hello all. i am still searching for a solution to using a set of dunstall cylinders on a 71 750 commando. research tells me the two largest drawbacks are thin sleeves and pulling threads. i have hope to use an 810 cylinder i have, sleeve it to 750 (73mm) and run either lo comp or flat topped atlas/early commando pistons. as for the thread pulling i have decided to run studs instead of bolts for the four head fasteners that run downward into the cylinder, thus halving the
load on the threads. mocked up so far head to cylinder fit isn't too bad. opinions please! cylinder to crankcase is where i am stuck at the moment. box spanner clearance to the cylinder on 4 of the cylinder base nuts is less than nil. i have 3 specialized
5/16-26 sleeve nuts that solve the problem but need a 4th. these nuts are the same in design as the stock cylinder head to cylinder stud nut for 850 commandos (06-3192). the nut i am searching for is 5/16-26tpi that is .86" lg x .48 dia (sleeve) with hex atop that fits a 3/8 american box wrench. has anyone ever seen this in a stock application, possibly on bsa or matchless? i need to get everything torqued down first before i go to step two and get it sleeved.
thanks so much, biggles

If you use studs in place of head bolts you likely will not be able to remove or replace the head while the engine is in the frame.

Other than just having a Dunstall cylinder, why would you want to use it? Especially if having to sleeve it to 750 and using low compression pistons. My opinion is sell the Dunstall part for a fortune and spend your money on a new alloy cylinder...they're out there and much better quality and design.

david, good points all but i'm extremely low budget. so far head fits on with engine in situ. dunstall cylinders don't seem to be bringing much and alloy 750 kits are quite dear. i'd settle for a used stock 750 cyl (commando or atlas) but i'm still faced with machining (atlas), sleeving, boring, or all three. it's the bird in the hand syndrome. i truly appreciate your input. so far this is still only in the r&d stage. never seen the nuts though?? thanks, biggles

Why not fit heli-coil/recoil inserts and use the standard fittings, these inserts spread the load much better than simply tapping in to alloy and should be easier and cheaper than some of the other options.

I've been out of the scene for many years and just getting back into my Nortons so maybe I've missed something. I don't recall that all Dunstall 810 cylinders were doomed. The thread issue can be addressed like Dave suggested with helicoils. As far as sleeving goes why not check into some new pistons and keep the 810. We have guys like Jim Schieder on this forum who are making pistons for 750's and 850's so why not 810? His pistons use a longer rod to give a lower piston deck height to achieve a big weight reduction of the piston so it costs some bucks but there may be other solutions for pistons that bolt in with stock rods. You could always lower the compression a bit if you were worried about pulling the threads. The weight reduction of the Dunstall barrels is nice to have.

been there done that with a dunstall cylinder. if i was to do one again i would helicoil EVERY threaded hole in it. not only is the 4 head bolt holes prone to pulling the threads but the 2 small stud's in the front WILL also pull. many years i had the privilege to talk with john Gregory of T C christinson and hog slayer fame and the 810 cyl came up (he was still at 750 and i had an 810 kit) i told him about the thread pulling trouble, well they did try the 810 kit's but when I seen them the top 3 fins were machined off and a LARGE weld was all the way around them along with deeper head bolt holes and helicoils.



windy

Personally I wouldn't touch a Dunstall 810 barrel with a barge pole!! of the 7 I've seen, all cracked all the way round between the 3rd and 4th fins if I remember correctly, shortly followed by the flange on the liner, so that when you revved it, you didn't need exhaust valves....all the gas came out through the cracks in the barrels!

thanks to all for your input. i'm trying to make do with what i have in the shed. this bike probabbly won't have a lot of miles put on it but it sounds like i need to save more money and go with stock cylinders if i can find a set. thanks again, biggles.

What is the condition of the 810 barrels and pistons?

I'd be interested to hear from anyone currently running an 810 as I know there are several on this board.

Folks like the hog slayer breed and other racers likely were running 11:1 compression and really pushing the limits.

Are there any success stories of guys with the bolt on kit and stock everything else?

I'm curious because I have a brand new kit I purchased in the 70's when they were all the rage. I only heard positive things back then.

rennie k if you could sir check your new kit and see if it doesn't include the special sleeve nuts (5/16-26) i have inquired about. possibly the reason no one has ever seen these is they were part of the original aluminum cylinder kits. i have two aluminum cylinders. one marked dunstall @ 810 bore and one unmarked @ 750 bore. both have three cracked sleeve spigots where they were "square" notched (as is a stock cast cyl) for rod clearance except for one sleeve that has been cut in an arc thus not giving the crack a place to start (inside corner of notch). the sleeves are a bit thinner than the guys i hope to have resleeve mine (750 set) but there should be enough meat on them to enlarge the o.d. a bit. thank-you, biggles

I took some photos of my kit minus the pistons that are still wrapped in oily rags etc.

This kit was purchased between 73 and 75 as near as I can remember. I have the original receipt here somewhere so I'll confirm the date when I locate it. It appears to have all the known fixes from earlier designs done to it like HD heli-inserts, longer studs and the circular rod cut-outs. Let me know if you need any other info.

Cut-outs


Front View


Fastener Kit


Head Fastening Set-up


Heli-insert The small threads appear to be heli-coils while the larger are inserts.


Liner Thickness and fastener screws


Check out the lifters!

Beautiful photo work!

Cut-outs


Awesome piece of history you got there . What does the tappets prevent from twisting

nortonspeed wrote:
Awesome piece of history you got there . What does the tappets prevent from twisting

There are flat plates that bolt in between the lifters to keep them from twisting. You can see them, and the screws holding them, in the picture.

Ken

lcrken wrote:

nortonspeed wrote:
Awesome piece of history you got there . What does the tappets prevent from twisting

There are flat plates that bolt in between the lifters to keep them from twisting. You can see them, and the screws holding them, in the picture.

Ken

So the tappets are like Triumph's but with a flat side on the stem to keep them from twisting