Board index Flathead Power-Technical Questions, Answers, and Suggestions 45 Flatties Increasing Size.. 45 to 74 ?

Increasing Size.. 45 to 74 ?

Moderators: Curt!, Pa

Post Tue Jan 25, 2011 7:34 am

Posts: 14
Can the 45 be bore and stroked to a 74? Or the 74 and 80 BT use different cases?

Post Tue Jan 25, 2011 9:52 am

Posts: 1668
Location: Interlaken, NY USA

The 45 can be taken out to about 63.5 cu in. 5 inch stroke, .100 overbore. That's with commercially available parts.
Frankie is currently 60.7 cu in with 4 13/16 stroke, .090 overbore. And yes, proof there is no substitute :D
Here's the chart with all the info:
http://victorylibrary.com/tech/ci-45.htm
Courtesy of our erstwhile friend Panic.
On the big twins, there's no reason to not use the standard cases, they're good for 150mph with the right stuff. And anyway, using anything else would be a hell of a job, seeing as the cams have to line up with the valves. Not going to happen on your run of the mill OHV case.
DD

Post Tue Jan 25, 2011 12:32 pm

Posts: 1646
erstwhile friend? :?

what effect does boring and stroking a 1930s sidevalve engine to that extent have on reliability, though?
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Tue Jan 25, 2011 1:25 pm

Posts: 1668
Location: Interlaken, NY USA

You know me, I won't throw mud at someone who can't answer for themselves :roll:
As for age, I just look at that 30's cast iron as well seasoned. Rumor has it, you know, that the factory used to cast up cylinders, then throw them out back for a year or so and let them go through a couple of Wisconsin winters before bringing them in. then they machined them to size and sent them out the door.
Frankie first went together back in '92. I just rebored from .070 to .090 due to my own stupidity. Let it run low on oil.
Surprisingly, it has been very reliable. Never left me along side of the road yet.
The only thing I've avoided over the years is winding it up and dumping the clutch from a dead stop. Other than that, It's been pounded plenty. :lol:
Let's face it though, although a hot flathead, it's still only about 42-44 Hp. I guessing at this as it always ran neck and neck with my buddy's Clubman's Goldstar.
The point being, not a lot of power to feed into basically a Sporty motor.
DD

Post Tue Jan 25, 2011 3:56 pm

Posts: 1646
I take it that panic has left the forum for reasons of his own?

have always reckoned that the original, iron XLCH was good for about 50-53bhp at the crank, 40-plus at the back wheel, as opposed to the British 650cc twins which generally make about 40bhp at the crank in original order. Goldies are quoted as anything up to 50-bhp but I would say it's nearer 40 in reality. Sportsters are heavier than British twins and that's where the difference in power goes

My 2-valve Jawa makes a genuine 50bhp at the crank with no special tuning, and it will knock any Gold Star into a cocked hat; even the 600cc, methanol-burning ones in so-called Pre-65 motocross struggle to keep up with the Jawas. 500cc JAPs make a genuine 45 bhp and they run away from the Goldies and Velos at vintage sprints.

I take your point about the actual power output but I would be more concerned about local heat build-up in what must be very short piston skirts and very thin castings, and lubrication issues. The 74" and 80" flatheads suffered from this, and my old M20 used to produce temperatures that I've never seen on any ohv engine, even a Squariel; the oil in the tank would be like tar at the bottom and yellow froth at the top after an hour or so, assuming the exhaust valve hadn't stuck that is.

however if you have a bike that's been running reliably in that state of tune for a fair few years, that's the proof of the proposition
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Tue Jan 25, 2011 4:22 pm

Posts: 1668
Location: Interlaken, NY USA

You're probably right about the Goldie's output, you've had more experience leaving them in the dust :D I had one years ago and sold it off, it wouldn't take the thrashing I gave it. However, surprisingly, my M20 did! I always rode that flat out, (you know, you have too!) and didn't have the exhaust valve issues most do. Think I replaced one maybe the second year I had it? 1970,71? I seized the piston once drafting a truck to get a little extra boost. Got too close, not enough air. Did a lot of drafting with that bike. She'd do about 65 if the truck in front would. :lol: You're right though, it runs at least as hot as my Squariel. That M20 sold me on flatheads though. Compared to the Goldie, stone reliable.
Panic was banned from the site, guess you didn't know. One of the threads got out of control.
He's over at the hydraglide site as kitabel alot these days.
Like the bit about the sea, a little culture never hurts.
DD

Post Tue Jan 25, 2011 7:34 pm

Posts: 3123
Location: Central Illinois, USA
I have never performed an extreme overbore on a 45" HD, but the limits can be guestimated easily by measuring the remaining wall thickness at the thinnest spots.
CYLWALL.jpg

If an OHV and a UL can survive in good health at .070" remaining wall thickness (approximately .155" overbore for an EL, .100" for an FL), then a 45" shouldn't suffer at .050".

The critical aspect is that the cylinder bores must be fitted while stressed with torque-plates.
I never made any up for a 45".

The hardest part would be to find as large of pistons as the cylinders would allow.

.....Cotten
You do not have the required permissions to view the files attached to this post.

Post Tue Jan 25, 2011 8:59 pm
Pa Site Admin

Posts: 5774
Location: Ohio USA

I could work around the top end but the bottom end would need a bunch of reinforcing. IMHO Pa

Post Wed Jan 26, 2011 2:25 am

Posts: 1646
65mph on an M20 is good going! Power output is quoted as 13bhp in some sources, which would be consistent with the 23bhp quoted for the 45, given the relative weights of the two machines.


Goldies can be made to go very fast but it costs in terms of reliability. The US dirt-track ones were probably the fastest of them all in terms of sustained high rpm, but they needed constant major attention and also suffered from transmission failures on a regular basis, not a problem encountered in European-style road-racing, although the last major success for the BSA singles was in 1971, in endurance racing as developed by Mead and Tomkinson. Velocette also had some success in this field, hence the "Thruxton" model name.

The BSA B50 also went on to become the CCM engine, which remained competitive in motocross for a good few years after the demise of the original factory.


talking about oversize pistons, there was a version of the BSA C15 ( generally referred to as a "three-fifteen" ) sometimes seen on grass-track racing. It used the 71mm piston from one of the Triumph twins to give 325cc on the original stroke, and was faster than the B40 model despite giving away 20-odd cc. It was quite reliable, mainly because virtually all such machines have roller-bearing crank conversions anyway!

There were various attempts to stroke Goldies for pre-65 motocross using M21 wheels, but they weren't a great success and the current ones use a special crank


I didn't know about panic.
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Wed Jan 26, 2011 8:25 am

Posts: 1668
Location: Interlaken, NY USA

Pa, I think the only obvious weak point would be the rods/crankpin. Using sporty components would take care of that. The cases themselves I think are up to it. The castings are of similar thickness to big twin, and of smaller overall dimensions, which should add to rigidity.
Cotten! What is that cylinder! The wall looks 1/2" thick. I've seen the Brits do that on their old 350 singles, by using the cylinder of a 500 and underboring it.
Nice to see you can improve your tools for "small change" :lol:
Speaking of which, having gone as far as shown in the picture, how do you "capture" that dimension for a definitive readout of wall thickness? That process has stumped me with the tools at hand.

By the by, Panic's chart lists displacements for pistons up to .150 over, but no source for them.
DD
Last edited by Frankenstein on Wed Jan 26, 2011 9:15 am, edited 1 time in total.

Post Wed Jan 26, 2011 8:57 am
Pa Site Admin

Posts: 5774
Location: Ohio USA

Thanks Doc. I see your points.

Cottens use of the hand calipers indictates he has a good feel/ touch, so to speak. Same feel when using micrometers is needed when using those tools. Instead of transferring the anvils of the caliper to a scale, you can transfer the anvil feel to ID mics, gauge rods or gauge blocks, to get a pretty accurate reading. Those are the types of measuring tools the first engine builders only had available for measuring. Stean ebgines and drives wre built. The first combustion engines also. Even the first micrometers !

Post Wed Jan 26, 2011 9:17 am

Posts: 1668
Location: Interlaken, NY USA

Duh! I never even thought of the inside mics! In this case though, when we're only talking about maybe .1 in or so, you'd substitute feeler gauges? Thanks, Pa
Dumb DD

Post Wed Jan 26, 2011 10:31 am
Pa Site Admin

Posts: 5774
Location: Ohio USA

Feeler gauges....anything as long as you have a good feel with them Doc. Even graduated micrometers are only as good as the users feel.

Post Wed Jan 26, 2011 10:52 am

Posts: 228
Location: Burlington Iowa USA
.100 over pistons are avalible for stock stroke. .125 over and up Indian Scout pistons can be used. The spiqouts start getting real thin but bore is good.

Post Wed Jan 26, 2011 12:55 pm
Pa Site Admin

Posts: 5774
Location: Ohio USA

I had a set of nos oem .070" oversize 45 pistons once. They were manufactured for servicar and were in amf boxes.

Post Wed Jan 26, 2011 1:11 pm

Posts: 1668
Location: Interlaken, NY USA

Pa, it suddenly hit me when I got notification of a new post, "Size Increase", you might want to consider having the thread renamed :lol: :lol: :lol:
Then again, it may just be my sleazy, evil, decrepit, mind at work.
DD

Post Wed Jan 26, 2011 1:26 pm

Posts: 1646
I have some +0.070" OEM pistons somewhere around.

the cases look plenty strong to me and Sportster rods and crankpins do, after all, handle Sportster power output, which is more than any 45 will ever produce. Provided the local heating is manageable - and I have seen plenty of M20 barrels with marked to severe heat-induced corrosion, but few 45 barrels in the same state - and the lubrication is up to it, I don't see why the cases shouldn't be ok


the quote is from Conrad, "Lord Jim". I had to abridge it a little to fit into the space available....
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Wed Jan 26, 2011 2:54 pm

Posts: 14
Frankenstein wrote:
Pa, it suddenly hit me when I got notification of a new post, "Size Increase", you might want to consider having the thread renamed :lol: :lol: :lol:
Then again, it may just be my sleazy, evil, decrepit, mind at work.
DD


I meant displacement... :oops:

Post Wed Jan 26, 2011 7:12 pm
Pa Site Admin

Posts: 5774
Location: Ohio USA

Frankenstein wrote:
Pa, it suddenly hit me when I got notification of a new post, "Size Increase", you might want to consider having the thread renamed :lol: :lol: :lol:
Then again, it may just be my sleazy, evil, decrepit, mind at work.
DD


Naw....we'll leave it as is Doc. The topic will confuse the googling wish they hads. :lol:

Post Wed Jan 26, 2011 7:24 pm

Posts: 111
I have a set of wldr cylinders that some old racer bored to 30 over sport scout. do the math on that. and they stil have spikets.

Next

Return to 45 Flatties