Adapting Briggs Build to accept bigger engine

[MEASURE TWICE]

I came across a newer Briggs 3.5 hp to use in place of a Briggs 3 hp. It has a short 3/4 keyed shaft and I could not use with my 3/4 bore clutch on it.

So it happens I had designed my build with a rear wheel drive sheave that was smaller originally, about 10 inch diameter. Then it broke and I got a Whizzer Clone Sheave to replace it and it is 16 inch diameter.

I was fine with slow speed greater torque as I ride trails. 20:1 ratio works with speeds under 20 mph. With the larger sheave I ended up having to connect two jackshafts together with 1:1 ratio to keep that 20:1 overall ratio. Now that I have a better engine I had to find a way to use it. I also figured out I could also swap back the older engine if I could figure a way not to mess with rearranging stuff permanently.

I figured a way to instead of having the 3/4 clutch that would not work on the engines short 3/4 shaft, I am now needing again the two jack shafts connected with smaller to larger gears for reasons you'll see.

I got bushing reducer 3/4 to 5/8 adapter to use the 3/4 clutch on the top jackshaft. The rectangular shape key I will make from 12 inch stock I got.

The engine 3/4 short shaft works with a 3/4 small gear. A chain instead of belt I had prior now connects engine shaft to the top jackshaft. As soon as the engine is started, the top jackshaft is also moving.

I can have 12 tooth to 12 tooth 1:1 ratio so that engine rpm is same as shoes on the clutch rpm.

Optionally I can have a 12 tooth to 13 tooth gearing to have higher engine rpm than the top jackshaft which has the clutch on it. Could be I would have more torque at clutch engagement speed the same as rated, but with engine rpm at marginally higher rpm.

When the clutch engages the clutch gear connect to a larger gear on the bottom jackshaft by a short chain. Then in line with the rear wheel sheave on the bottom jackshaft I still am using an adjustable diameter pulley with AX Belt connection.

1st picture shows by mistake, but not one that got very far, it was 12 tooth to 28 tooth gear connection. The 28 tooth gear is reserved for use between the clutch top jackshaft and the bottom jackshaft with the 28 tooth gear on it. If it had been assembled with the mistake I don't think I could ever get enough engine rpm to engage the clutch.

In some pictures you cannot see, but I am temporarily make shift piecing extra jackshaft to see what it would look like. I eventually slid the parts into position and all I need now is some 3/4 ID washers to shim the bolt at the end of the engine crankshaft. I would also use split collar on it, but there is not enough room.

You see the back idler pulley on the adjustable tension brackets in the photos, but it in no way interferes with routing chain instead. In case I need to swap the smaller engine back all its still left to do it.

 

[MEASURE TWICE]

Got the recoil starter fan shroud drilled out to move the guides for the throttle cable and then bolted the parts to the bigger used 3.5hp Briggs Engine that is in better condition.

When I have time again an not raining I will see about testing out the new bearing chain and jackshafts.

Once the bigger newer 3.5 hp Briggs is all together and runs OK on the same bike frame and engine platform, I think I'll split the crankcase to see what happened to maybe rings or such in the old Briggs 3hp. It sized up, but then freed itself. I oiled it from the cylinder head off, but what the rings look like or in the crankcase I'll be interested.

Both of the recoil fan shrouds are very much the same, just the 3hp old cover was needing modifications to attach it the old block since attachment points had damaged threads. Those modification that were made has it not usable on the newer 3.5hp block.

There are a different number of fins on the cylinder on the bigger 3.5 hp. I think it has 1 extra fin. The most difference I see is from the base of the crankcase to the top of the cylinder head fins it a bit taller on the 3.5 hp Briggs.

The spark plug is a short one I've been using. Still it is necessary to remove the screw top of the spark plug nub, or remove the spark plug all together and reinstall it after engine is fitted into frame and platform.

Then I wondered then how the recoil covers are the same for different horsepower. I don't think just valve timing or smaller head space. Anyway I am not going to take the head off the newer Briggs just to see the piston diameter and the stroke length, I expect with the serial and model info I'll get it online.

The older Briggs I think I have the original cover with the serial and model, but can't be too sure. Since it is that I'm going to open it up anyway, I'll take the measurements in the process.

Then I can see what the difference is in the displacement.

MT

 

[Tony01]

Since you have a rear sheave why not get rid of the box with the 1:1 and just output a pulley clutch? Only thing is that with the reduction your stock clutch will engage later so you may need to consider putting heavier shoes in and/or a lighter spring.

For single speeds I have always liked the idea of the clutch on the jackshaft. Makes it easy to run a belt primary from the engine and keeps the width of the engine to a minimum.

Btw the 3.5hp Briggs is a 9ci (148cc) and the 3hp is probably an 8ci (131cc).

 

[MEASURE TWICE]

There was at one time the jackshaft top to bottom connection was with different size gears to help get the low ratio for off-road riding torque.

It then became no longer necessary when the rear wheel sheave pulley size was replaced with a larger diameter one when the prior cracked.

Then the 3hp Briggs seized but I got it running again, but do not trust it till I open the crankcase and see the rings.

I bought a replacement 3.5hp Briggs of newer used condition at a reasonable price, but made a mistake in not realizing the shaft diameter and length was not 5/8 diameter and enough length for a clutch. If I new I could have asked the seller to allow me to take the pulley off the engine to measure it.

A way around was to put a sprocket on the engine crankshaft and run it to the top jackshaft. The top jackshaft runs a 1:1 with the engine crankshaft.

Since the engine crankshaft is too short for a clutch, now I can instead put a clutch on the top jackshaft an it spins at the same speed as the engine crankshaft.

Then a connection of a short chain to the bottom jackshaft larger diameter gear for more torque.

Then from the bottom jackshaft a small adjustable diameter pulley to the rear wheel sheave. This is where it was before and still is, underneath the banana seat and inline to connect with the rear wheel sheave.

The clutch on the jackshaft cannot fit inside of the top and bottom jackshafts under the banana seat as I measured it.

Although the cover I made to go over the left side of the bike for crankshaft output sticks out a bit, it again is still necessary to cover dangerous moving parts. It will still look the same from the outside, nothing to change for the cover.

I would have narrowed the profile of the cover if I could have. If some of the gearing down could have remained between the two jackshafts under the banana seat, it would have enabled me to do this. It is not possible as the clutch is two big to fit in the space.

If I never thought I would use the old Briggs 3hp on the bike again, I could take the angle grinder a cut away the welded brackets for the belt back idler pulley, but it is far enough forward it makes no difference in helping straddle the bike.

It is just a bit further back by the foot pegs a narrower profile for the cover would help.
Back past the foot pegs is where the clutch on a jackshaft stick out just as far as before when using belts and pulleys, so the cover is a massive thing I know and stays as is.

One day I may see about getting a crankshaft replacement with enough length to mount a clutch on with this engine, but the cost would most likely not be economical. Just trying to find if the 3/4 short crankshaft engine like this can even accept a crankshaft that has 5/8 diameter with a stock part is a nightmare. Ebay is great for used parts sometimes, but mostly you'll not find a machinist as a seller who know his $hit.

MT

 

[ragdolldude]

When I was a sophomore in high school, I made a mini bike out of a 20 inch bike frame with smaller tires. I found a V-plex clutch for a v-belt, but the shaft of the PTO shaft of the engine was 3/4 inch and the clutch was made for a 5/8 shaft. With the help of my metal shop instructor, I turned down the shaft to 5/8 on a lathe and we put in a keyway into the shaft also. This might be an option for you too.

 

[MEASURE TWICE]

There is not enough length though on the shaft so that is the real problem.

I have a 3/4 clutch and I know some people may just put a drilled piece of steel shaft for a bolt to go through into a tapped shaft, but it is not advisable using about 1/2 the bore to support the load.

Welding some more shaft on to lengthen it, if you take the shaft out of the engine someone might attempt to do I know.

All the shaft extenders that clamp with keys and such cost more than another good used engine so I'm not thinking about doing that. Using the shaft extender to get more usable shaft does require the shaft to be offset further from the crankcase wall which is a problem also. The offset is the area that clamp onto the existing crankshaft.

MT

 

[MEASURE TWICE]

Since you have a rear sheave why not get rid of the box with the 1:1 and just output a pulley clutch? Only thing is that with the reduction your stock clutch will engage later so you may need to consider putting heavier shoes in and/or a lighter spring.

For single speeds I have always liked the idea of the clutch on the jackshaft. Makes it easy to run a belt primary from the engine and keeps the width of the engine to a minimum.

Btw the 3.5hp Briggs is a 9ci (148cc) and the 3hp is probably an 8ci (131cc).

I'm not sure I know what you mean by the box with a 1:1, as it is just two jackshaft with chain connection on sprockets.

 

[MEASURE TWICE]

I fitted the cover with the throttle linkages to the carb just like on the older engine and then went to work on the tranny.

Oh, forgot to mention I found that this newer engine has no points. The magneto has no need for them and has a nice hot spark. I already knew it ran well from the seller testing it in front of me when it was on an edger. Just now how well does it run with a different carb and air-filter that I'm not changing, as its form factor and the use of gravity feed gas tank I have to keep.

For the tranny, I got the new jack shafts and bearings (extra bearings I swap in after each year just to be safe).

The clutch which I had to shim with those adapters so the 3/4 chain type clutch fits on the 5/8 jack live jackshaft works well. I took apart the clutch and removed rust and lubed the bronze bearing inside.

I have to cut the jackshaft length some maybe to fit within the covers when they go on last. The chains will not need any half links as the engine can move forward and aft and it make chain tension adjustable that way.

The bottom jack shaft has adjustment up or down to set tension on the chain going from the clutch on the live jack shaft to the bottom jack shaft.

Then the bottom jack shaft has the adjustable diameter pulley for the belt tension adjustment. When I use the temporary screw tightener to pull the axle on both side to the rear drop outs, then the belt tension to the rear sheave is set.

Will be making threads in the end of the top jackshaft to put a bolt to help better retain the clutch on one side. The other side I have it easier, I just use a split collar. The split collar if I used it at the end of the jackshaft is just to far outward to fit under the cover.

I'm going to later look at the old engine and see about the rings and what is inside the crankcase now.

I forgot to remove the back idler pulley from the bracket that is welded to the frame. I'll do that later and keep the parts with the old engine and have it as a way to swap the old engine back in once it is fixed up if ever I need to use it as a quick spare to swap back in.

 

[scratchbuilder]

Het I've got a 3.5. Needed a mag. I got a 4.5 vertical lawnmower for 5$. Coil fit, aluminum flywheel fit...rotated each eng. both looked to be opening closing the same...so put it together...fired right up. A lot lighter also. So I'm wondering if the 3 hp crank would swap with the 3.5???

 

[MEASURE TWICE]

Het I've got a 3.5. Needed a mag. I got a 4.5 vertical lawnmower for 5$. Coil fit, aluminum flywheel fit...rotated each eng. both looked to be opening closing the same...so put it together...fired right up. A lot lighter also. So I'm wondering if the 3 hp crank would swap with the 3.5???

I think it could be a possibility.

In my case I got an engine that was used that was just 1/2 hp bigger, but later unknown when purchased was that it was a 3/4 shaft.

That was not all, the shaft is not long enough to put any clutch on, just a pulley or a sprocket.

I yet really don't know if I could take the 5/8 crankshaft from the 3 hp and swap it into the 3.5 hp which now has a 3/4 crankshaft.

I heard the term journals, and the diameter of those have to be the same. All what else I will probably one day know.

Ebay sellers have some Briggs parts, but none showed any way of knowing all the measurements of the crankshaft.

On a separate sort of note, I was just checking the valve/tappet clearance with feeler gauge and the intake had none. The exhaust had 0.008 inch. I adjusted the intake valve on the older engine by grinding the stem a little, but since this new used engine runs fine I will leave it be unless it does not perform well on the bike.

I did check the piston position in relation to the valves opening and closing and they look right. I have I hard time understanding how that can be with the intake valve without any clearance. I yet have to look up the specs, but the older engine said like somewhere I remember around 0.006 to 0.008 inch for both valves. The smallest feeler gauge I have is 0.006 inch and it would not fit, nor could I see any gap with strong light on the intake.

Once I have the old engine crankcase open, I'll see if the rings are bad or broken any of them. Also lower down on the cylinder wall where I could not check with just the head off.

I also found the rings I saved from when I changed them a long time ago. They probably were OK. It was mostly the reshaping of the valves and seat angles by having the professionally cut at a machine shop for just 10 dollars a surface. This made the valves seal way better and higher compression feel.

I have a Briggs 5S I had the valves also cut the same time, total 40 dollars for all work.

Maybe if it is the rings are bad I will just put back the origonal ones in the old engine and check the wear on the crank and connecting rod etc. I'll make measurements of the crank shaft that I'll keep a record, just if maybe one day I find out if it will fit in the bigger engine.

I'm just not about to fiddle with a known good working engine where I am changing none other than the carb and air-filter as necessary for it to fit my bike. If for any reason it does not work it is easier to conclude what could be wrong with less changes made or parts removed and reinstalled.

 

[scratchbuilder]

I know what you mean about taking apart good runnig engines. Mine too has that short shaft and I had to also run a pully with belt lever clutch. Maybe I to will someday see if the crank's are swappable.

 

[curtisfox]

If the model # are the same series it should it should change over, Just have to try. Even if they look different, Briggs prolly used the same crank for a lot to save coast............Curt

 

[MEASURE TWICE]

I have checked the crank shaft where it steps up in size right by the crankcase wall before it meets the seal. Both the 5/8 and 3/4 step up to 7/8 inch at the seal. If they call that the journal (think supporting in place surface) for that side by the output shaft, then probably the same 7/8 inch on the side as it leaves the crankcase on the opposite side by the flywheel.

My newer used engine has no points and I have seen the space where the point cover would have been under the flywheel if it would have had points. I don't think that would have any consequence except where I have not yet compared is the area where the crank shaft is shaped to work in conjunction with points movement if it were of that kind.

With 1/2 a hp difference I was wondering what made the difference. It will be later for these answers as I have a good used 3/4 shaft 3.5 hp and only if it does not work with my bike will I be checking more to swap parts.

The height of the cylinder looks to be just a bit taller, but only due to the taller fins on the head. That does not give any extra displacement though as it is outside the cylinder. The diameter and piston throw which is created in conjunction with the connecting rod and crankshaft and the head space gives the compression.

If it turns out the compression is the same with both the 3 and 3.5 hp, I have heard of this in other engines where the cam shaft have a bigger bump to allow longer intake of more fuel and for example an 8hp becomes a 9.9hp.

I think if the camshaft is something as well that can be swapped along with the taller fins on the two Briggs engines a 3 may be a 3.5 hp.

If you look up Easy Spin Start there is the opposite in what I believe is the bump on the camshaft for the intake valve that means a longer intake opening, but during the compression stroke. This leaks out some of the compression and makes it easier to start. It was created when law imposed lawn mower blade to stop if you release a handle. When it stopped with this brake it could have the crank in a compression stroke making it hard for some to start.

I myself would take the high tension lead off and turn the blade back to get some slack and then put the high tension lead back on to start easier. Anyway this reduced the rated horsepower I think. Conversely I heard that at starting speed the engine only looses the compression mostly, as the compression cannot escape as quickly at running higher speeds.

When I get this engine that had seized, but freed by only taking off the head, I'll see what damage must be in the crankcase. This is where I can't otherwise. Then I'll be measuring the crank of this older engine and also going to see about the bump on the intake cam that I hear might be there as part of Easy Spin Start.

 

[curtisfox]

A lot of times the hp difference in hp is just the carb size, Like Kohler engines K 321 ( i think that is the number ) The block is the same, the only difference in 12 and 14 hp is the carb size...........Curt

 

[cannonball2]

Lotta times the difference is the decal. Snapper was known in the past to "derate" Larger engines to lesser HP to compete in certain niches. This was legal but not the other way around. While working at a Snapper dealer an entry level 28" rear engine rider showed up with a 9hp L head engine. Was actually a 12hp. Never was sure the reason but it forced a bunch of buyers to 33" 12hp models for just a bit more which were overstocked at the distributor. Next spring we received 12hp decals to be applied to any of the "9hp" remaining stock. With a price hike of course.

 

[MEASURE TWICE]

That is interesting about just for show numbers HP rating and psychological effect for sales.

The problem I had with the Easy Spin Start is even if the HP was same at higher speed, I was seeing some reverse flow of fuel past the venturi at the airfilter. Paper airfilter after getting wet would stop flow of air into engine and choke to stop. It could have been that the valve wearing into the seat lost all of its gap and combined with Easy Spin Start it was over board with 1/2 the upward compression stroke and the intake valve then just closes.

I made a larger clearance gap at the valve tappet to stop that, but another way may be if I see a little nub of metal and relate it to this Easy Spin Start on the in take cam (aside from a normal lobe), I may grind it away.

But if it then has too large a gap, I'd have to cut the seat as the valves adjust by grinding either the end of the valve stem or the valve seat to adjust.

Overhead valve engines with real adjustable valves are nice and I have a generator that may give up and salvage the engine for a motor bike, but it is only 2hp. I'd remove the governor from that engine as it probably only has a range of adjustment to keep the 60 Hz and runs fast with no idle at start up.

 

[curtisfox]

You will find out that generator engines have special crank and parts that make it hard to salvage for normal use. The newer small ones have the charging system right in the flywheel, so look it over good before you start. ..............Curt

 

[MEASURE TWICE]

I might have had you thinking I was taking apart the generator's engine from the set. But the cranks were just from very similar Briggs engines.

The gen is a Colman and is Chinese made. The engine is probably good, but the tank is rusty. It is hard to start even after cleaning carb and tank but runs.

I bought a new magneto when the old one was intermittent which had my arm almost come off. The guy who gifted if to me said it works see. Yea he pulled the cord like 50 times.

Still the generator part which is separate has its own issues even after making new connectors all around and cleaning it.

Probably not part of MB stuff, but if you know what could be going on I'd appreciate it. I already looked for answers at Smokestack website where there some real informed folks there that work on portable gens and ones the size of Mack Trucks.

See what happens was at times the overload light comes on, but I did not draw excessive amount. It is also alternator type without permanent magnets. This presents some strange thing that sometimes is necessary called flashing the coils to put a minute amount of magnetism back in them so the gen can excite and put out the full load. I've not risked doing that yet. It requires turning by hand or by drill the gen and actuall having voltage applied to the output plug from another source. Yeah.... strange!

The diodes I checked and seem OK. There is a capacitor I bought a new one as it had some contacts burnt also so connections would not be good.

At most I spent 25 dollars on parts. I used it to power some 120 vac super bright 500 watt halogen work lights and so for a while it works. I forgot what it is rated at but it like 700 watt gen.

 

[MEASURE TWICE]

I could have used an even smaller pilot drill, or at least find a small pilot drill bit that has a more pointy end. This was brand new out of the package, so it is just the style that is not optimum for staying put in a center punched metal. My center punch made a mark that had the bit wobble around. Then I had to correct it or use the other end of the jackshaft and start over.

I only need one end with a thread on the jackshaft so I could have started over, but it is fairly well centered, not exact though and it will work.

I forget what they call the bits, but I bought some at Sears that self center. There is a pointy section below the rest of the normal diameter of the bit and I may look for one like that in the future.

Multiple pilot drilling with smaller bits using maybe 3 before the tap drill bit could have likely avoided the wobble. To set up a drill press would have been ideal, but that is hind sight.

The tap worked great, I only used it once before and it is a 1/4-20 with a 7 drill bit set by Vermont brand.

 

[curtisfox]

https://www.google.com/search?q=dou...=8lBNWOHTAoPHmQHCw6P4AQ#imgrc=u1p0ePsSp8SS9M:,
This is what you should get for starting holes like you are doing. MSC or others have them, all different sizes, and different tapers 60 degree is most common. I got a package at swap meet cheap, from tool guy vender. ............Curt