Short Stroke 750 Build

Well, I've been inspired by the build threads by Paul and others, so I'm starting my own. I'm just starting the rebuild on one of my short stroke 750 engines. I originally built it as a mild road race engine for AHRMA Formula 750 racing. After a fair amount of that, I freshened it up a few years ago and ran it in my featherbed at Bonneville. Now I'm going through it prep it for road racing again, and planning to do some upgrades. I'll keep adding updates as I go, but it might take a while. This isn't my highest priority project right now, so I'll only be working on it sporadically. The first seried of pictures are disassembly pictures, so the parts are sometimes a bit dirty. I do plan to clean up the cases and such, and will post pictures when I do.

These are shots of the MK3 crankcases showing the welded reinforcements.





This is a shot of the interior of the drive side half, showing how the bearing race is locked to keep it from spinning. It's a technique detailed in Paul Dunstal's tuning book, and I've used it for a long time. My experience has been that the bearings eventually spin in stock cased in a race bike, and Locktite won't hold them after they get loose. I haven't seen the same problem in the new cases from Steve Maney, which hare cast from a better alloy, but I don't have a lot of experience with them yet.


That's it for now.

Ken

nice welding!

pelican said:

nice welding!

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Thanks, but I can't take credit. I did this back in the '80s before I had my own TIG welder, so I had a local welding shop owner do it. He was the welding instructor at the local community college, and was really skilled. I did this same mod to several crankcases. The only problem with it was some distortion. The main bearing bore would shrink a bit from welding the plate around it, so I had to bore the hole to size after welding. I also had to clean up the cam bearing bore on a couple of the cases. I had the cases welded while the two halves were bolted together, and that kept warpage at the joint dowm pretty well. I used Locktite Master Gasket as a sealant when assembling the cases, and never had a leak.

Ken

These are pictures of the crankshaft assembly. It's the first short stroke crank Dave made. He wouldn't do it without a sample, even though I told him it was exactly like a stock crank, but with a different stroke. Fortunately, I had business trips to England fairly often back then, so I carried a stock short stroke crank over in my luggage for him to see. It was one that had cracked in the race bike, so I didn't mind leaving it with him for a while. He agreed to bore the two large holes in the flywheel to bring the weight down, and to make it in the wider MK3 width. The factory short stroke cranks were standard Mk 2 width, but I wanted to use the newer MK 3 cases, because I thought they would be stronger. I'm not sure I was correct, but that's what I felt at the time. The rods are Carrillo, and are the longer length and larger wrist pin bore used in the short stroke engines.





Ken

These are the original Omega pistons that Norton used in the short stroke, but I machined the dome to a more rounded contour so I could push it up higher in the combustion chamger for more compression. The short stroke had a full hemi combustion chamber with no squish band, so I machined the piston very carefully to create a squish area between the side of the dome and the combustion chamger wall. The only drawback to doing so is that it reduces the top land enough that the top ring can overheat and lose it's tension. I had JE make some pistons in this design, but with the to ring groove a bit lower, and that's what I'll use for this rebuild. I'm also planning to use a different head with even larger valves, and the original piston had a problem with the valve pockets getting too close the back of the top ring groove when enlarged for bigger valves. The first set I used burned through into the ring groove. The lower ring groove in the JE pistos should solve this problem too.



Theere is very little color on the piston, mostly because the only time on them was from a couple runs at Bonneville and El MIrage. I think it was also running a bit rich at the time.

Ken

Thanks for starting this thread and sharing these excellent photos and dialog! I'm sure it will be appreciated by many.

Ken,

That's some mighty fine jewelry!

How'd you do in AHRMA back then?

...and WHEN was "back then"?

Very nice stuff you've got going on.

"Back then" was actually "way back then". I started racing in 1972 at the age of 30. I bought a used Production Racer from Dan Gurney, that he'd been riding on the street, and went racing. I raced mostly local club races through the '70s and '80s, did some AMA Pro Twins racing in the late '80s. I raced my PR, by then highly modified and running 920 engnes, and a Commando engined featherbed in the 750 class. When AMA killed the modified classes, and AHRMA picked them up as BOTT classes, I started racing the featherbed in AHRMA. I think my last race was in '93 or '94. I was a middle-of-the-pack racer, but had a lot of fun at it. I started crashing a lot, had some serious injuries, and decided it was time to quit. After I quit racing, I kept the bikes and had friends race them. That's when we started winning a lot of races. I guess I was a lot better mechnic than racer. I still take the featherbed to an occasional AHRMA race, with a friend riding it, just for nostalgia. I went to Bonneville in 2001 with the featherbed, and came home with records in three classes, and I was hooked on the salt. I'm trying to get our Norton-pwoered streamliner sorted, but have had disappointing results so far. I plan to keep doing LSR bikes for a while.

OK, enough rambling. Here is a picture of the valve train stuff, with an Axtell #3 cam, mildly lightened lifters, Alloy-Tech pushrods, and a Norvil vernier sprocket.



And a couple pictures of the current head. It's an RH7 short stroke head with KPM Black Diamond valves and springs, mildly lightened rockers, and cleanup on the ports. I'll probably replace it with a more modified one when I put it all back together.




And, just for fun, a picture of me on the featherbed back in '93, at Roebling Road.



Ken

Racing history & pedigree; cool.

I never won a race, but had more fun in one year racing than the previous 30 years of riding put together.

lcrken said:

OK, enough rambling.

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No, more rambling AND pictures! This is great stuff.

That's beautiful work!

This thread is just what ive been looking for, im also planning to convert my mk3 commando engine to a short stroke race motor.
I assume that most of the factory made short stroke engines were exported to the states.
How much difference is there in the short stroke comapred to the standard stroke engine? more rev happy? style of vibration?

ken,

Thank you for posting this, I've already done a Commando motor or two and find that one looks much like another, so you sharing your unique experience of non-standard modifications and engineering enhances all of our knowledge. Please keep posting these photos.

At Mosport last year, I talked a bit with Herb Becker, he built an ultra short stroke motor, he could rev that thing to 10,000 rpm, unfortunately, it fragged itself, the valve train just would not follow and a valve contacted a piston wrecking the whole thing. There is only so much that can be done with a 60+ year old design. But anything done on a Norton is cool so go for it

Jean

Nitrosmurf said:

This thread is just what ive been looking for, im also planning to convert my mk3 commando engine to a short stroke race motor.
I assume that most of the factory made short stroke engines were exported to the states.
How much difference is there in the short stroke comapred to the standard stroke engine? more rev happy? style of vibration?

Click to expand...

All my short stroke experience is with race bikes, so take my comments with that in mind. Vibration? I ran both regular 750 and short stroke 750 engines in my PR, balancing both at 62%, and I can't say I noticed any difference in vibration. The isolastics made vibration a non-issue. On the other hand, unless the bike shakes at high rpm (like it does when a crank starts to crack), I wouldn't notice it much. I did run the short stroke once in my featherbed at Bonneville, but again, it's all high rpm. The tach just sits at 7,500 rpm for over a mile. There was no vibration problem at that rpm. I used to use a 7,000 rpm red line for the long stroke, and 7,500 for the short stroke, occasionally running a little over if necessary. That was back in the day of stock cases and cranks, which would both eventually crack and break after sufficient racing, even with the usual crank mods. With modern aftermarket parts avaialable (Steve Maney, etc.) I would be quite comfortable at significantly higher rpms. My experience was that both engines tended to have their horsepower peak around 7,000 rpm with the usual racing mods. The extra 500 rpm with the short stroke was handy because it sometimes reduced the number of shifts going around a circuit while still staying in the power band. If you build both engines to their maximum capability, the short stroke will deliver maybe 8 or 9 more peak horsepower. The max I've seen for a long stroke 750 was 76 rwhp, and for the short stroke 84 rwhp. That was a long time ago in Axtell's shop, and those engines had a tendency to grenade regularly. I believe people like Herb Becker and Steve Maney are currently building 750 engines at those levels, and even a horesepower or two higher. I know Jim Comstock put out some really serious horsepower from his long stroke 750, back when he was racing it regularly. But it takes really extensive work and lots of dyno time to reach those levels. I think there is even more potential in the ultra short stroke (75 mm stroke) engines that Steve's parts make possible, but until someone (Steve, Herb, Jim, whoever) sorts out a valve train that will allow them to breath better at higher rpm, I don't expect to see them make any more power than the short stroke (80.4 mm stroke). HIgher rpm only makes more horsepower if you can flow enough air up there to keep BMEP from dropping too much. There's a lot of effort going into that area right now. Several people are experimenting with more radical cam profiles, light weight BSA lifters, stiff pushrods, beehive valve springs, etc. Personally, I think the limiting factor will be that long, skinny camshaft. I think we'll have to have a center bearing for it to make the really aggessive cam profiles work. It's been done, but its' still not a very common mod. Guys like Steve Maney, Herb Becker, Jim Comstock, and probably lots of others I'm not as familiar with, have continued to develop the old Norton twin way past what it was designed for.

With so much interst in the subject here, maybe we could talk Jim into doing a thread on his development work. I can guarantee that would be entertaining.

Ken

Ken
I met Herb Becker last year at his shop and had good look at his bikes that he set up. Mostly Steve Maney Crankcase/ barrels lots of go fast stuff.
I got to meet Doug MacRae at the Vintage Road Racing Association booth who rides Herbs Bikes at the Toronto International Center SuperBike show last weekend.
Got to see the New Norton Commando 961 right beside Our Ontario Norton Owners Club booth. I was great to see the old and the New Generation of the Unapproachable Norton.
Here are some cool Utube videos
http://www.youtube.com/watch?v=s71KQdn3Yh4 Herb Becker starting the bike

http://www.youtube.com/watch?v=HhDCqhkx2xg Doug MacRae

http://www.youtube.com/watch?v=EuYrHKk9STA and this is fun….you never know how it going to end. LOL http://www.youtube.com/watch?v=funNlm0XIFE
enjoy.
Keep the rubber side to the road.
Regards,
CNN

These are pictures of the alloy cylinder from the engine. If you look closely, you can see some of the Bonneville salt still on the front fins. I've since cleaned them up a bit more. I used to buy these cylinders in batches from Drew Robertson in England, import them into the US, make a few mods, and sell them. The quality of the cylinder castins was great, but the liners weren't so good, the base holes weren't in quite the right locations, and the liners were frequently not all the way into the cylinders. I had to replace a couple that had the liners drop several thou during races and blow their head gaskets. From then on, I had to heat the cylinders, press the liners all the way in, and then flycut the deck a few thou to get it level with the liners. I also had to put the cylinders on the mill and enlarge the base holes towards the right locations. It just wasn't worth the effort, so when Steve Maney started making really good quality cylinders, I quit. I still had a few in stock, so used one of them for this engine. Now that I'm planning to go from standard to +.060" pistons, I don't think I'll use them again. Boring them +.o60" will leave a sleeve thickness of .062" or so, and I don't think that's enough for iron liners in aluminum cylinders. I think I'll have LA Sleeve make up some new liners with a bit larger OD, and fit them to another one of the alloy cylinders that already need new liners. The other alternative would be to use stock iron cylinders, and I might consider that. Decisions, decisions.





Ken

Nice videos, CNN. The Roebling Road race is pretty entertaining. I recall that there was a spot where you'd get a small river running accross the track in wet weather. Made for interesting lines.

Ken

What kind of lifters do you use in those cylinders?

I'm curious if anyone has experimented with coated aluminum alloy liners and pistons for the alloy barrels.

It just seems the steel liners in spite of their durability seem to be problematic and I would guess due to the differences in expansion and contraction rates of the different materials. Further more the temperature differences at the top of the cylinder compared to the bottom must compound these expansion/contraction rate differences and could explain the types of breakage's experienced with the Dunstall cylinders and liners.

I'm wondering if alloy barrels, liners and pistons with nikasil, ceramic or nitrated surfaces would work although I have no idea of the cost of putting it together. I know it wouldn't be cheap until someone starts putting a kit out (if it indeed could handle it)

How bullet proof are the Maney kits?

Hi Ken

I met Drew & his dad at Donnington 2 years ago & he was looking to get out again on a Norton Commando again. He even talked of making titanium con rods.
He was the one who told me how to set up my engine in 96. Freely helped me out. I had never touched a Commando engine until then.
tIf I need help know I always ask John Conroy (Seeley 920 on this site)
Jerrys site has become the one to ask questions on.
All the best Chris