Convert 850 Commando to 750 Commando

OK. Once you've all settled down the laughter, please let me explain...
I race a Commando sidecar outfit, that has an 850 engine, P3 cam, Omega Pistons and Thunder Rod con rods. This is all coupled to a stage 2 Maney head, and a Hemmings cr gearbox.
As the engine is 850, the unit gets classed way above it's punching weight, into the group with up to 1200cc engines...
I have found the engine to be reliable and strong, and with an acquired MRE chassis, very able, but I can't run with the big boys.
I have an 850 cylinder that had been bored out to 920, but that seized a while ago, and it looks way past repairing, with a hole and cracks in the liner.

So. I want to sleeve the existing 850 cylinder to allow for 750 pistons. The reason for doing this is I have an 850 crankcase and head, so rebuilding and fitting will be relatively straight forward. The only fly in the ointment is the new 73mm bore with a 77mm plus aperture cylinder head.

How to get round this safely?

Duh... keep the 77mm bore and build a short stroke motor...

With a short stroke motor with a billet crank, you should be able to use more revs. The short stroke 750 motor would probably be faster than your long stroke 850, because you can move the power band up 1000 RPM, without destroying the motor. More revs with the same bang on every stroke, means more power. Where you might lose out a bit is in torque. Which with a sidecar, might mean something. But if you are racing in a 750cc capacity class, that might not matter as much.

Thanks for that. Very green here as to how to short stroke an engine, but I guess from your reply it will be a crank and rods?

Champy72 said:

Thanks for that. Very green here as to how to short stroke an engine, but I guess from your reply it will be a crank and rods?

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Yeah, you just missed a short stroke crank on eBay!

We have a couple of experienced short strokers on the forum, if you start a thread about it you’ll get some seriously solid input...

Duh! Typical! (Missing out on Ebay)

In your first post on this topic, you said we would probably laugh. If I was serious about racing and there was a suitable 750 class, I would do what you are now considering. Try MAP.

http://www.mapcycle.com/performance/

acotrel said:

In your first post on this topic, you said we would probably laugh. If I was serious about racing and there was a suitable 750 class, I would do what you are now considering. Try MAP.

http://www.mapcycle.com/performance/

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Thanks.

I started with an 850 and had a machinist install sleeves to get the 73mm bores. The machinist left a spigot to fit into the head and I ran with a solid copper 850 head gasket.

As mentioned above by Fast Eddie, a short stroke is a viable option and you would save boring out the 850 barrels for the sleeves. For the short stroke you are looking at a crank, rods and pistons. The factory short strokes were 80.4mm by 77mm bore. To realize the potential of the short stroke you would want reangled and oversized intake and exhaust valves, larger carbs and manifolds and different exhaust and likely a different cam: also consider fortifying the drive side of the engine case or pay for a race crankcase if you can find one for sale - slippery slope.

With a proper Norton short stroke you loose nothing over a conventional long stroke all while extending your useable power band beyond the factory stock 7,000 rpm red line - this from firsthand experience.

You are at a branch in the road. The simplest alternative is to sleeve down to the 750 bore but you will miss the opportunity to make some real trackable power.

Time Warp said:

The 850 is really 828 cc ? rounded down is 800 cc so what is another 50 cc's for the pot, 750 decals and done.

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It always seems to be OK if nobody protests. But these days anyone who protests is usually regarded as an idiot. I am of two minds - if you win with an oversize motor, does it mean anything ? But does complying with the class rules mean anything anyway ? Who does fuel checks and engine measurements anyway ?- we don't usually play for sheep stations. In Australia, we have 'Little Athletics' - every kid gets a trophy.

I have got three lovely trophies sitting on my shelf looking impressive. However they do not mean much to me, because I did not win them in the days when I was racing All-powers C grade where every race was the charge of the light brigade. If you have a win against stiff opposition while complying with the rules, you have actually achieved. Racing costs too much to be simply mucking around when you do it. Having fun is one thing, but you can do that by thrashing around a field full of tree stumps on an old MX bike.

Time Warp said:

The 850 is really 828 cc ? rounded down is 800 cc so what is another 50 cc's for the pot, 750 decals and done.

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I wish. But the 850 barrel is significantly different to the 750. If you're not doing too well, nobody cares. If you start winning or getting podium positions, somebody will. A strip down will expose the cheat, and cost. Really want to be kosher on this one.