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Boring a taper?

Or tilt the head.

That's pretty funny. :)

Personally, i think the lathe is best, probably followed by tilted rotary on mill if you do not have big enough lathe; and a D-bit reamer is not at all difficult to make if it devolves to that and you have access to a surface or T & C grinder.

However, i have noticed that quite a few of us have Tree boring heads in the 'ol tool crib...."just in case".
And might or might not be susceptible to watching/supervising you use it if you were nearby & sufficiently dedicated.

I think a Universal Precision (brand) boring head could be contrived to bore a taper by means of some light modification to use a suitable gear and rack along the path of feed. Or even a wire and pulley. They tend to sell for under $300 (Make sure it is complete) but i don't have a need significant enough to test my hypothesis.
 
"And then bigger valves, bigger ports which should get me up to 30HP. On a good day I may be able to go 80MPH."

Unasked, but that's probably *way* too fast to be travelling on a BSA that was originally about 3.5 HP (after the load from the lucas electrics kicks in...).

Pictures, or it didn't happen!
 
OK, in reply to multiple people.

Yes the flywheels are steel (actually forged, I thought they were cast). BSA also made cast iron flywheels for the same bike. It just depends on what year it was made.

The motor is a M20, so 500cc, side valve with 13HP at 4200 RPM. It has now been turned in to a M21 motor which has a longer stroke, so it was 500cc and then 600cc and now 720cc. Can I make it 840cc, we shall see.

I also have a BSA M21 which was 600cc from the factory. It will also be bored to a 90mm to make it a 720cc. At the moment it is still 600cc. I ride this bike all the time and it will not be bigger than 720cc. I am not stroking this motor.

So the M20 motor I am modifying is just for fun. When people see the bike which in factory condition can only do 55 MPH they look down on it as it is so slow and who would want one? It makes it fun to pass guys who think their bigger British bike will leave me in the dust. If I can get 30HP out of it, I will be happy.

A guy in the Netherlands has made a 860cc M20 that has 40 HP. He says that is as big as you can go.

I am familiar with stroking motorcycle crankshafts. On a Japanese bike with a straight crank pin it is easy. Weld, bore, hone and you are done.

Doing it on the lathe, I would need to remove the main shafts to hold it on a face plate. The main shafts are riveted in and not easy to remove but it can be done. As you can't hone a tapered hole, I could use my tool post grinder?

On a mill, I don't have a tapered boring head but a reamer could work?

Let's pretend it is 1950, S&S does not exist yet and you can't buy a stroked Harley crank shaft. How did they do it then? There were plenty of Harley with motors bigger than they left the factor with. And they had tapered pins.
 
Let's pretend it is 1950, S&S does not exist yet and you can't buy a stroked Harley crank shaft. How did they do it then?
The mainshafts are tapered also. Pop them out then easy to just offset the flywheels in a four-jaw.

For mine, I just made new flywheels, but used straight press fits and made the whole thing. De-stroking was a little bit more of a problem, the nuts interfere so on the early XR's the mainshafts were welded into the wheels on the back side. It was not ideal. For the alloy update, the wheels and mainshafts are a one-piece forging and the pin is a straight press.

Don't know your engine but one thing to think of is, these pressed-up cranks are so wiggly that the mains don't live well. They made a big step forward with XR's when they quit trying, and changed over to spherical rollers instead of ball/roller bearing mains. Quit trying to constrain it, because you can't. Just let it flop around and use bearings that can handle that.

People nowadays kind of sneer at stroking but in some cases ... due to the rod lengths and breathing capabilities and so on, a 3 1/8" bore, 4 5/8" stroke sportster is 77 inches and runs like a bat out of hell. Long strokes can work really really well.
 
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Don’t understand why you need to remove the main pins to turn on the lathe.
Just make a plate with an offset bore to take the main
Hold that plate on the 4 jaw.
Or bore a hole in a faceplate hot the main pin.
Don’t see a Problem.

Cheers Ross
 
Don’t understand why you need to remove the main pins to turn on the lathe.
Just make a plate with an offset bore to take the main pin.
Hold that plate on the 4 jaw. Don’t see a Problem.
I don't know his motor but on a sportster, that'd be several inches. On the drive side the mainshaft is long enough to go through the bearings and all the way past a triple-row chain and a shock-absorby thng, then has a nut past that.

A bigger lathe won't mind but on a 13 or maybe even 15" lathe, it'd be awkward. Probably still easier than doing it in the mill but if the flywheel is heavy and the lathe is small, could be less-than-optimum ...
 
I don't know his motor but on a sportster, that'd be several inches. On the drive side the mainshaft is long enough to go through the bearings and all the way past a triple-row chain, then has a nut past that.

A bigger lathe won't mind but on a 13 or maybe even 15" lathe, it'd be awkward. Probably still easier than doing it in the mill but if the flywheel is heavy and the lathe is small, could be less-than-optimum ...
Counter balance.
Don’t see a home brew reamer or endmill swarf cutting producing a finish to hold the taper.
Aren’t you only working on one disc at a time?
How much off center are we looking at?
Cheers Ross
 
Counter balance.
Don’t see a home brew reamer or endmill swarf cutting producing a finish to hold the taper.
Aren’t you only working on one disc at a time?
Yeah I know, but usually the flywheels are not small, either. Again, sportster and vincent are about 8" diameter, 74's even bigger, so he'd need a reasonably big 4 jaw ... agree with you, if you have the bigger lathe then there's no question but .... so far he didn't say what he's using. Most people don't have 20" swing Monarchs in the garage.

And yeah, finish won't be as pretty but guys did more with less in the past. It was pretty common to lap the shafts into the flywheels in the old days. If he was going afm racing one would suggest against this but for a 10 hp flathead just for fun ....
 
One can do the rotary table tilt. That is messy to nail.
Tapered endmill... those do not make just what you want.
Reamer? not cheap in a custom.
This just screams really,really good lathe or ID grind to me.
I am also darn sure you can do this on a 50 year old B-port mill.
 
" these pressed-up cranks are so wiggly that the mains don't live well. They made a big step forward with XR's when they quit trying, and changed over to spherical rollers instead of ball/roller bearing mains. "

Yeah, bmw 's hotter 1960s vintage 600 cc motor has the rear main as a spherical roller, whereas the milder bikes used caged radial bearings front and back. Many of those also had problems with the rear bearing seat in the engine casting wallowing out a bit. These were, as you might guess, pressed-up cranks with roller bearing big ends.

Really they knew they had problems as they kept re-designing the cranks over the production lifetime of those motors, and finally gave it up and used forged one-piece cranks with plain bearing mains and big ends, with a proper high pressure trochoid oil pump, starting around 1970 or so. (the R90S that pridmore campaigned used a stock crank, after an earlier attempt to run a lightened one resulted in a catastrophic failure, again a well-meaning attempt to squeeze out performance that went a teeny bit too far)

One of the inherent problems with pouring more hp onto an older motor design is, sometimes another part isn't up to the task.
 
British bike ramblings:

I had a BSA, a twin cylinder 1968 650 I think, and it sure would go way over 55MPH.

I had a couple Norton 850's but never liked the bikes.... an Interstate and a 'regular'...I'd rather have a Triumph 750 any day of the week.

The new film out on Phil Vincent is well worth watching. I looked at buying a Black Shadow around 1994 in Chicago....guy wanted $25K which I would have been stretched to afford. The good news is now I make a lot more money. The bad news is the prices have gone up on Black Shadows such that I am no more able to afford one now than 30 years ago. Could there ever be a better looking bike engine than the Vincent? Not a chance.


https://www.amazon.com/Speed-Expensive-Vincent-Million-Motorcycle/dp/B0CFP5KCBW
 
Although only a single cylinder bike, it has four main bearings. Both sides of the crank have a a ball and roller bearing. As it was designed to haul a sidecar and be abused constantly pulling lots of weight the crank assembly is way over designed. So the crank does not flex at all with the power it makes. The crank pin bearing was also used on other BSA's making more than twice the power of my bike.

I just want to move the pin out a few mm. As for boring it on a lathe, yes the main shafts are gonna be a problem unless they are removed. Both main shafts are long as they have to go through two main bearings each and on the drive side the shock absorber. On the other side, two bearing and a gear and bushing and nut.

Luckily M20 crank shafts are common if I fuck one up. BSA made 126,000 of these bikes during the war.

I am not worried about holding the crank in a four jaw as my biggest lathe can handle a 23" diameter part.
 
"I had a BSA, a twin cylinder 1968 650 I think, and it sure would go way over 55MPH."

That was my *second* motorbike: BSA A65T (single carb version) and when it was working well (weather not dark, hot, or damp) it would really scoot.
 
That was my *second* motorbike: BSA A65T (single carb version) and when it was working well (weather not dark, hot, or damp) it would really scoot.
Cycle did a road test on the Rocket Three once. Rocket Three was this same engine with one more cylinder stuck on. Last paragraph was something like "So what to do if you're out riding and a Honda four comes along ? One more cylinder, no leaks, $500 less, real electrics ... I just twist the throttle and go past him."

They were actually pretty fast. Sounded good, too.
 








 
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