Author Topic: 12 hp flatty cranks  (Read 4611 times)

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Offline MowJoe

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12 hp flatty cranks
« on: April 21, 2009, 08:08:18 am »
Hi all I'm new at this engine building stuff but have been working on engines all my life. My question is I'm building a 12 hp flat and the crank I got I was told was a non syncro 12 hp but after checking the stroke in the 12 hp block it has a shorter stroke and the piston don't come all the way to the top. Is it possible to used a different rod or piston to make this work or should I just use it in a 11hp block. Any help would be great I don't have a bunch of $$ to do this build but was hoping to make something work.
PS. The class I run in all parts have to be stock B&S. π
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Offline wardee1977

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Re: 12 hp flatty cranks
« Reply #1 on: April 21, 2009, 11:48:55 am »
what did you take the crank out of, an 11hp?.  I don't think that will work in a 12hp 28ci flatty.  If you got a complete 11hp, I would just run that.
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Offline MowJoe

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Re: 12 hp flatty cranks
« Reply #2 on: April 21, 2009, 02:45:56 pm »
I got the crank from a lawn and garden repair shop I used to work for it was on the shelf of used cranks marked 12 hp.  I think I will just build a 11 hp I have a bunch of them.
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Offline Rooster

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Re: 12 hp flatty cranks
« Reply #3 on: April 21, 2009, 03:02:40 pm »
There you go, blow them 25's up figuring out how to build one then go to a 28! Isn't what Briggs made those for, practice?
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Offline MowJoe

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Re: 12 hp flatty cranks
« Reply #4 on: April 21, 2009, 04:42:43 pm »
Ya I guess that's why I have so many kicking around for practice. Do you know of a non syncro crank that will fit my 28? I would much rather run that but I'm kinda running out of time 1st Race is on May 2nd and I still have to finnish the mower. 
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Offline wardee1977

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Re: 12 hp flatty cranks
« Reply #5 on: April 21, 2009, 05:45:39 pm »
I'm not sure, but I read maybe somewhere that the old 12hp's didn't have a syncro in them.  Don't take my word for it.  You might as well dig in to one of those 11's you got instead.  You don't have much time by the sounds of it.
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Offline MowJoe

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Re: 12 hp flatty cranks
« Reply #6 on: April 25, 2009, 08:16:43 pm »
Got my engine done and mower finished today and took it for some easy laps to check steering and other things . I have to say the 25ci don't sound to bad I think it will work for now while I work on my 28ci. I have a question can someone tell me the difference between the 25ci and to 28ci other than the stroke I already found that out. Thanks I been learning a lot on here.
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Offline Rooster

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Re: 12 hp flatty cranks
« Reply #7 on: April 25, 2009, 10:59:50 pm »
Pics?
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Offline Snowman18

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Re: 12 hp flatty cranks
« Reply #8 on: April 26, 2009, 12:39:37 am »
 Well, you already know that the crank is different (longer stroke), so to accommodate...the 28 has a taller deck, a slightly more open head and one extra bolt in the oil pan also most of the flywheels are two piece (separate plastic fins). The worst thing is the fact that they never made it a horizontal. I almost forgot, longer valves because of the taller deck height and the tins are different.
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Offline MowJoe

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Re: 12 hp flatty cranks
« Reply #9 on: April 26, 2009, 08:18:34 am »
So I see there is not a huge difference. I used the 28ci cam and did a little porting on the intake and exhaust I also used the 28 ci head after trying the 25 ci it feels like it runs better with the 28. I also chamfered the
block from the valves to the piston.  I will post some pic's of it when it's painted .  I have to tell you this when I first put it together and started it the magnet in the flywheel came out and busted my coil and destroyed the blower housing and that was at a idle of about 1200 rpm,s  so I'm pushing for our club to let us run the ARC flywheels. They say they want to keep the cost down but I say its a safety issue I have seen a lot of flywheels blow up in my 20+ years of working on small engines and it's not a good thing.
J & K Yard & Sport, and used parts emporium.
Home of Mowjoe racing
Former President of the Barry County Grass Clippers
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When the green flag drops the Bull stops!

Offline MowJoe

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Re: 12 hp flatty cranks
« Reply #10 on: April 29, 2009, 03:42:09 pm »


Well here it is my first build it runs out real nice maybe not the fastest in the bunch but it handles like it's on rails. I will continue to work on it through the season to get it where I want it but it's a start
J & K Yard & Sport, and used parts emporium.
Home of Mowjoe racing
Former President of the Barry County Grass Clippers
2010 Sportsman Points Winner W/ Grass Clippers
2011 Mod-x Points Winner W/ Grass Clippers

#55 JD 111 GPK
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#67 Snapper Mod-x/CP 

When the green flag drops the Bull stops!