Port polishing a Briggs & Stratton L-head

This is something I have only done to a few engines, but with positive results. Most people aren't willing to do it, or have never thought of it. I'm going to give a quick explanation for anyone who would like to try it out. Of course it is a procedure that should be done by someone with at least a basic knowledge of the engine, but you don't have to be a guru!

First off have the engine on a clean bench, the oil should be drained from engine to prevent a mess as well. You will need a good set of sockets/wrenches of course, some pliers / cutters/ a die grinder or some small round files ( powersaw files) some different grades of sandpaper, 220, 400, and 600 would be a good selection. A valve spring compressor is also handy. Compressed air is also a help but not necessary. Wear safety glasses if you are using compressed air and while handling springs. Some small stiff wire brushes and some varsol / paint thinner will also be needed.

First of all, remove all external pars from engine, shroud, coil, carb, throttle cable, etc. Then you will have to remove the head, and valve cover / breather, being careful to not damage the gaskets as they may be re-usable, although I recommend getting new ones for how cheap they are. Then you will have to remove the valves, (remember safety glasses!!) a spring compressor will be the best bet, but a good flat screwdriver and a steady hand can carefully compress the springs enough to remove the keepers. Be very careful with the springs, as they can fly out if you slip. Be careful also not to lose the keepers and not to mix up the intake / exhaust parts. Now slide the valves out and set them aside, preferably in separate plastic bags along with their respective keepers / springs to keep them clean and organized.

Now that the engine is bare, make sure the piston is at TDC to keep debris out of the cylinder. If you are experienced with engines you may even want to dismantle the base and remove internals to be safer. Now you can take a wire brush and remove the loose carbon that may be in and around the ports, on the piston and combustion chamber (cylinder head) and valve faces if necessary, being careful not to gouge the metal as it is only aluminum, and especially careful not to damage the valve seats!! Once it is clean, you will notice how rough and angular the ports are, which is very restrictive on fuel / air flow and exhaust flow. Whether you have a die grinder or files, start by lightly grinding away and the roughest edges, Try to round them off, slowly, take your time, and do not go too deep. I recommend only taking off 1/16" to 3/16" (one sixteenth to three sixteenths of an inch) as you don't want to damage the casting. After rough grinding it till you are satisfied, you should clean the filings out. Then you can take some 220 grit sandpaper and start smoothing it out more , and working your way down to 600 grit to give it a fairly smooth finish, remembering NOT to touch the valve seats!!

If you have a newer Weber carb with the separate intake pipe, you can also perform this procedure on that pipe, as it is also very rough inside. You can even polish the combustion chamber on the head, as it is usually rough. This will not only help flow better, it will also prevent a lot of carbon deposits. Motors I have performed this to stay nearly carbon-free even after hard use and oil burning!

Now you should thoroughly clean out all debris and residue with compressed air (wear safety glasses!!!) or paint thinner, take a light colored rag afterward and rub the areas down till they stop dirtying the rag. Make sure to get inside the valve guides as any leftover debris will quickly destroy the valves!! Use a small dowel with a rag wrapped around it and push it through both guides many times till they are spotless. You can even use a good vacuum around the piston and ports to suck up anything that was missed. If any debris does make it's way around the piston, carefully rotate the engine till the piston goes down a bit and carefully remove the debris, do not let the piston back up till all debris is removed as it may damage cylinder wall / rings / piston.

Now comes a tricky part, (wear safety glasses!!!) hopefully you have a valve spring compressor ( I don't lol ) Place one valve at a time, coating them lightly with engine oil, remembering which valve goes where and use the proper keepers for each valve. This may take a few tries if you are not used to it, having a buddy help is very useful here. If you DON'T have a spring compressor, here is what I do, I don't recommend this method, but it will work if you have no other choice. Place each spring in a vice carefully compressing it, and use some small solid copper wire (telephone cable size works) or small zip-ties, tying the spring in 2 places on each side in the compressed position. Then you can place it over the valve, insert the keepers and while holding pressure against the keepers, snip the wires / ties with a pair of side-cutters / snips being careful not to let the springs jump out. Once again, if at all possible use a spring compressor! And always be careful with springs!

Now that the worst is over, you can re-install your head and external engine parts. Torque head bolts to specs if you have them, if not just tighten them as evenly as possible in a criss-cross pattern ( they are not very fussy) Make sure everything is assembled properly, install motor on machine, and fire it up!!

You may not notice much difference, but it will effect performance, it may idle a little roughly at very low idle, but it will have better acceleration and smoother running in the long run. If you have to re-adjust your carb for proper mixture, do so.

Well I hope this is helpful for anyone who wants to try, it is very time consuming but nonetheless very worth it. If I have missed anything, or if anyone has suggestions please feel free to say so! Also for anyone goes through with this, let me know the results! And also remember, you are doing this at your own risk, I cannot be held responsible for any engine damage that may occur during this procedure. Good luck!

Sounds awesome. Nice tutorial. The valve springs put me off doing this a little though

I read at a lawntractor racing site that when polishing the intake, it should be left a little rough- least on briggs motors- can polish the carb all y want till its smooth inside. Something about briggs liking a little turbulence flowing in.

I also herd a 10HP head will fit a couple other motors- to increase compression. Yeah with an aluminum motor, you can wreck it in a hurry if youre not careful. I considered minor polishing the intake on my 18.5 opposed twin- to make it flow better- but with the cast intake manifold i cant see wasting my time. Id like to make a custom intake to run my scooter carb- bet that would flow awsome.

Good info, saved it to the harddrive for future use. It made me curious though as to what all could be done to massage an opposed twin, flathead/L-head.

I've never had the chance to do an opposed twin (I call them 'boxers' like a Porsche lol) but I'm sure it would work just as well, just double the work!

It might be easier though. The valves are in the block right? So you shouldn't have to remove them, just pull the heads off and take it away from the rest of the motor. Less cleanup that way.

Jamus wrote:

It might be easier though. The valves are in the block right? So you shouldn't have to remove them, just pull the heads off and take it away from the rest of the motor. Less cleanup that way.

But to polish the ports the valves must be removed.

DOH!!! Well that probably turns it from easier to harder real quick doesn't it, haha.

if i done this i would only do it to a I/c Engine for as Briggs aluminum bores may last a good while, but i don't like that type of cylinder i like a cast iron bore, last much longer and is very shinny when well used and you can re hone it too unlike aluminum you would have to have it re hardened for as honing would wipe the hardened surface right off.
Also to increase compression MIll the head, thinner head gasket and some high grade bolt to have good torque. I snaped my briggs 3.5hp head bolt with a 3/8 drive ratchet, though i really didn't try to and the bolts were 19 years old.
Also I have a set of DUAL valve springs but idk what i want to use it on. I can't do the twin cause its a set for a single cylinder but i could get another set to do the twin. but the dual valve springs just help with keeping pressure in the combustion chamber, faster snap back for higher rpms, which would then result in thrown rod unless i put in some racing rods. but thats alot of money
also i would like to see roller lifters like in my car except my car is hydraulic but have solid roller lifters,
but idk

It all comes down to 2 things : money and power- you can stuff the motor full of racing parts and dump a ton of cash into it- a tech i used to work with had a wheelhorse pulling tractor with a 8HP kohler - he said he dumped at least $8000 into the motor alone. OHV motors would be easier to mod then flatheads , with the valvetrain outside the motor.

Alot of tech tips can be had from lawntractor racing sites - i went to a lawntractor race a few years ago, the super mods were running 22HP v twin vanguards bored out to well over 70HP - things were way to fast for that small track.

But also with speed/power comes expensive breakage- alot of highly modded motors tear them selves apart after a short time.

I suppose its one reason that id like to see how fast i can make a tractor go with a stock twin cylender running the governer- its possible to get in the 50mph range with the right gearing.

And usually a better way to get more power- get a bigger motor- i went from a single 14.5HP OHV briggs to a 18.5HP opposed twin on the mod murray and that thing flew- too fast for a nearly stock tractor.

That reminds me of another question I have had for a while now. There are many seemingly identical motors in lawn tractors with dfferent power ratings. My opposed twin fr example only makes 14hp, while I've seen the same engine as high as 19hp, maybe higher. Are they larger displacements internally? I come from a car/truck background and can understand the same block having different displacements and therefore different power outputs, compare a Chevy 305 with a 400 small block.

Jamus wrote:

That reminds me of another question I have had for a while now. There are many seemingly identical motors in lawn tractors with dfferent power ratings. My opposed twin fr example only makes 14hp, while I've seen the same engine as high as 19hp, maybe higher. Are they larger displacements internally? I come from a car/truck background and can understand the same block having different displacements and therefore different power outputs, compare a Chevy 305 with a 400 small block.

You got it pretty much right on. I've got 10, 11, 12, 12.5 and 13.5 engines all look the same, but have longer blocks / different cranks. Most pistons/ con-rods are interchangeable. I've seen people put cranks from an 11 into a 12.5 block and wonder why it has no power, well the piston was coming short about 1/4" from TDC!! hahaha

Same bore, longer stroke, good to know thanks.

Probably why there was a lawsuit again tecumseh- people were suing over the 'actual' horsepower ratings were under what the motors stated - alot.

Yeah all the small briggs motors- 10 to 13HP all seem to be nearly identical - why alot of parts swap between them.

Ive got a 13HP on my 99 murray ( not original) and a 16.5HP on my 2003 murray ( original)- oddly the 13HP seems stronger then the newer motor- i dont really like the plastic shroud on the 03, if the flywheel ever comes apart- that plastic wont stop it, least with metal it has some sheilding.

I have a few twin opposed motors- makes me wonder why theres a 17HP, 18HP and an 18.5HP - probably isnt much difference in actual horsepower between them- id really like to see what power my 20HP onan will do- IF i can get it running ( cranks- no spark, just have to tinker with it)- itll be the biggest motor in the fleet.

Seems like all the twins have about 20 horsepower, even the 13 horse twin. Just different stickers.

Twins are awsome for modifed purposes tho- my twin had plenty of power when i went for test runs - at half throttle and 3rd gear , it was moving good.

With the singles you have to rev them out to get any serious power- with a twin, you have double the power and can get alot more from it.

My 12.5 I/C Gold that I'm running now, that is heavily ported has tons of very low end torque and revs freely as well. Even in 6th gear I can go from barely above idle to full speed in a matter of seconds, over 30 MPH.

well i may do some port cleaning somtime in the future but my 19.5hp I/c twin has plenty of power for what i want because my 11hp moved my mower around pretty well so 19.5hp will be plenty.
also my trans is on a hault for a bit, got 2 oil shaft seals ordered i lost the other one and the one i had was too big

Ford Boi : Yeah my 14.5 seems sort of slow - the murray has 6 gears , i used all of them ( back when it was stock with just swapped pulleys) thing could barely go in the 20's - when i put the 18.5 on , i actually was nervous to go over 4th, even with my helmit on - thing was just too 'darty' and loose . I never had a chance to see what WOT and 6th was speed wise- i imagine in the upper 40's even 50.

Im thinking about trying to adapt that scooter carb to the mild mod MTD to get more power- its also getting a larger motor pulley .

I've had good luck with the Yardman I'm using now, goes strait as a car even on pavement, wide open, stock front axle with new bushings. Never crashed it yet! (YET) lol . The Rallys and Craftsmans I've had always were scary at high speed tho.

Just the simple no cost mods ive made to the MTD ( profile pic) have made it handle excellent - i can do donuts all day long on that thing w/o worrying of flipping over.

I have a 18HP twin that i got with no intake/carb - if i get something fixed up for it, thatll wind up on it.

Someday ill redo the murray ( was my first long time mod) - i did use an 85 craftsman ( like your rally) as a first mod- that thing was scarier then the murray - the clutch would contact the engine pulley, no brakes - then i locked up the trans when it broke several teeth on the pinion gears- that ended its run as a mod.

Yup those Craftsman/ Rally etc. platforms are some sketchy, I use them alot tho since they are simple and common, altho I have smashed up quite a few over the years, the MTD and Yardmans are much safer, I've only crashed mine once in 3 years, and hat was in the snow. I think I am going to lower my seat like you did, I love he way it handles now and I'm sure I would like it even more. I can whip donuts on dry pave as is lol. Hopefully I can find a big single or twin to replace the old 12.5 tho, after driving my buddy's Craftsman with a 15.5 I want MORE POWER !!!!!

I have a 97 mastercut i actually mow with- virtually the same as my mild mod, but stock with another 14.5HP motor- and that thing boogies . Only real issue with that is it tends to over rev a little - might swap a 12HP back on it , while im at it ill do the zerks in the trans ( it whines horribly).

Yeah stock seating position on a fast tractor bothers me- you sit too high and its too unstable ( least for me)- why i hated my murray and rarely drove it - next round of mods ill put lower profile tires on and lower the seat- itll probably get the 14.5 off the mastercut or maybe the twin.... ill see when i get to that point.

I do have a spare craftsman chassis i might use for a mod in the future - i used parts for my other tractors , so its nearly a bare chassis, cept for the front axle - might even use that for my wheelhorse mod....

Believe me- a larger motor on it and youll be like " holy crap! " - that pretty much summed it up when i went for a first test run with the big twin on the murray after the anemic 14.5. Another thing i noticed was using tapered bering hubs on the murray - i could practically roll the machine ( complete with motor) with two fingers - it rolled really smooth - could be why it seemed to run tons faster then with stock wheels.

When i take a head off I always scrape the carbon off and clean everything up nicely with some carb cleaner and compressed air.

Took my opposed. I have a valve compressor thank god. Had a diagrinder and a multicarb bit. Grounded the ports of intake and exhaust. Made it smooth and polished it. Polished the head. Carb is polished intake it grounded out and polished. Runs like a beast. With regular gas and methanol mix. Doing this to your motor does improve torque alot