Building new short stroke engine

Hey. I’m from Norway and has worked with Commando for six years and restored a fastback -70 and working on my Rodster 850 Mk IIa -73 now. Total dismantled . I have not posted a thread before, but I have followed this site for some time.
I have great respect for many people in here. In particular the thread Head Flow Testing posted by Jim is very good .
I will build (new) bulletproof short stroke engine for the Mk II. I will use the bike for touring and fast road use. I have bought a new crankshaft from Nourish Racing Engines (80.4 stroke).
Long Carrillo rods, lightweight piston (9.3:1 C.R.), gapless rings, complete radius cam kit (stage 1), Beehive valve spring & valve kit and PWK carb kit. Thank you Jim, it's nice parts!

I also purchased Power Arc Ignition systems from Old Britte. Throwing away Boyer.

I have not started assembly yet. This is because I wonder if the crankcase is strong enough for my use? Some says that the Mk II is the strongest Commando crankcase?
Or should I go all the way with Maney crankcase? Are there other suppliers that are between the standard and Steve Maney?

In step two in my plan is to install Full Auto head, maybe next year. I will feel the extra power from a new Full Auto head. Maybe Jim Comstok can get some extra power by adjusting the ports? :wink: I also have thoughts of 81 mm pistons, then there will be 850 short stroke.

I hope someone can give me good advice on the selection of crankcase.

There is no question the Maney cases are stronger but they are also considerably heavier.

For a street engine I think you would probably be fine with the MK2a cases especially with the Norish crank which is particularly kind to cases. You will want to keep the reciprocating weight as low as possible also.

For a short stroke motor you will want to do a big valve conversion on the Fullauto head to get the most out of it. I am doing two conversions right now. Jim

Hello Kvinnhering, Welcome to the forum. I'm jealous of your build, as your putting in all the parts I'd like to put in mine, but I'll have to be content with the stock parts that I have for the moment. Maybe when the kids move out of the house I'll have more money. Looking forward to more of your posts about your bike. Cj

TC's dragster 850 cases held back 150 hp @ 8000 rpm. Its much more work than money but factory cases can have plates welded on and stress risers relieved, refliled and beat back to seal w/o the ma$$ of both haves of Maney cases. Of course its rpm more than raw torque that injures cases and supporting Maney's shop is good for all of us. How hi rpm are short strokes able to tolerate? Would the short stroke have to be geared down to enjoy in day to day traffic?

What the reasoning behind only 9.3 CR pistons? Higher CR would help improve lower rpm civilzed operation and agressive cams need more CR rather than less. Cams with more duration/over lap than standard grind, tend to stifle detonation on current octane gasoline.

Kvinnhering,

I concur with Jim as to adequacy of the Mk2a cases. The weight difference between a Maney crankcase and a Mk2a is a fart in a wind storm.

One thing to consider is that you are sinking considerable money into this build; the foundation is the crankshaft and the crankcases. From your post you are looking at future improvements; my hunch is it may leave you wishing you had a race duty crankcases in the future when you go with an enhanced head.

The scourge of the crankcases is vibration and high RPM. For street use you will not likely approach anything near the abuse of a race bike. Furthermore, as Jim said, the Nourish cranks are kind to crankcases due to their mass. Go with the Mk2a if available but for a few more bucks you can have piece of mind.

I would stay away from welding up the crankcases for fortification; this was done out of desperation when there was no viable alternative.

If I read that correctly he already has the Jim Schmidt pistons and con rods. That should remove most of the concern about vibes. And wouldn't that put less strain on the crankcase?

Dances with Shrapnel said:

Kvinnhering,

I concur with Jim as to adequacy of the Mk2a cases. The weight difference between a Maney crankcase and a Mk2a is a fart in a wind storm.



I would stay away from welding up the crankcases for fortification; this was done out of desperation when there was no viable alternative.

Click to expand...

>>>10+ lbs difference. That is the same as the weight difference between cast iron and aluminum barrels.<<<<
>>> Agreed, welded up cases often end up weaker than stock.<<<

Probably better to spend the money on a 5 speed gearbox , so theres less of a gap between the gears . Therefore the cases are stressed less ,
as you dont have to extend things as far .

This sounds interesting, we'll need photos soon! : )

swooshdave said:

If I read that correctly he already has the Jim Schmidt pistons and con rods. That should remove most of the concern about vibes. And wouldn't that put less strain on the crankcase?

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Those rods and pistons have that effect on a stock Norton crank but what about the Nourish? Is this an off set crank? Jim's rods and pistons swap into the commando engine well because the shorter deck height of the pistons is compensated by the longer rods which produces other effects with piston speed. Will you be using a shortened Maney barrel to account for the short stroke or are your rods custom sized? I'm trying to see the whole picture. I'd go with the bigger pistons too, why hold it back.

IIRC Jim's rods are 6.4", so 80.4 mm stroke gives 2.021 rod/stroke ratio, which is in upper extremes used/attempted.
Here's article to think about the cam profile your engine might perfer.
http://www.stahlheaders.com/Lit_Rod%20Length.htm

Hi

One of my race engines has a set of Andover Nortons crankcases. Good quality good price & strengthened by a company that new where the faults were.

Worth considering.

Chris

I would have to agree with Chris that if you want new cases the Andover Norton ones would be a good bet and they are designed to take the primary cases you want on a road bike, unlike the Maney cases I have for my short stroke race engine build. I have a Maney crank and I have the 10:1 pistons and longer rods from Jim.

MkIIA cases were the ones I always saw cracked across drive side the main bearing in race bikes in the '70s!

Not figured cam choice yet, might go with Jim's radius set up, but I am also considering full auto head, interesting to see Jims comment about big valves.

As always, all I need is money....

Again, it's a matter of the foundation of the motor which is the crankcases and crankshaft; nothing to skimp on if you are going to push the motor. Mk2a cases should be fine with moderate street use. Once you start building from there (big valve heads, higher compression, more excursions to high rpm...racing) you need to look at more than factory cases. The only street application where a case split in half was on TC Christianson's daily rider and I wrote that off to TC being TC. I am not saying there are not more instances, it's just that they are not apparent to me.

As for the Andover cases, it may be worth looking at if they are new and not welded up. In the case of the crankshaft, material of construction makes a world of a difference, this may be the case with new crankcases when compared to factory cases. I don't know enough about it. I seem to recall Steve Maney stating that he uses a "better" alloy and if Andover is making new cases I would presume they are doing the same.

From discussions with Dave Nourish and seeing several of his crankshafts he is clearly a proponent of ample mass (heavier cranks) and this is beneficial as it dampens overall loading of the main journals thus "kinder to the cases". I don't recall whther the subject crankshaft was offset of 360 degrees. I presume it is 360 degrees as it makes cam selection much simpler.

There is no question Maney cases are the heaviest duty cases going. Steve will trim an inner primary cover to take factory alternator with 40 mm belt drive and seal it up air/oil tight if needed like me. I double checked with Maney, Dreer and other belt drive vendors - that there is no need to vent the primary for alternator or belt heat. Not a bad idea to vent of course, unless in grit and messes like me and Wesley. I'm confused, [shut it] on reasons to go with short stroke over long stroke with a crankshaft that is allergic to Norton 'hi rpm', which is barely into power band rpm of sports modern engines.

There are a number of riders/racers that regularly ran factory strokes into the hi red zone 8500-9000 rpm and even Dances here is going with stroker motor's bigger bang displacement and less max rpm. Jim said the FullAuto chokes a bit at top rpm but would it matter on short stroke smaller breaths? Decisions Decisions to make with someone else's money is fun.

The short stroke is where you need the larger valve size. Jim

The short stroke is where you need the larger valve size. Jim

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This makes sense with the hi long rod/short stroke 2:1+ ratio that beathes better just after TDC than lower rod/stroke l/r ratios, ie: more valve slot area at partial lifts near TDC. What confuses me is that about the only advantage of short stroke I know of is to reduce the piston's TDC acceleration jerk apart for higher rpm tolerance, therefore more breaths per time unit > more fuel burnt power and gear ratio advantage. If 89 mm stroke can take over 8000 rpm, especially with lighter stronger piston kit, what advantage will the 80.4mm stroke have if crank&cases can't take being rev'd higher than this. Valve train must not float either of course, which larger valves hinder some.

Its annoying as heck to have to blip engine extra to leave lights or creep in traffic and not just bog down or stall. Any feedback on short stroke race like engine in public use? It enjoyable as heck to have to be careful easing on throttle from lugging rates, at least in real life.

The advantage is the ability to turn high RPMs without suffering from the power loss from piston friction.

Friction loss goes up on a very sharp curve when the piston speed gets above the recommended maximum speeds. Friction loss becomes a serious player on a longstroke at around 6000 RPM. Somewhere around 8000 rpm with a longstroke motor, it becomes impossible to make additional horsepower gains from RPM. Any possible gain will be eaten up by friction loss. Jim

The advantage is the ability to turn high RPMs without suffering from the power loss from piston friction.

Friction loss goes up on a very sharp curve when the piston speed gets above the recommended maximum speeds. Friction loss becomes a serious player on a longstroke at around 6000 RPM. Somewhere around 8000 rpm with a longstroke motor, it becomes impossible to make additional horsepower gains from RPM. Any possible gain will be eaten up by friction loss. Jim

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Ok that's an un-expected answer, friction not piston acceleration or crank fling limits. So 8000 rpm is practical power-friction limit in factory long strokes. What redline are these shorts strokes expected to reach for their advantage to shine - reliably?

With modern materials it isn't hard to turn a long stroke over 9000 reliably. I have run one to 9500 with a reworked stock crank and Carrillo rods. It took me a while to figure out why the horsepower was on the downward spiral over 8200 no matter how it was cammed, carbed or flowed. Then I learned about piston friction and how it increases with piston speed and it all became clear.

Depending on gearing and the track it may be an advantage to rev it beyond the power peak but don't expect a power gain up there. A short stroke generally adds an extra 1000RPM. Jim

Thank you for your many good posts. Sorry for the late reply.

Comnoz: Very interesting with a big valve conversion. I will contact you when it is approaching. And thank you for sharing your knowledge and experience with us.

Hobot: I would not weld plates on the crankcase. I want be able to return to the original look.
I will not test out the max rpm for a short stroke engine. I will set rev limiter to 7500 rpm.
The original 850 head will give 9.3:1 C.R. with piston I bought from Jim. My head is milled down 0.5 mm so I can raise the CR later by reducing the thickness of the gaskets. I now have thicker Cylinder head gasket (062") and base gasket (021"). I consider not to install the base gasket, then the CR is 9.8:1.

RennieK: The crank is 360 and I use the original height of the barrel. The rods are longer to compensate for smaller stroke. I have the original cylinder and bought +20 pistons from Jim.

Matt Spenser: I already have four-speed close ratio gearbox from RGM . I have read that this is stronger than five-speed.

Chris: Many thanks for your hint. I will investigate this. I have original MKIIa crankcase, if I go for a new crankcase there is relatively small price difference between Andover Norton and Maney. Maney can machine the crankcase to suit the primary cover.

Ludwig: Yes, I can probably get TTI box, but way. I have sold one of my bikes to fund this once in a lifetime stunt .