Dunstall Barrels

After recent 'mixed reactions :roll: ' regarding the Dunstall 810 barrels, do the same issues apply to the earlier 750 barrels, and what are they?

I'm about to put a set on my 750 PR with a 'stage 1' head and 4S cam. I'm not planning to race this motor, so it will only be getting a mild spanking from time to time

B+Bogus said:

After recent 'mixed reactions :roll: ' regarding the Dunstall 810 barrels, do the same issues apply to the earlier 750 barrels, and what are they?

I'm about to put a set on my 750 PR with a 'stage 1' head and 4S cam. I'm not planning to race this motor, so it will only be getting a mild spanking from time to time

Click to expand...

While the 750 barrels have their limits (as do most things mechanical) they are nothing like the Dunstall barrels. Dunstall was pushing the envelope ...again with their design which originated with his racing endeavours. The cylinder liners were minimal with no option for reboring so they could still drop into the 750 cases. When Norton designed the 828 cc engine they beefed up the cases and used a wider opening in the cases to accommodate the wider cylinder liners that were also thicker and reborable. The 810 gave Dunstall a considerable weight reduction, improved lifters and the larger displacement. This likely suited him on the track where longevity was not nearly as important as track times. The resulting kits were designed as a bolt on to the 750 cases. Once others you raced against also had 810 kits everyone looked for other ways to push the envelope even further and I'm sure this contributed more to the failure rate of the kits.

We have one member here who has logged an incredible amount of miles (for any bike) on a Dunstall 810 engine in street and hi-way use.

NOTED FAILURES:

Bottom of liner cracking and breaking off. - FIX - #1 Use a rounded cut away for the rods instead of square. May have reduced the occurrence but not eliminate it. #2 Use of 750 cc liners which were thicker and more durable.

Base Flange Cracking and Breaking off. - (no real fix other than detune)

Cracking at top 3 fins and top of cylinder separating and staying with the head. - (no real fix other than detune)

Threads wearing or pulling out of the top of the cylinder. - FIX - Install heavier threaded inserts

Grade of aluminum, thinness of liners, stresses from high compression and vibration, and expansion rate between steel and aluminum differences are all factors that can result in failure with hard use.

Failures of the stock 750 cylinders I've seen mentioned on the board here were the base flange splitting off usually in conjunction to 11:1 pistons and the like. Perhaps others can elaborate.

I ran a set of Dunstall 750 cylinders (same as the 810 except for thicker liners) in a race bike for a while, and my only real problem was that the alloy was so soft I had to check the fasteners every race, and the head bolts pulled out enough to cause head gasket leaks. I quit using it, but at least it never broke. I built an 810 engine for a friend, who raced it for a number of AHRMA races with no problems, until the liner broke, dropped into the case, and trashed the engine. As a result, I'm among those who don't think much of the Dunstall kits.

Ken