Flathead 45 Solo Speed?

How fast are these bikes in stock form?

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Flathead 45 Solo Speed?


Post by RooDog » Mon Jun 10, 2019 1:12 pm

Now, I don't want to open a can of worms here, but having no personal experience with the little Flatty I would like to know, for general knowledge, just what kind of sustained cruising highway speed can one expect from a stock solo commercial W series 45" Flathead. Max top speed of same, and what have you guys actually gotten from your hot rodded versions. Not interested in Bonneville specials, just what y'all are doing in the real world. I know what big twins are capable of, just wondering about their little brothers.

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Re: Flathead 45 Solo Speed?


Post by kitabel » Mon Jun 10, 2019 2:06 pm

My earliest attempt in 45 tuning, 40 years ago, speed equipment noted *.
Chassis stock weight w/o front fender, no windshield, bags, or crash bars
34 tooth engine sprocket
stock WL 120-36/361 cylinders (no porting , relieving etc.)
stock military pistons
stock (very loose) deck clearance
* KNS big intake valves
stock #5 aluminum heads
* open exhaust
* 34mm Mikuni VM on the stock WLA manifold
stock breather timing, cams, valve springs, ignition

Timed over 90 mph by Suffolk County State Police

What I would have done (if the owner had more money and I was more experienced)
auto-advance breaker
* intake cams
* stronger valve springs
* extended breather timing
* relieve cylinders
* modify valve shapes
* radiused intake manifold
* milled cylinder deck to close quench to .035"

Easily 100 mph

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Re: Flathead 45 Solo Speed?


Post by liberator » Mon Jun 10, 2019 6:34 pm

According to 'Harley-Davidson WLA 1941-42 service manual' page 1; "...top speeds obtainable are around 85 m.p.h. and the machine can run as low as 2 or 3 m.p.h. in first gear...".

My stock (M88,30 teeth sprocket,19" ural rims) fully laden type VII is capable of doing a genuine 70, but doesn't feel comfy in doing so..
My '41 type II (M88, 32 teeth,18" rims) does 75 BUT...isn't as heavy as my '45 ex-russian steed.
She feels much 'crispier'..

That said, I don't push both of them beyond 55-60, where they're very happy to stay all day long.

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Re: Flathead 45 Solo Speed?


Post by Blind Melon » Mon Jun 10, 2019 11:43 pm

As I recall, my 1946 WL was rated @ 68-70 mph with as delivered gearing... They would actually go about 75 mph with no wind and flat level road way near sea level...With stock gearing.. Change the trans sprocket and you could go a hair over 80 mph.. With a BSA gearbox and raised gearing my 1946 would just touch 100 mph if you waited long enough and had a slight tail wind... So, my '46 was balanced had stock cams, dual mufflers, polished valves, relieved cylinders... Nothin' fancy.. The gearbox, hand clutch, and me weighing less than 180 made it all work.. The era was 1970..

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Re: Flathead 45 Solo Speed?


Post by kitabel » Tue Jun 11, 2019 1:01 am

The unfortunate part is that with this tall gearing (which is needed for low RPM cruising and high top speed) the "crawl ratio" (1st gear take-off power) is poor. The cure is a big twin 4-speed ($$$ + special frame) or a British box ($$$ + fabrication), some of which have a more useful 1st gear (2.70:1 etc.).

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