Briggs Flatty Low-Buck Buildup

Decided to spend my afternoon doing some engine work. I'm taking a Briggs 12hp flathead (281707) and build it into a snotty little single, to plop in a speed machine for my son. The idea is to have something that's going to perform better than stock, without breaking the bank with a bunch of speed parts. DIY is the key word here.

Having torn everything down, I got to work on the block and head today. I began by decking it (perfecting the head-block mating surfaces really), according to the info provided in theis lovely little how-to written by my good friend Rusty. https://www.atltf.com/t3005-eliminate-your-head-gasket I will indeed be running this without a head gasket.

Now- with a flathead, any time you deck it, run a thin gasket (or in this case NO gasket), you decrease the flow in the head. So, I've shaved the eyebrows, creating a flow path about 0.070" deep. This will more than make up for the loss of "headroom". It's not so deep as to risk popping a ring, I've still got room there.

While I was gettin' jiggy wid it, I honed the cylinder, mostly just to refresh it. While much of the original crosshatch was gone, the bore was in really good shape.

Dremel in hand again, I proceeded to start with the port work. It's mind-boggling how flawed these castings are. Amazing the damned engine could breathe at all. Lots of room for improvement here. Note that I did NOT increase port diameter, just carefully ground away flash, casting flaws, and sharp edges, as well as radiusing the corners. I still have a final cleanup of the ports to do, but I had to give it a rest. The flexi-shaft extension on the Dremel is light and manoeuvrable, but a couple hours on a Dremel is still gonna cramp your hand. So, port pics when I'm done with them.

I will also consider port-matching the intake side, and possibly dealing with the exhaust port threads. I'm more likely to build the exhaust using a bolt-on flange, and those threads ain't nothin' but trouble for flow. I may fill the threads with JB weld and then grind smooth. If I simply grind the threads off, the port will end up too large and reduce exhaust velocity.

Sounds like we'll have some cool pictures to look at. Wouldn't jb weld melt/burn out if you used it in the exhaust port? I have heard of people using it in the intake ports to match them up for different carbs. I guess it's pretty rugged stuff.

I'm hoping it holds, but I'm not incredibly concerned. If it lets go, it's being blown AWAY from the engine. That's okay by me...

On further thought, I'm leaning towards threading a 1" pipe nipple in, cutting it flush, and radiusing the edges.

Doc Sprocket wrote:

I'm hoping it holds, but I'm not incredibly concerned. If it lets go, it's being blown AWAY from the engine. That's okay by me...

On further thought, I'm leaning towards threading a 1" pipe nipple in, cutting it flush, and radiusing the edges.

Right there is the way to do it. I did the same when I didnt have a setup where i needed the thread. To get it out, just take a die grinder and slice a notch and go around it with a chisel to loosen it out. Definitely are running ungoverned correct? I'd recommend finding yourself an OHV Rod for cheap, or a used arc rod. They tend to want to go.. E3 plug is the way to go as well, I think I got a E3.10? Nothing will make your engine roar more than a cam and carb work. Keep your valve springs stock or close to it so you dont actually blow your wheel at 6500k, or watch it close with a good tach.

TroyBilt Pony wrote:

Keep your valve springs stock or close to it so you dont actually blow your wheel at 6500k, or watch it close with a good tach.

Umm- I'm guessing you're not quite aware of the fact that I wrote this article, then- https://www.atltf.com/t2575-governors-flywheels-and-an-internet-full-of-crap

LMAO- but thanks for your input!

I will be tweaking the governor up to around to begin with. After that, we'll see.

Should be an interesting build. I look forward to seeing it as it progresses

Hey Guys, T-dub13 newby here.  I'm following this thread because I'm doing a very similar build as this one.  I can't decide on a connecting rod though.  In Murrayracer's thread about RPM's, he mentioned a 494504S rod.  Can anyone confirm that this is the rod to use from the OHV motor?
Looks almost the same as the stocker except smoother finish (possibly indicating forging instead of a casting)???
Also the same 28 series OHV motor that the 494504S rod came out of lists a 692419 rod after a certain date where it looks like B&S changed a ton of internals. (looking at a 287707 motor for reference).
Does anyone know if these rods have the same length and diameters?  I'd rather use the newer rod, looks much stronger...
Thanks and I'll put up a newbee post soon and show my tractor.
Stock Rod

494504S

692419

Sorry about the Hijack Doc!    
I got a little excited because we're doing the same build.
Anyway, I've pretty much confirmed that it is the 494504S that I want in my motor.  This is the one that was installed in the first gen 14.5 and 15hp OHV motors.  And is purportedly "Far better than a stock 12hp rod".  That's good enough for me.     
FWIW: I'm installing an ARC Dogbone too because I really don't want to hatch this motor.   

No worries! And if the new rod specs out, then great! Let me know how it goes.

I called all over the place looking for a 494504S in stock so I could measure it, but nobody had one in stock. So I just ordered one on ebay and I'm hoping it will arrive by Thursday since I've got the day off.

The stock rod measures: 1.250" on the big end, .400" On the small end and 4.750" Distance between centers. If you're measuring the inside-to-inside distance, it's about 3.725".

I will update the group as I find answers.

So Doc, I've got a carb question for this build / forum: I've got a two piece setup and a one-piece setup. I think I like the single piece old style carb because it has an "adjust-on-the-fly" low and high speed screws. I'm leaning towards this carb, but wondering if the two piece tilly would make any performance difference (or enough of a difference to give up adjustability). Any preference?

My preference is always the pre-EPA fully adjustable units. I have a precision drill set and am fully capable of doing my own jetting- but why do something the hard way if you have the choice?

I was hoping you'd say that!    I never really liked the non-adjustable two piece carbs.

So, on the old carb with the elbow, I may smooth out the inside radius. I also think I'll try to run two stock filters: Extend the studs and install one on top of the other. I will have to cut the top out of the lower one though.... Good idea?  

In theory, yes. More filter surface area= more flow. I think id rather go with an aftermarket reusable high flow filter such as a K&N though.

Oh no doubt, a K&N would be great! But I'm going extremely low-buck here. Meaning: If I have two stockers I can stack for free, then I'm trying it...  

Now my Billett Dogbone is stuck in Georgia due to some ice storm or something! And I'm expecting the Con Rod to arrive by Friday.... Fingers crossed!

I'm so bored here at work, I just want to go home and build a motor (or beat the dog outta that 6hp Quantum motor)   .

I guess I can do a little more build prep, like a little more work on the eyebrows. I didn't go as deep as Doc did with the cut. I'm wondering about the flame path hitting the side of the piston at TCD I guess. Any concern with this?

From what I can tell (research) it'll be okay. If I'm wrong, well- I have another block and piston... Experimentation, right?

Doc Sprocket wrote:

From what I can tell (research) it'll be okay. If I'm wrong, well- I have another block and piston... Experimentation, right?

Makes a nice lamp base if its not working   

Now there's a neat idea! I was thinking to make the PTO side cover of a Tec H40 into a clock.

Doc Sprocket wrote:

Now there's a neat idea! I was thinking to make the PTO side cover of a Tec H40 into a clock.

lol, ye why not  

Are you gonna make your own cam or buy one?

I don't think it will be a problem.  Hopefully it won't experience piston knock.    

I'm planning on running the stock cam.  I really want good low end grunt.  From what I've researched, the cams offered are tuned for mid to top end.  But I've only found two (I think) cams offered: G-team and somebody else, I forget the name.  I'd be interested in trying one, but that will be after the initial build.  I'm waiting on experimenting with timing advancement too.  Anyone have a cheap used cam?    

What cam are you going to run Doc?

Stock cam for now. If this engine ends up getting used for it's intended purpose (which will basically be Jr. beer-league racing) I can't lose any low end. This won't be and insane revver, either- so the stock profile is probably best. I may use the spare camshaft to experiment with altering the profile, a gentle grind just to make it a bit snappier in the top end.

Are all the stock cams the same? I've got cams galore. I was thinking the same thing about grinding down the "valve closed" part and combining with taller lifters to create more lift. There's an insane amount of math that can go into profiling a cam though... Hmmm, back to the 80-20 rule possibly?    

I might be short one cup of coffee this morning, but I'm not seeing how taller lifters are going to help anything. With lash correctly set, lift height is entirely determined by cam lobe height. If the cam lobe baseline is zero (valve closed) and the lobe peak height is 1/2" (example), then the valve will lift 1/2". Did I miss something?

Doc Sprocket wrote:

I might be short one cup of coffee this morning, but I'm not seeing how taller lifters are going to help anything. With lash correctly set, lift height is entirely determined by cam lobe height. If the cam lobe baseline is zero (valve closed) and the lobe peak height is 1/2" (example), then the valve will lift 1/2". Did I miss something?

He's talking about grinding the duration of the cam down, setting lash, and the taller lifters that make up for the added valve lash will provide more lift on the ramp, and peak, on the love of the cam.