Awesome guide to modding Briggs 5 hp flatheads

#1
Ok , so this is a guide I ripped off of DIY go-karts forum and I thought It would be some good info to post here.
CREDITS TO oscaryu1 (from DIY gokarts) FOR WRITING THIS ENTIRE GUIDE!

Cheap power:

First of all, get out your dremel, and your little barrel like sand paper.

1) Port. Clean up any rough casting when it came out of the factory. Mine had huge amounts of this, just sticking out. Sand all that down, then round any sharp corners you see. Don't sand the seats though!

Do the same with the exhaust port. The intake can be a little rough, but the exhaust port should be much smoother.

Then, cut the eyebrows. (Pics can be sent for an example upon request). These are the bits of metal sticking inbetween the valve/seats, and the cylinder hole. By sanding these down, you increase flow by a huge amount. This isn't offset by the amount of compression you lost either. The amount of flow gained is great!

Just a tip on the porting - Don't go wild. You want to slightly port it out. Anymore, and you'll lose velocity. It'll have horrible acceleration, ect...

Rods:
Stock steel dippers hold at around 4200-5000RPM. The rod goes at 5000RPM.

A Raptor THREE (3) rod can hold up to 7000RPM. The dipper is integrated into the rod, so no worries there. A ARC Billet connecting rod or any other can pass 8,000 with no problems. They can also withstand bursts of 10,000RPM.

There are many combinations of rods. The stock length Briggs 5HP rod is 3.875". There are much longer ones too, such as 4.475", ect. These are rods made for special pistons, Wiseco for an example. Longer stroke, "shorter" piston.

EDIT - If you want to stick with the stock rod, go the safe route and get a nylon dipper. This will only prevent dipper breakage, it will NOT prevent the rod from breaking at 5000+.

Heads:
You want a good flowing head. Not a high compression head, as those lack flow. Have you heard "Put on a 4HP head! It gains huge amounts of compression!" WRONG. The amount of flow lost offsets the amount of power gained. Stick with the 5HP head. Get a Raptor BluePrinted head for $30 on eBay, and if you get a milled head, or one milled yourself, don't ever buy one or get one milled over .030. Any more than that, and again, flow starts to decrease. You won't notice any more speed from this, but more torque and power in the middle end.

An easy way to spot these heads are by the numbers on the heads. If the head has a "10" on the top, it is a 4HP head. A 5HP head will have a number "13".


Cams & Springs:

Ahh cams... the best way to gain power. As you probably already figured out, more lift = more power. However, not always. You have to focus on other things, such as duration. To make it short, the most popular cam sold today is the 94ss "Cheater" cam. They retail around $60. There are many more, but the 94ss is the most popular cam around.

By getting one of these cams, and revving high, you'll need better springs and retainers. You could get a whole new dual spring set, with retainers and keepers, or just new springs, with hardened retainers.

Any performance cam will do fine. The only thing difference, is the higher the lift, the heavier the springs will need to be. Also take in mind the heavier the springs, the more power it will suck. Getting a [for example] .265" lift cam, and using springs for .600" lift or so, will be a huge waste.

The 94ss cam is recommended by me, but any other cam do. Just take in mind you may have to cut (relieve) the valve area of the head if you go too high lift of a valve. Now to 2 terms you'll probably encounter:

Coil bind - When the coil cannot be compressed anymore, but the engine wants it to... (basic definition), = parts break.

Valve floating - Valve float is an adverse condition which occurs when the poppet valves on an internal combustion engine valvetrain do not remain in contact with the camshaft lobe during the valve closure phase of the cam lobe profile. This reduces engine efficiency and performance and potentially increases engine emissions. (Wikipedia)

In other words, the valve does not fully close in time when the piston comes up, or the valve starts "bouncing". This can cause (on OHV engines) bent valves. On flatheads, it will just not rev anymore. Either way, you'll notice it when you start revving higher.

Valves:
You want light, and good flowing valves. I'm guessing you have seen those "blueprinted" valves on eBay. Some people says it makes no difference just that it lightens your wallet, while others say that it makes a good gain. The truth is, well it can be both. Poorly "blueprinted" valves can yield no gain, while quality BPed valves can see a gain. So don't go buying $10 BP valves from a random auction.

Valve lapping - The most common action done when wanting the valve and seat to have a good seal. You start off with a rough compound, and then either (for water based ones) add water to get a smoother compound, or use the "smooth" compound already supplied. You will usually not need to use a drill to do this. Doing this too many times is also hard on the seats.

3/5 angle valve seat cutting - The "race" cut. Many motorcycle shops or other shops do this. Many people say they saw a bigger gain with a 3 angle cut than bigger valves. If you can get someone to do this cheap, go for it! Usually they're "Competition cuts" or something on that line.


The Block...:
The very last thing you want is a worn or cracked block, no? The most common place for breakage is the lifter area. The exhaust lifter area to be more specific. To prevent this, due to high lift cams and such, many people weld more material on to the lifter area to increase strength. Which leads me to another area, lifters. Try and get your hands on BILLET lifters. They're cheap. I'm going to grab 3 sets today for $5. Stock length. Longer ones can be found (which aren't needed unless you have a special cam) for other prices.

Remember, you want valve lash! You don't want to end up with a burnt valve. You can grind the lifter a bit to increase lash.

Aluminum Bore - Also known as the "Kool Bore". These blocks are very forgiving during the normal use.

EDIT: I was incorrect, Kool Bores do NOT have a sleeve - these are only to I/C motors (steel/cast iron sleeve motors). Kool Bores are popular, cheap, but wear down and must be re-sleeved if you wish to re-use it after you've gone through all the "oversize" sizes.

Steel Bores are loved for their durability, lack of using a chromed piston, ect. They generally come with bearings, unlike Kool Bores with bushings.

Another thing to take into mind, aluminum bores (Kool Bores) require a chrome plated piston. It isn't the chrome like you see on mufflers or grills. You can identify this by (Briggs and Stratton pistons only, 5HP only) a 4 on the bottom of the piston. A 4 indicates chrome plated, while a 6 indicates it was made for a cast iron sleeve. Kool Bores also wear faster, and do not last long in racing. But honestly, as long as you keep up with oil changes, don't go throttle crazy half the time, you'll be fine.

Cast iron sleeves - Also known as "I/C" engines, or Industrial/Commerical engines, have a cast iron ("steel") bore. These can use both the non chromed or chromed piston (4 AND 6). These CAN be honed and bored, but recommended not past the sleeve, for the reasons mentioned above. These are "heavier duty", and last longer than the Kool Bores. These are the most common block found in 5HP flathead karting.


Crankshafts:

As you know, bushing cranks are usually what most people have. These will handle fine up to around 7,000, where problems start occuring (oil starvation most likely).

And as you have seen, there are cranks with bearings. These cannot be easily removed, and are not slide on-side off bearings. These are pressed into the crankshaft, and require special tools to remove. They come in 2 standard sizes, the bigger ones, and the smaller ones (how easy it that? ). If you wish for a bearing crankshaft, you will need either one with a big/small bearing, and a side cover that corresponds to your bearing size.

Flywheels and Coils:
Cast iron flywheels are the most commonly used ones to date. And also the most dangerous. Never spin one over 7,000. Over 6,000, they are the danger zone. People have spun the Clone's flywheels at 6,500. One's shattered at 5,800.

Billet flywheels save lives. ARC has (JMO) the best. The Clone one has been lab tested to take 12,000+ RPMs. This can probably be said for the 5HP Billet ones also. These are around $200 shipped. Also I'm experimenting with a much cheaper alternative. ($50 haha!)

Coils. I'm hazarding you've seen HOT Coils around? Again, people have had positive experiences with them, and people with no differences. IMO, the stock coil is fine. All you need is a spark to get the mixture to ignite.

I've given most of my learned information to you. Use it well. Hope this helped you in something!
 
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#4
Wow, that just makes me want to run up to my garage and get into more trouble with my wife. :grind: Thanks for the post It will come in handy for sure.
 
#7
A pretty comprehensive guide to a nicely modded 5hp, the standard of the industry's workhorse as minibikes go! As well as but not limited to go-karts and lawn and garden, concrete equipment etc. There are still a decent supply out there if ya look, and there is much support! Such as your fine guide here! Thanks for sharing your method with us! It is straightforward and makes sense, like it should! Great work!:thumbsup:
 

Neck

Growing up is optional
#8
One thing that wasn't touched on was the Briggs aluminum flywheels, like what comes on the vertical shaft engines. Would they make the engine accelerate quicker? require faster idle speed? Are these any good for extended RPM's
 
#9
One thing that wasn't touched on was the Briggs aluminum flywheels, like what comes on the vertical shaft engines. Would they make the engine accelerate quicker? require faster idle speed? Are these any good for extended RPM's
If the aluminum flywheel came off of a lawnmower's vertical shaft, then you cannot use it. Vertical shaft lawn mower engines use the grass cutting blades as second flywheels. The aluminum flywheels off of lawnmowers will not work because they are to light.
 
#10
If the aluminum flywheel came off of a lawnmower's vertical shaft, then you cannot use it. Vertical shaft lawn mower engines use the grass cutting blades as second flywheels. The aluminum flywheels off of lawnmowers will not work because they are to light.
NEVER USE ONE.ITS A TIME BOMB WAITING TO GO OFF.if you prefer,use the cast iron one 3 or 3.5 hp. with a stock unmodified engine,they'll last as long as you keep it under 7000 rpm.[completely stock engines won't rev to 7k anyway.]
 
#15
the 3 or 3.5 hp flywheel is a bit lighter than the 5hp "light"flywheels. the cast iron wheels[3-3.5] will make the engine rev quicker,but also not idle down as fast.the down fall of this is inertia, where the lighter flywheel may have difficulty keeping the engine running at idle speed,700-1000 rpm. most clutches engage at 1200-1500 rpm,so it may be hard using this wheel to keep the engine from engaging the clutch.
3-3.5 hp wheels weigh about 5lbs,most 5hp wheels weigh 5.5-7lbs.
 
#16
the 3 or 3.5 hp flywheel is a bit lighter than the 5hp "light"flywheels. the cast iron wheels[3-3.5] will make the engine rev quicker,but also not idle down as fast.the down fall of this is inertia, where the lighter flywheel may have difficulty keeping the engine running at idle speed,700-1000 rpm. most clutches engage at 1200-1500 rpm,so it may be hard using this wheel to keep the engine from engaging the clutch.
3-3.5 hp wheels weigh about 5lbs,most 5hp wheels weigh 5.5-7lbs.
Thanks for the info!
 

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