Dunstall cylinder re-use

I have a Dunstall cylinder that has been stored away for many years and am interested in re-use possibilities.

I measure the cylinder ID at 75.9mm (Dunstall stated the bore/pistons as 76mm) and measure a STD 850 piston skirt OD at 76.9mm.
Standard Norton 850 bore size is 77.0mm.

Are there any options to use lightly machined down 850 pistons on the 810 cylinder barrels to give a lower CR than the original 10:1?

Or is it best to leave the 810 an interesting paperweight?
The Dunstall cylinder to head bolt threads are the really odd sizes!

I'm unable to offer any advice on bore size matching, but I do know that Norvil list a set of fasteners to suit these barrels

http://www.norvilmotorcycle.co.uk/065656SS.htm

Not cheap at £165 though!

I've just went through this on over bored 750's to learn what matters is getting the piston to wall clearance between .004 - .0055" depending on how hot you'd run it and special fuels that might add extra heat. The other factor is ring gap which manual says shoot for .01" std top ring and hot rod experience says 20% more on 2nd ring. TotalSeal rings can also be used but their street sets have normal gap 1st ring and overlap no gap 2nd ring. It is very common for special engine builders to order pistons that need a bit of narrowing to get the bore clearance acceptable. Heck Jim Schimdt even trims off skirks and drills holes in his already light pistons to get less mass yet. Alloy barrels need less cold piston clearance d/t bore expansion factor.
No replacement for displacement is ancient motto to seek more joy.

What is most dangerous on trimming pistons is removing too much crown to bring down CR or prevent head of valve clash. Always an issue to keep the dang things fastened down.

I have a set of good condition Dunstall 810 pistons I took out of a well running bike if that might help you out.

B+Bogus said:

I'm unable to offer any advice on bore size matching, but I do know that Norvil list a set of fasteners to suit these barrels

http://www.norvilmotorcycle.co.uk/065656SS.htm

Not cheap at £165 though!

Click to expand...

Well, I ordered a set of these for my 'latest' and sad to say, they were sent back in short order; simply not fit for purpose.
The three front base studs were OK (standard Commando), but that was about it :roll:

It was suggested to me that the too-long head bolts were too long because I had early Dunstall barrels.
Er... really?!



Anyone know of a source for the 'right' fasteners?
If no joy I'll run a small batch up myself. I hope they'll be a bit cheaper (and more useable) than the competition.

Do they still use cei threads in the alloy barrel?, if so you want to arshole them out and find a more suitable thread for aluminium any way

splatt said:

Do they still use cei threads in the alloy barrel?, if so you want to arshole them out and find a more suitable thread for aluminium any way

Click to expand...

I don't know if mine have been molested in the past - I suspect not; the threads are BSF, and are solid brass inserts into the aluminium (note we both know how to spell ).
The bolts I got were the right thread, but would have needed shortening a good bit for these 750 barrels. I don't know if the 810 barrels have a much deeper thread?

Since I got these barrels I've managed to acquire 2 sets of Dunstall pistons. manufactured by GPM.
I don't know what compression ratio they are, but I'm hoping they'll go well in the Proddy Racer I'm building (4S cam, 41mm inlet valves).
Just hope the cases can take it :wink:

It's not the cases that will fail, when I was discussing the use of my Dunstall barrels, the consensus was the base flanges let go, hense the need for thru bolting like the 850's plus the addition of 2 more at the rear .

Check pg 3 of this post. Apparently Ando has gotten 80,000 miles on his dunstall 810. That's street miles and I don't suppose any other radical modifications.

post129951.html?hilit=dunstall#p129951

The 810 kits seemed to work ok if they were the hop up. It's when you started hopping up the hop up kit that you ran into trouble.

Thanks for the link Rennie - I missed that first time around (must have been playing with D****is at the time).

I'm hoping this will be good enough to hold together for road and occasional track-day use; if I ever get serious I'll replace the barrels & cases with some Maney kit before wringing its neck properly.
The base studs which came with the kit I just bounced back were 1/4" too short where they screw into the crankcase, so maybe they were intended as a 'frangible link' to protect the barrel flange when they inevitably parted company from the cases

the problem that I had with dunstall barrels was not the base flange but the helicoils for the head bolts pulling out of the cylinder. A machine shop fitted some oversize thread inserts for most of the bolts and made up some threaded bushes for two of the studs. Since that time (most of the miles that I have done) i have not had any problems.
ando