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Old 10-27-2011, 09:05 PM   #1
checcohd-1
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Default Big twin flathead motor question

HI,
i'll rebuilt my flat... or better... built my flat..
i have standard 1947 cases and rods and cams.. i need to buy cylinders and flywheels...
i'll buy T&O torquemaster stroker flyweels and
ULH 13fins cylinders..
i don't understand some thinks... what pistons i need .. i need stroke plates(whi?)... what is teh final volume.. and if is a good idea....

somebody can help me and teach abput flathead... is my fist flat...
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Old 10-27-2011, 10:15 PM   #2
Rubone
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Default Re: Big twin flathead motor question

If you use stroker flywheels with the correct stroker pistons no plates are necessary.
Why stroker flywheels? Standard 74 and 80 engines work fine. Depends on what you want to build it for! Flatheads are not street racers!
Robbie
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Old 10-27-2011, 10:49 PM   #3
Flathead4Ever
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Default Re: Big twin flathead motor question

Isn't it true that the stroker pistons have shortened skirts, to clear the flywheels at B.D.C.? I have read that this can significantly shorten top end life.

I have had the chance to compare my stock Harley U (74") to a stock Knucklehead EL (61"). I was a bit surprised that the Knuck didn't seem that much faster...

Like Robbie said, depends on what you want. The stock U is no speed demon, but feels like it's loafing at 60 mph. I assume that higher compression UL heads might give a little more oomph.

Good luck with your build. I love my U.
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Old 10-28-2011, 12:21 AM   #4
flathead howie
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Default Re: Big twin flathead motor question

i am running the stock flywheel and and stock bore on my 80" flathead and like anyone will tell ya it's not a racer and she could really use a 5th gear but it makes for a real nice cruiser when you are in no rush to get anywhere. if it is speed you are after... sell your flathead hahaha good luck on the build... throw up some pics... let's see the progress pictures!!!
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Old 10-28-2011, 12:23 AM   #5
flathead howie
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Default Re: Big twin flathead motor question

if you need to buy new cylinders and all that you should check out www.45flatheadservice.com.... the only place i have found new 80" cylinders... hope you can find what you are looking for at a decent price!
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Old 10-28-2011, 07:14 AM   #6
checcohd-1
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Default Re: Big twin flathead motor question

thanx...
ok i'll run standard flywheels...
and 80" cylinders..
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Old 10-29-2011, 09:58 AM   #7
Scott McKelvey
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Default Re: Big twin flathead motor question

i'll preface by saying that side valves are new to me and i'm not a motor expert either, but i'll share the following.

Got my first side valve on the road this summer. It's a UL that I picked up. The lower end was already put together with S&S 4 5/8" wheels and stock rods, but the top end was off. They guy used 80" STD pistons and cut just enough off the top so they were flush w/ the top of the cylinder. (he said this was .060). I'm not aware of any mods on the lower part of the piston. The baffles were removed from the front cylinder, but I can't remember if the specs on the wheels required clearancing on the cases, or whether this was just someone's choice.

I wasn't sure if there was a real reason the motor wasn't completely put together, so I went through everything, and mathematically it seemed like it should work, except for piston to head clearance, so I milled a bit out of the head to get back to factory spec for clearance. crude calculations put it at like 6.8:1 compression.

It runs really strong, more so than I expected. But since it's my first, I don't have anything to compare it to, yet. I have a 40U that I'm close to firing up, so I'm interested in comparing the two. The stroker is fun to ride, but unless there's a dramatic difference (which I don't expect), I wouldn't think it's worth the extra work (I'm not racing it after all). Consider the following:
  • using stroker plates will jack w/ your intake manifold
  • I've seen nothing at this time in the way off off the shelf stroker pistons (if someone knows of some, please share), which means you have to get creative
  • case modifications, if necessary
  • there are inherent cons to increasing stroke, even if negligible
  • for me, one last consideration was tuning w/ the M-51 w/ the increased cubes. Since there aren't other manifolds available (of which I'm aware), another hurdle was to adapt another carb that I felt allowed more options and ease of tuning.
If it were me and I was starting from scratch, i'd do stock wheels, 80" pistons/cylinders, hi compression heads, and porting; call it a day. but that's just opinion based on my limited experience, so take it for what it's worth.
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Old 10-29-2011, 10:06 AM   #8
checcohd-1
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Default Re: Big twin flathead motor question

thanx
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Old 10-29-2011, 09:59 PM   #9
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Default Re: Big twin flathead motor question

when you run your engine you should always throttle up and then let off never hold the throttle open for a long time you will burn your pistons
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Old 10-31-2011, 11:42 AM   #10
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Default Re: Big twin flathead motor question

I may not remember this correctly but using the stock wheels and reboring them for the step crank pin gives you a 1/4 inch more stroke and don't bore your good 11 fin (74") cylinders out to fit 80" pistons, you're cyclinder walls will be to thin.

By the way Paul Friebus at American Cycle Fabrication, Inc. says he does the step pin thing all the time and who knows more about flatheads then he does.

Mutt
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Old 10-31-2011, 01:29 PM   #11
Rubone
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Default Re: Big twin flathead motor question

If re-boring UL flywheels for the '41 type OHV pin the only way the stroke is increased is by boring them OFFSET. Otherwise the stroke stays the same as the centerlines of the shafts do not change. When bored offset the thrust washer faces in the wheels become offset to the pin. It is cheaper, easier and stronger to use T&O flywheels. Robbie
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Old 11-03-2011, 12:58 PM   #12
checcohd-1
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Default Re: Big twin flathead motor question

there is any company taht makes pistons for flathead stroker applications?
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Old 11-03-2011, 02:27 PM   #13
jerseymutt
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Default Re: Big twin flathead motor question

Quote:
Originally Posted by Rubone View Post
If re-boring UL flywheels for the '41 type OHV pin the only way the stroke is increased is by boring them OFFSET. Otherwise the stroke stays the same as the centerlines of the shafts do not change. When bored offset the thrust washer faces in the wheels become offset to the pin. It is cheaper, easier and stronger to use T&O flywheels. Robbie

Rubone,

Easier and stronger to use T&O wheels I would agree with. I don't know what the machine work is worth. I have talked to Paul about this and he says he does it all the time when he rebuilds U's so maybe he is set up for it and it doesn't take him very long. There is also a right up about doing this in one of the Easy Riders Tips (4 book set) book. The new hole is started at the bottom of the old. This moves the pin out 1/8 of an inch giving a 1/4 inch stroke. Givin that U flywheels are 63 years old or more T&O might be the best bet.

Also the corvair piston (someone correct me if I am wrong) was the piston that was used for stroker U's but I am told they were not a good choice and didn't hold up. I think on the S&S / Flathead power site there is a pretty lengthy discussion on this subject and a search of the archives there might be a good idea. Again I am no expert.

Mutt
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Old 12-15-2011, 02:27 PM   #14
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Default Re: Big twin flathead motor question

If your going to build a UL stroker engine , you can use stock chevy corvair pistons .Make sure your cylinders are bored to 80ci . Buy the early model corvair pistons .

I have a real fast UL . 84 inch , 3/4 cams , port job , aluminum HC heads , etc etc.
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