How to Port & Polish, Shave Eyebrows: Briggs 5 hp Flathead Performance Mods

How to Port & Polish, Shave Eyebrows: Briggs 5 hp Flathead Performance Mods

Hey everybody, it’s KartFab here, and today
we are going over some performance mods for your briggs and stratton 5 horsepower flathead
engine. The two modifications we are going to do today are 1) shaving the eyebrows, and
2) porting and polishing the intake and exhaust. So let’s go check it out. (shave eyebrows)
So what does it mean to shave the eyebrows on a briggs and stratton 5 horsepower flathead
engine? Well, basically, all it is is removing material between the intake valve and the
piston, as well as the exhaust valve and the piston. What that will do for you is that
it will actually increase your flow so you can have a better performing engine. The downside
of that is that it actually decreases your compression a little bit. You can overcome
that through some things we are going to look at in another video later down the line. The
first step is to put on your head gasket and mark where you want to have your eyebrows
shaved. It is very important to shave only where the headgasket does not touch, and that
you just do a gradual slope. Your tools that you need to have is a rotary grinder. I just
have a dremel with a sanding disc, and a polishing disc. Go very slow, just making a side to
side motion, and you only want to slope up to the top of the cylinder. You do not want
to cut out where the cylinder is or you might ruin your block. You do all the grinding first,
vacuum it up. That is the initial grind with the heavy grit. Then you put on a finer grit
or a higher number grit sanding drum. You just do the same thing. Smooth it all out.
The idea here is just making sure everything is smooth. The final step is taking the polishing
disc, and making sure everything is super smooth. You can do this with or without the
valves in it. I did it with the valves in place because I am going to be replacing them.
You don’t want to run into the valve seats. That is it for shaving the eyebrows. When
we are talking about porting and polishing, we are talking about intake and exhaust. The
engine, when it is built, has castings in it that need to be knocked down. I am not
going to do a very strong port and polish job. It is going to be very mild just to knock
down the castings. The tools I used to port and polish the intake and exhaust were basically
some bits for carving wood. A sanding drum, and a polishing drum. I also used tape to
tape everything off so you don’t have all this junk going into the engine. So this is
the exhaust. It has all the junk (castings) in it. The intake. All these little bits of
casting and sharp edges are protruding. As I mentioned, you don’t want to have stuff
go into the engine. I just broke some q-tips in half and shoved them down the valve guides,
and put a rag in there as well. I used this bit here to grind out the casting . I roughened
up the intake, polished up the exhaust here. This is just for demonstration purposes. I
used this bit mainly to rough out those sharp edges there. Just remove the casting burrs
and edges. This is the exhaust before and after. This is the intake, before and after.
You notice I removed just a little bit of the burrs. Kinda filed that down. You don’t
want to run into the valve seats when you are grinding. That is very important. This
is the intake here, make sure it is not super smooth. That is it for porting, polishing,
and shaving the eyebrows. Hey everybody, thanks for watching this video. If you like it, please
give it a thumbs up. Comment below. Let me know in the comments if I did a pretty good
job. This is the first time I have actually ported and polished a small engine, as well
as shaved the eyebrows. I have seen a lot of different pictures on it, and I wanted
to make sure I am doing it right. The next video we are going to look at is tearing this
engine apart, and putting all of these awesome performance parts in it. We are going to go
over the flywheel installation, arc billet rod installation. We are going to look at
cams, as well as installing valves with higher tension valve springs, and putting on a thinner
head gasket. Please subscribe if you haven’t already, and we will catch you next time.

About the Author: Michael Flood


  1. Good job. I have only 4 engines now. I find it much easier to use the grinding stones attachments. Cone shaped, cyclinder shaped, and frisbee shaped.
    I did however grind down my guides to create a much smoother transition. I saw it done it pics online and it didn't have any ill effects on the engine.

    I practice a few times on two of the 97 mL engines since they're so cheap.
    It helped build my confidence.

  2. i think u did a pretty good job and its funny im acually in the process of doing the exact same thing to and older 5hp briggs myself

  3. plz do the other video i just got my hands on a 1980's briggs 5hp out box never ran and i want to mod the shit out it

  4. i have a engine that as no compression so it wont run but everything is rebuilt so what could it be

  5. would be interesting to hear if there is any way to identify a kart engine or any other small engines, i'm working on a small engine that had no indication as to the make and model but it looks exactly like the one from the video with the exception of the intake and exhaust ports being swapped, so being able to identify a small engine would be rather helpful in my case >.<

  6. How much power do with these modifications?
    We have to do the two things or flat head enough?

  7. Hey BBQJOE here. Great videos! You're easy to understand, the lighting is good, and you're relaxed and not nervous. All those things make for good watching, whatever the topic.
    Keep up the good work!

  8. Porting is specifically useful at higher RPM. During high RPM the gases have less time to enter and exit the cylinder so better flow is critical. Porting does not help so much during mid to low RPMs.

    Nice Video!

  9. looks a good job i ported an old ford 2.0 litre cast head using your same principle….. as long as you dont take too much material off or go too deep to upset flow characteristics you dont need a flow bench(where a specialist tuning company measures the port flow during porting making sure the flow works as if done wrong the engine will lose power at certain rpm?this is what i was advised…but could be wrong?…great video well presented….thanks for sharing

  10. built a 3.5hp when I was a kid and did all that and more man that go-kart was so fast but I want to make one for my kid like the video.

  11. Most gains will come from rounding off "inside corners" in the ports. I see you left them untouched. There are also gains to be had from rounding off the top edge of the cylinder (about 1/8" radius) in the area where it meets the combustion chamber. This allows gasses to flow more easily over the edge of the cylinder as the piston falls.

  12. Lots of comments from experts on port & polishing. Wonder how many ever operated a flow bench to see whats happening and actually seen the results on a dyno from their work? My guess is they are just regurgitating what they seen on other amateur vids or weby backyard forums.

  13. The problem with doing many mods at the same time is you have no idea what actually gave you how much improvement. Your only going by what the manufacturer says, which is often BS

  14. I love your videos and I think their awesome but could you try to be more thorough. sometimes I end up a little confused. thanks

  15. actually im not sure if you want to make it "super smooth" , advised by professional engine tuners you should have some texture … theres a scientific explanation but im not 100% on the details so i dont want to confuse.

  16. I think you made a fine video. Starting out slow and not overdoing the ports is smart. You can't go back. Remember there's people on here that all they want to be is toxic. I owned a custom Harley shop and built lots of engines . Did all my own machine work and I can say you did a fine job of explaining the process.
    Good luck.

  17. Don't make the exhaust any bigger. The exhaust port on a flathead Briggs 5hp is too big from the factory. Smoothing out the casting lines is fine.

  18. Did a pretty good job. I did the almost same thing on my kids 5hp Briggs, I took a little more off in the runners on mine. It really woke up the motor. If wondering, made low end rpm way stronger, but I have a stock cam also. I love cruising down the street and then just hammer on it, LOL.

  19. Nice video sir, I just joined diygokarts and saw that you are a member too, I still see some flashing inside , can I ask why you did not totally remove it ?

  20. The only way your going to see improvements from your work is with a flowbench and ccing the chamber with a burrete with liquid? That is the proper way to build a motor and also degree the cam with a degree wheel! Lot of work for a motor of this size you bet but just going off of videos your shooting in the dark and chasing your tale! Get a good performance book and speak with vendors and always trust your own work with solid numbers and procedures? Never although informative base your work on u tube videos! Sound working knowledge and procedures puts you in the winners circle!

  21. i bought one of these for my mini bike and when i took the piston out of the engine the piston was damaged really badly but luckly the cylinder walls can be repaired

  22. You can grind the tops of the valve guides it's not going to hurt them. Your not removing enough of the guide to affect the valve life.

  23. In fact the exhaust isn't where the gain is at all, it's more in the intake, it's still a bit rough normally you want to decrease the angle of attack, that's what I Dit with a Honda engine (gx) and fill the port with some filler stuff. But I am gonna try this in my own briggs. Good video by the way

  24. Great Channel Man! Not sure if you will ever see this but Epoxy on the Intake 90 Degree Turn would be a good idea and I bet would gain about a good 1/2 HP or so.

  25. R.I.P my old Briggs 5 hp the old owner of the engine ran it without oil and it overheated the piston a couple of pieces of metal off the piston got melted onto the cylinder walls might take it to a machine shop someday and have the cylinder walls repaired

  26. Silly putty or just modeling clay to seal up those ports instead of rags, works like a charm. I like silly putty for its non stick qualities.
    Great vid brother.

  27. ive heard that when you grind the inside of a engine block you have to use lube so it dont rust. is that correct

  28. I'd love to see some dyno numbers on a stock flatty and then with each mod done. I've seen some vids but they never seem to show the numbers for a bone stock b.f.h.5. we all know it's not 5hp.

  29. You can buy carbide cutters for working with metal. For aluminium, get a coarse cutter and use a coating of grease in the port to keep the aluminium from plugging the flutes. As the grease fills with filings, wipe it out and put on another coat. Wash the grease off before using sanding drums. The grease is messy, but makes the job easier and gives a better finish.

    One major area to promote flow is the "short-side" radius. On the Briggs, this is right where the roof of the port turns up to the valve. You want to make this a smooth curve with no sharp edges.

    The intake should be finished with a coarse sanding drum to give a slight roughness to promote turbulence along the port wall which helps eliminate fuel sticking to the port.

    The exhaust should be as smooth as possible to promote flow and minimize carbon buildup.

    Unless going for high RPM, making the ports bigger can be detrimental to power, so go for smoothing the port and eliminating sharp edges rather than removing a lot of material.

  30. The inside walls of both the intake port and exhaust port needs to be pulled back to more inline with the entrance to the intake seat & pulled inline with the exit from the exhaust seat. What was shown in the video won't help at all. Lol

  31. I like what you did, but I would have left much more of that that center ridge untouched, the part between the "eye brows" , kept it a little higher, that actually helps to channel intake gas into the cylinder head. Question, how much HP did you gain ?

  32. Great video grant im gonna port and polish my first small engine (briggs flathead) please feel free to follow the build and offer advice .. thanks for great content

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