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DIY Home Built Motorized Bicycle (non kit) Post all about your home built rides here. Weedwacker motors, lawn mower engines ect. This area is for non kit builds

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  #1  
Old 08-06-2016, 06:55 AM
MEASURE TWICE MEASURE TWICE is offline
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Default Adapting Briggs Build to accept bigger engine

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.
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Last edited by MEASURE TWICE; 08-06-2016 at 06:58 AM.
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  #2  
Old 10-23-2016, 10:28 PM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

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
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Last edited by MEASURE TWICE; 10-23-2016 at 10:47 PM.
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  #3  
Old 10-24-2016, 01:57 AM
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Default Re: Adapting Briggs Build to accept bigger engine

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).
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Old 10-24-2016, 03:48 AM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

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

Last edited by MEASURE TWICE; 10-24-2016 at 04:11 AM.
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  #5  
Old 10-24-2016, 09:08 AM
ragdolldude ragdolldude is offline
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Default Re: Adapting Briggs Build to accept bigger engine

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.
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Old 10-24-2016, 02:44 PM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

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
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  #7  
Old 10-24-2016, 02:47 PM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

Quote:
Originally Posted by Tony01 View Post
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.
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Old 12-01-2016, 07:28 AM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

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.
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Last edited by MEASURE TWICE; 12-01-2016 at 07:32 AM.
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  #9  
Old 12-01-2016, 04:46 PM
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scratchbuilder scratchbuilder is offline
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Default Re: Adapting Briggs Build to accept bigger engine

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???
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Old 12-01-2016, 09:33 PM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

Quote:
Originally Posted by scratchbuilder View Post
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.

Last edited by MEASURE TWICE; 12-01-2016 at 09:38 PM.
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