is the MK 6 250 Astro....

Most of the 250 heads here have a flat bottom and not recessed. Here is a comparison photo of M 49 (250) and 3 plug hole (325) so it must not be off a 250? Could the 105 mean something else?

It’s the compression ratio 10.5 it’s just how the person has stamped it.

Thanks David and Greg.

I guess conditional on the head fitting, the other issue is compression ratio. My guess is that if I measure the volume of the existing head ( let's say fill it with fine sand) and compare this with the DT250 decompression head, I'll discover any concerning differences.

Alastair.