OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Hello Everyone

Ahh yes, once more a pic provides brilliant illumination!

The different journal diameter is of no consequence, as far as mounting the clutch half goes. It's positively located rotationally/radially on the PTO by the key, and axially by the set screws.

The difference in the step diameter is due to the ball vs plain PTO side bearing - so that a normal-sized ball bearing can be used. For a given ID, a ball bearing will have a much higher load rating than a plain bush.

Well how about this.... light bulb moment.

Hello xhall, Mod Gadge, members and guests

That's brilliant. You plan to run one engine without a crankshaft!
xhall, you are doing a great job here.

I just want to interrupt to say how this post is very helpful to members and very well looked
after by Mod Gadge. I think the topic heading should be changed, and, perhaps, the topics split
between the frame issues and the engine restorations.

There just seems to be a lot of good photographic evidence and moderator advice to make
this an important set of postings.

Cheers
------------------------
JACK

Hi Guys,

There are only the two size clutch halves available, there has never been any other size.

Cheers,
BB

Jack, I agree with your suggestions. Repairing the frame is on the agenda and that will start to happen once I get closer to completing the engine rebuild. Hoping to do as much of the restoration work myself. BTW, thank you to everyone who has provided input along the way.

Update,

I went to my local lawnmower shop with both engines a couple weeks ago. The chap took a look at the crankshaft of the orange engine with the damaged keyway. He said he has seen worse and it would be salvageable with a longer key. Apparently you can get em' cut to any length. They don't do rebores, but sent me to some performance car shop were they do. They asked for $140!!! (to machine that sucker). But first they need to see the oversized piston, to measure the new diameter. They also said they have seen worse and that a light hone and new rings might do the trick. But it could smoke a bit. Bugger if I am going to spend close to $200 just to get the internals back to spec.

Anyway, all the parts on the orange engine are painted. I will be putting everything back together in the next few days and depending on the weather, shall start her up and see how/if she goes. What this rebuild has taught me, if anything, is that a lot of meticulous effort, time, money, tools and parts are needed to restore a beat-up, run-down, rusty, small, 30+ yr old engine to its former glory. Some people will also mention, that it will never be the same as it was, when it was new. So true.

I think the usual view is that unless the bore is badly damaged, at most a light hone is called for, but far more often, the bore should be left alone. Briggs does not recommend deglazing their aluminium bores. Most times you only need to clean it, measure the ring gaps with new standard rings, and if they are OK, lap the valves and put it back together. Considering you can probably get a similar engine in good condition for about $20, more elaborate procedures are both a waste of money, and less likely to give a good result than the simple approach.

The position is sometimes different with the large Briggs engines, if they have the optional cast iron bore liner. However even in that case, you always need to plan the job after finding out the cost of a good used engine. Remember Briggs built side valve engines at the rate of millions per year - they are not rare or special.

Grumpy, in the end it will have been a good learning experience (at least for me) in what to expect when fixing or restoring a small engine. It is interesting learning how things function and to see how well you can handle a rebuild with limited knowledge, skills and tools. This is my first attempt at disassembling and repairing a four stroke engine.

The only other time I had a go at fixing an engine, was when a 2-stroke Husky 125LD trimmer wouldn't start, not even after fifty attempts pulling the recoil starter. Then I noticed the primer bulb was cracked, replaced that and it still wouldn't start. I tried replacing the fuel lines, tank grommet, fuel filter and it still wouldn't go. Not wanting to take it to the tip (it did seem in pretty rough shape and quite old, a 93' model), I persisted in completely pulling it apart, cleaning the piston, head, exhaust port and muffler of years of carbon buildup. Purchased fresh gaskets, two new piston rings (bore was in great shape), new Walbro carb kit, deglazed the clutch shoes using a soda blaster and even replaced the bearings (which were quite noisy when running) in the aluminium trimmer head; that houses the bevel gears. Could you imagine the joy I felt after 6 weeks (which included waiting for parts to arrive) when she started after a few pulls (no more than 4 or 5). Anyway, now it starts swiflty, runs really smooth, idles nicely, blows a bit of white smoke when cold (but almost none when warm) and it doesn't bog down one bit with full throttle. Could have easily tasted defeat by scrapping that trimmer and going out and spending good money on a new one that is not half as good, but I am glad I didn't.

Here is a side by side comparison of the carb before and after painting. Carb was completely degreased, then soda blasted and washed and fuel passages were sprayed with carb cleaner to dislodge any remaining dirt. New diaphragm is installed. Engine block and sump cover painted. Unfortunately, I found that paint chips really easy near sharp edges and corners. Pretty sure they painted these engines after being fully assembled. Oil seals were a real pain to remove and replace. Used a 15/16 socket to tap them in evenly/gently. A 22mm socket would have been better though. Need to finish assembly and also get some oil. Internals go back in tomorrow, but first I have to find torque values for the sump cover, conrod and head bolts.

It was a nice sunny day today, so I started to put the engine back together. Used lots of assembly lube on the journals, conrod/cap, bushes, cam and tappets. Quite thick, sticky and the stuff I used is coloured with red dye. That way you can keep track of which parts are lubed.

Here is a comparison of the old vs new ring gaps, checked using a feeler gauge. All rings were replaced.

- old. new.
compression ring - 0.66mm 0.50mm
oil scraper ring - 0.65mm 0.49mm
oil ring - 0.83mm 0.47mm



The new compression ring doesn't have a 45 degree chamfer on one side like the old one did. Very peculiar.



Using my bare hands I roughhoused the piston and rings into the cylinder. A piston ring compressor would have been handy, but managed to save myself $40. Best way to lift up those tabs that lock the bolts and prevent them from rotating, is to use a ratcheting spanner. Very easy this way. A drive pin punch and hammer is OK for flattening the tabs, that is what I did so I could remove the bolts. According to the manual, the specified torque for the conrod bolts on this engine is 100 inch pounds or 11.3 Nm.





Markings on the cam gear and crank gear were aligned before putting on a new gasket and torquing the sump bolts to 85 inch pounds or 9.6 Nm.




Also decided to remove some of the rough casting from the intake port using some needle files.


The valves, keepers, springs and head will go back in/on tomorrow. I'll Probably reset the points gap, put the coil and the flywheel back on and check for spark. Putting a new recoil spring in too, that'll be fun.

Going to keep my eye out for a cheap 2nd hand engine just like mine. Has to have a crank with a good keyway. Then I should be able to swap them over with out any problems. Until then, I am going to proceed with caution and get pulley key stock and cut it to length. The original is 1.5*0.2*0.2 inches. I'll probably find some at a bearing shop and cut to length at about 2". Then there is no room for the key to move.

Final random rebuild pics. All I have to do is top up the oil and gas, then try to start it. I guess it's time for the chassis resto now.

It's ALIVE!!! Started first thing this morning.

Had to mount it back on the frame to use the throttle lever. It was a lot easier to start having the added stability of those large rollers and the leverage you get with the engine being down low.

Damm vibrates a lot at full throttle, can see why those frame rails cracked.

Link to a short 4sec video of the engine on the lawnmower.

Briggs is finally working!

Hi,

It certainly doesn't help when there's no weight or resistance attached to the PTO shaft.

But then how else do you think these units got their nick name of "Briggs & Rattlers"

Cheers,
BB

I agree with BB, Briggs engines usually rely on the flywheel effect of whatever is attached to the PTO shaft to provide even basic operation. Many is the person who has tried to start one without the clutch (or on a vertical shaft Briggs, the blade plate) and experienced a quite unpleasant kick-back through the starter cord. It is not legitimate operation of the engine.

Isn't kick-back caused by pre-ignition, from bad plugs or advanced timing? I.e. firing inside the cylinder before the piston has reached TDC. Or, possibly not pulling quickly, hard or far enough to get past one of the compression strokes, at the time of ignition. Thus, causing the engine to backfire. Like the people who were unfortunate enough to have arms and wrists broken, apparently trying to hand crank Ford Model T's.

Anyway, I only started it for a few seconds, just to see if everything was working. At the moment, I don't have a clutch half that'll fit on the PTO because it's worn and damaged, to the extent that a key won't fit at all on one side and on the other side, it is completely loose. Someone on this site also mentioned, that I shouldn't run this mower at all in this state, until I repair the cracks and sag in the frame because I risk further damage to the rails, clutch and PTO.

First and most important, you should certainly not run the engine attached to the frame until the frame is in good condition. You may do additional damage otherwise.

Kickback is not caused by pre-ignition: pre-ignition only happens when some deposit or surface in the combustion chamber is quite abnormally hot, which won't happen when the engine is not running. You may have been thinking of detonation, which can occur at more normal temperatures, but at least for a petrol engine, it is quite unlikely when performing a cold start provided the fuel being used is petrol. If you substitute kerosene or paint thinner for petrol, and particularly if you are making a warm restart, it is quite possible however, and could be very unpleasant.

Petrol-fuelled engines always have their ignition timing set to before top dead center, unless they have variable ignition timing, which few mower engines have. So, the timing is a compromise between what would be needed for maximum power and fuel economy, versus what enables the engine to start without kicking back excessively against the starter.

In the case of Briggs side-valve engines in particular, very great attention has been paid to minimising the weight of the engine. As part of this process the size of the flywheel has also been minimised. The manual warns the operator against starting the engine without having it attached to whatever it is supposed to drive.

Grumpy, thanks for the advice and the clarification. Indeed, the first time I tried to start the engine, the pull cord retracted violently.

I have since discovered that the PTO does need some weight on it. This is to provide the needed momentum to carry the piston past TDC. Also, the pull-start mechanism on this Briggs is not very smooth or robust and it hasn't held up very well with age. This surely doesn't help.

Finally got around to disassembling the frame today. Tried to remove the bed knife by heating it up. Could only get two screws loose. I will try some WD40 before I have had enough and drill them out.

What do you think of the reel, can the dents be straighten out with a dolly before sharpening? What about the bed knife, replace??? Looks like it was sharpened on an angle.

The frame doesn't appear to show any cracks at all from underneath. But the rear rail does need straightening. Bearings on the reel are shot and need replacing. Most components look in reasonable condition.

Why are you trying to remove the bedknife from the sole plate? You have a very good chance of stripping the thread in the soleplate, and you can't achieve anything useful by removing it anyway. The bedknife has to be properly fastened to the soleplate to be able to grind it. Since it appears you don't have a suitable clamping facility to properly remove the bedknife, you don't have a facility to install it either.

Normal practice is to deliver the complete soleplate (with bedknife attached) and reel to a reputable, specialised machine shop and have them sharpened together. If the bedknife needs to be replaced, they will do it.

I was thinking of replacing it with a new one that is pre-ground. Seen them for sale on ebay for about $40. Then repair, powdercoat and sharpen the reel because they are really expensive to replace. I own a bench vice and some g-clamps that might work.