The 135 Mk8 250 engine as already stated has the same stud pattern (transfer port size) and barrel, head crank etc as the Mk 7 engine and so qualifies as a roll on model. The last Bultaco engine legally eligible for pre 75 (360) must have 121 engine numbers. The centre cases looked slightly different as they were rounded in the area above the countershaft sprocket.It had the Left or right side gear lever option.It also used the newer shape clutch cover.ĭifferent weight flywheels and ignitions were used both engines, these can be swapped as everybody does to change engine characteristics,no big deal. The only difference in the gearbox of the MK8 was the use of a needle roller bearing instead of a brass bush on the output shaft.The ratios and clutch were all the same. The Mk 7 121 360 engine (narrow stud pattern) has an actual capacity of 352cc.
As Firko suggests it would be too easy to just bolt on a 1978 or later cylinder and head,not exactly fair in the pre 75 class. The 136 engine uses the wider (later) stud pattern which is the same size as all the later engines,it also gave a fair power increase. The main difference between the Mk7 121 and the Mk8 136 and why it was not allowed was the size of the transfer ports in the engine cases. It was a matter of having to draw a deep line in the sand somewhere and the "must be the factory designated 1974 model" was included to conteract the "75 model manufactured prior to December 31 1974" anomoly. These hard decisions were done to keep the sport on as level a playing field as possible and although many of the decisions weren't popular at the time, most racers went along with them and within a few short years we had a good clean sport with very few eligibility hassles based on model designation. I seem to remember Jim Pomeroy telling us to use as late a model cylinder as we are brave enough try and get past scrutineering!
Apparently (and I'm open for correction here) the Mk7 and Mk8 barrels differ in appearance but the Mk 9/10/11 cylinders are externally identical to that of the Mk8 but feature gradually improved port design. I know I seem a little hypocritical allowing the 250 but not the 360 but I do remember the heated debate of the time and the Bultaco experts mapping out the many differences between the two models and suggesting that we'd be opening a can of worms by letting the 360 through as it would open it up for owners to fit Mk9/10/11 cylinders without any external signs that it had been done. For that reason I don't think the Mk 8 360 should be allowed. A lot of years have passed so I'm a tad rusty on the details but I remember that the porting arrangement is much improved on the Mk8 and I believe the crankshaft flywheel weight is quite different. The 360 however is a different situation as it features many different engine internals when compared to the Mk7. I agree with this on the 250 version of the bike as they are mechanically identical except for some minor details and the previously mentioned left/right gear change position. Today, with the exception of Peter Lawson in WA and one or two other anal retentive officials, most people turn a blind eye to the Mk8 250 as long as the Mk7 swingarm is used or the Mk8 version is retro modified back to Mk 7 specs. That's allowed as long as the correct number cases are still used. Many Bultaco riders converted their Mk 7 right foot change bikes over to left foot change using Mk8 parts. At a national level, Mk8s were not permitted at all. Depending on the importance of the race meeting, many organisers turned a blind eye to MK8 Bultacos fitted with MK7 swingarms basing their decision on the assumption that the only difference between the models is the back suspension and the placement of the gear lever. However, after much consultation with Bultaco experts all over the world we acepted the fact that the Mk8 is actually the factory designated 1975 model, no matter when it was purchased. The main bone of contention was that the Mk8 was released in 1974, as many an owner and even Kevin Fraser, the big NSW dealer of the time argued. This particular model caused more drama in the early days of VMX than any other bike. In a nutshell, the Mk8 Bultaco is ilegal for pre '75.
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