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Interesting.

So as an academic exercise, lets say I was to take a these 2420's, we'll call them basic cams, and have them reground, what would be the optimum grind (lift and duration) for a ZB engine with the smaller, more restrictive head?

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Consider racing your A65 in classic 60's 650. We had 8 bikes on the grid at Barber. The class is growing and we're starting to see some some interesting bikes out there. The 650's run with the fast 500's too so lots of good racers in that wave.
And there's a pesky guy from California that needs to be kept honest....

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Bones: I do not know what refs you have. The BMS books shows ZB/BB 500 Inlet 2444 and Ex 2446 for Racing (petrol). The Bacon Restoration shows Inlet 2438 and Ex 2436 for 1950-51 Racing, The bacon books shows that for 1952 2438 became 2448 and 2436 became 2450. 1952 was the introduction of the quieting ramps but with the same specs as the earlier cams.

The B31/33 2420 cams were never stamped with a code. I do not think there is enough metal on a 2420 cam to make anything out of it without a lot of building up of the lobe. It would be much easier to find what you need I would think.

What is the cast in part number on your head and what is the size of the carb mounting opening? The carb opening on my early ZB34GS head is about 1 7/32 "

Gordo

Last edited by Gordo in Comox; 12/20/20 7:08 pm. Reason: added info

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Mike,

While the engine in my A65 is a 1968, the chassis in a 1971 Oil-in-Frame, and does not meet the spirit of the Classic Sixties class. The argument could be made, but the reality is when I come out of Prod Heavyweight, I will likely run it in BEARS. That seems to be a better fit for that machine.

Gordo,

As far as references, I have access to the internet. Which means is access to the whole of human information and recorded history. but that is beside the point. I do have some older racing workshop books, that as more general information. I do have digital copies of the factory manuals, which are all part numbers. I work as a CAD designer, meaning that while part numbers are interesting, actual dimensions make more sense to me. So adding material onto a cam might be completely impractical. More likely, because I have a pattern to model off of, doing a set of custom cams on a CNC might be a way to go. Regardless, the exercise is theoretical at this point, so the question still stands, what would be a perfect cam profile for a fairly stock ZB33?

The head casting you asked for is 65-1383. Intake port measures 1.18325" (al little more than 1 11/64") and exhaust port is 1.6245" (1 5/8")

Last edited by bones_bir; 12/20/20 9:37 pm.
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Go with the late 2442 inlet and 2446 exhaust and you won't go far wrong. If you run the lift figures through some of the newest sftware I'd be surprised if they were the perfect fit for inlet valve head size and port size but they were the factories ultimate versions for the 500.

Practically, almost none of the OEM cams time-up even close to the book figures. Even 'in the day' the cams that did were allegedly passed to favored riders. What this means for you is that a slightly 'off-spec' late set of cams might be a very good fit for your bike. (After the fact, I found that this was the case in my iron B33.

The 1 5/8" exhaust port is not a limitation - the slightly narrower pipe will drop the rpm for peak torque slightly (perhaps 500rpm). Use the book pipe lengths, or ever so slightly longer (to suit exhaust cams without the pukka timing figures.).


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Update - In the Spring I tore down the engine to inspect and evaluate.

I had the crank looked at by someone smarter than I and it was determined that the existing crank is actually flawless. All tolerances we re well within specification. no cracks dings or otherwise detrimental issues.

Cleaned, blasted and polished the cases. got a gasket set and put it back together.

[Linked Image from images.i.thechive.com]

Now on to the Gearbox, electrics and fuel delivery.

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Assembly pics

[Linked Image from i.pinimg.com]

[Linked Image from i.pinimg.com]

[Linked Image from i.pinimg.com]

[Linked Image from i.pinimg.com]

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Bones: Nice to see the progress, fine looking engine.

Looking at the assembly photos reminded me of how well brother Wilf's BB34A Clipper survived being scrambled with high compression piston and GS cams. The standard flywheel stood up quite well. Road racing is most likely more strenuous but you should get some good fun out of it.

Gordo


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I agree with Gordo, fine looking engine. But, looking at the fly wheel it appears you are using the original OEM crank pin, and for that matter complete bottom end? And an undersized B33 one at that? I would say that would be an inadequate vintage racer. Sorry to be harsh but the first thing a racer wants to accomplish is to finish the race and you need to think more about giving yourself the best chance rather than being just adequate.


Bill B...


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I am always bemused by those that assert "you gotta drop 15K into that engine or it is going to..."

By that rational there would be countless stories of BSA singles grenading on spirited road rides. To the point that there would be none of these bikes left in existence. While I am sure that has happened, the reality is that many of these bikes are still on the road or operational.

The great thing about these bikes is that the only computer on it is the one between the riders ears. Much like flying a Huey (I'm a retired helicopter pilot) the operator does the thinking, not the machine. Doing the thinking also means having the self discipline not not wring its neck at every opportunity. How you race is probably a lot different than how I race. Most aim for top speed and quick lap times, while I strive for smooth and perfect lines.

Besides, the stock flow on the ZB head is so anemic, that I would have to work at over-revving it. That is where the computer comes in.

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There have been countless B31/B33 engines road-raced with standard wheels and pins. The bigger threaded portion nd larger nuts on the GS crankpin is all very wheel, but worse than a stock crank if the GS crank is tired.

These days the problem is fit of the pin and security of the shafts being riveted to the wheels. If the fit of everything is good and RPM limited to sensible numbers, the bottom end will be fine.

Run a little hour meter off the ignition. Cheap to find and easy to fit. Run the bottom end under racing conditions for more than 50 hours at your peril.
KW


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Originally Posted by Kerry W
Run a little hour meter off the ignition. Cheap to find and easy to fit. Run the bottom end under racing conditions for more than 50 hours at your peril.
KW

That is a sensible solution and you are right, the condition of the crank is key. As previously stated, this one was checked over and deemed to be in excellent shape.

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Originally Posted by Kerry W
There have been countless B31/B33 engines road-raced with standard wheels and pins.


Kerry, do you suppose these were used wheels and pins of unknown history and hours?


Bill B...


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