OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk My Briggs 11.5HP rebuild

I have started a couple of threads on how to go about this but thought it better to put everything in here as I go along. It's taking me a while so it could be painstaking but time is scarce. Probably not helped by the fact it takes place at a friends residence who has the available space.

The story is that the Cox mower was a one owner which looked to be in excellent condition. It was owned by an elderly couple. The husband passed and the wife continued to use the mower without service and the failure was a combination of low oil and sloping ground.

The rod let go and there appears, to my untrained eye that there is very little associated damage. There is the slightest bruise on the crankpin hardened surface. You would have to wonder if that bent the crank. It is a spot about 1mm sq if that but it is a slight indentation into the hardened polished surface.

This is the remaining portion of the rod


This is the soup that came out of the casing


Armed with this paper. I chose 800 grade and found it to be more than suitable. I used only the one piece as I found when new it was easily abrasive enough and didn't want to take too much material away from the crank pin.


And this as a lubricant


In this fashion


Over time with careful application and patience I aimed to remove this aluminium deposit




Till I slowly came up with these results

The piston looks good with only one major mark on it evident in this photo


The rest look good






The cylinder still has good cross hatching. Hard to photograph but easily visible to the naked eye.

The casing looks umarked


As for the valves, well I can't tell as I haven't seen that many mower set ups


I wish the mower had an hour meter to give some indication. I really think it is a low hour unit. Tyres, blades, belts etc seemed to be original and in suprisingly good condition.

I suggest you remove the rings and clean the grooves, Aldo - there could be some rubble in there. Be careful not to remove any metal from the piston, of course. You might measure the ring gaps, with each ring in turn pushed down about 1 cm from the top of the bore. Use the top of the piston to push them down square to the bore. If they are any more than halfway between new and maximum allowable gap, it makes sense to replace them while the engine is dismantled. Incidentally that tiny score on the side of the piston is of no consequence in a 4 stroke. You do need to linish across it with some fine emery or wet and dry though, in case the ends or edges of the score are raised up.

I've seen crankpins that looked like yours, after your cleanup, that seemed to work quite satisfactorily. It doesn't look excessively rough. So far as the divot out of the crankpin is concerned, it depends where it is. The most sensitive spot is near the highest part of the pin when it is on TDC, because that is where the maximum pressure is applied to the rod by the piston. In essence, I wouldn't be even inclined to try a divoted crankpin on a modern car engine, but on a splash-lubricated side valve engine with a maximum speed of 3,000 rpm and a compression ratio of 6 to 1, I'd try to re-use it myself, and be hopeful of success. I can recall a friend hearing a relative arrive in an ancient Ford 10 (1940s/50s side valve with splash lubrication and white metal cast into the rods, no bearing shells) and noticing that one big end was clattering horribly. My friend took out the rod and soldered up the big end with a plumber's iron using ordinary solder, then scraped it a bit and put it back together. By the time the relative had finished socialising, it was ready to go, and it apparently lasted for years. There's something to be said for low tech, low stress engines. Edit: By the way, crankpins are not usually hardened. I can recall when Austin Healey made a racing version of the first model Austin Healey 100 in the early 1950s (the racing model was called 100S), one of the engine modifications to make it suitable for the Le Mans 24 hour race, was to nitride the crankshaft. To wander even further off topic, the 100S engine also had a different cylinder head from the 100 model - a conventional port design instead of the extraordinary, perhaps even insane, design of the standard head, which was taken straight from the Austin A90.

Please be sure there is no aluminium deposited on the bore, that would be unlikely to work well. You can easily polish deposits off the bore the same way you cleaned up the crankpin.

I can understand the problem with the old lady keeping the place going after her husband passes on, and not doing the necessary mechanical maintenance. I can recall an old guy buying a tiny Japanese car new, then dying a couple of years later. His widow kept driving it, and putting petrol in it. After 3 or 4 years it finally seized. Still hadn't had its 1,500 km free service, let alone had anyone look at the dipstick.

Thanks Grumpy. It is amazing where metal flake has ended up. Upon removing the valve spring cover I found metal had made it's way into there.

I thought that a light hone might solve my issues with metal flake in the bore. I guess quite a lot of pressure put it there it seems to be caked hard in some areas. One spot on the crank weights required removal with a screw driver.

I need to source some removal tools for the valve springs so I can give the case a good wash down.

I was going to buy new rings but may take your advice on checking the old. The motor hasn't turned in a good 2 years I guess so I have no idea if anything works, starter etc. I don't want to invest a great deal on an unknown engine.

Most honing seems to be done with cheap tools that scratch the bore and leave it in a considerable mess, especially if you don't have a way to rotate them at around 200 rpm so you can pump it up and down the bore fast enough to get the desired 30 degree cross hatch of hone marks. If you have a bench drill you can make a simple hone out of a wooden cylinder barely smaller than the bore, so it fits very easily down the bore when the drill's motor is not running. Use a nearly-radial saw cut down the full length of the cylinder to anchor one end of a suitable sheet of abrasive paper, cut to just barely make one circumference of the cylinder (not counting the bit that goes into the anchor slot) without overlap. By rotating it as slowly as your drill will run (which is fairly slowly if it is a double-reduction type of drill), whipping the drill handle up and down rapidly with one hand, and trickling kerosene down the bore with the other hand, you can get a professional job, without buying a tool that isn't any good anyway.

So far I'm finding it easy to remove mower engine valve springs with my fingers if they have the keyhole style of retainers, which most Briggs and Honda engines do. You can also use Joe Carroll's method, which involves a pair of plastic cable ties, positioned on opposite sides of each spring.

With regard to the valve chest cover, remember it is also the Positive Crankcase Ventilation valve, and it probably has metal swarf inside it so it needs to be cleaned out properly.

Howdy. I thought I would jump in and let people know that I have ditched the rebuild. The lower counter balance weight bearing ended up being too close to the wear limit for my liking. The crank eccentrics were not replaceable. It seemed the wearing surface was the crank eccentric and not the weight white metal surface. The costs involved in rectifying that issue and the intended use I required of that motor didn't end up being cost effective. I didn't want to chance reassembling the engine and having a knock that would annoy me.

I hope at least the shots of cleaning the crank pin and the suggestions offered by others are useful to someone.

Thank you for keeping us posted, Aldo. I understand your concern. Problems with the reciprocating weight type of balancer seem to be a common reason to scrap those engines. Perhaps it happens because at some time they've been run with more sludge than oil down there in the crankcase - the problems I've seen reported are usually following broken connecting rods, like yours. You've made it clear that wear in the crank eccentrics of the synchrobalance linkage is the most important problem in repairing them, at least in your case.

Will you get another Briggs synchrobalance engine, or will you update to OHV? The reports that I've seen on the Briggs Intek engines so far have been worrying. The Vanguards seems to be less troublesome than the Inteks, and perhaps the Hondas have not been troublesome at all. However all of those options seem likely to cost a lot more than getting hold of another old side valve Briggs.

Hi Grumpy. I haven't made a decision on this yet. I will keep an eye out for a Briggs. I like the old side valve engines. I am not interested in the OHV models. I sold the engine in bits. I really should have kept it and perhaps used it for parts if I found another. The rest of the engine was very very good. Ah well rushed decisions.

It sounds as if you just needed a second-hand crankshaft to get it going again. An alternative might have been to get the eccentrics re-machined and then bush the matching links down to suit the remachined crankshaft, but all that might cost as much as a second-hand engine.

I hope you find an engine soon and at a reasonable price. Please let us know when you have news - I'll leave this thread open for now.

I have managed to source a second hand 11.5HP Briggs. It is out of a Craftsman I think so I will need to chase down some parts, muffler, tank etc. Tank was frame mounted and muffler is one of those front of chassis exit jobs. I will post some pics once I find these bits. I have yet to start it so I have taken a bit of a gamble. I have simply spun the motor over by hand via the pulley as there is only electric start.

Out of curiosity, would you remove the head or the sump to just have a look see or simply wind it over and see what happens?

It's a 28D707 made in 94.

I'd check the compression - generally I don't remove a cylinder head unless I need to. I only remove the sump if I have some reasonable suspicion of trouble, such as too many, or too large, metal bits in the drained sump oil, strange noises, or jerky movements of something. For compression testing Briggs advises you should rotate the engine backwards until you find compression, then spin it backwards and see how well it bounces off the compression. With any engine but a Briggs, I'd spin the engine with a geared-down electric drill and measure the compression pressure, but Briggs' Ezyspin feature makes that a bit pointless.

I ended up with anothe similar engine, an 11.5 28D707. This one was out of a Craftsman I think so it didn't have a tank or a suitable muffler.



After sourcing all the right bits second hand I started to check the old girl out. I found the first issue...



Due to me needing the correct head bolts to hold the front tank bracket I pulled the head to investigate the problem.

The head bolts around the valve area confirmed the leak issue.



I did a dodgy check of the head with a straight edge and it seemed to be ok.

I put in a new gasket and hoped for the best. The bore had some marks which I ignored once the head went on;)

I torqued the head down once I picked up a long 1/2 inch socket so I could get over the tank bolts.


I fitted the rest of the gear and pulled the carby bowl



You might see my mistake in the above picture

Aldo, it would have been worthwhile inspecting the valve seating while you had the head off. I agree that head gasket was leaking, by the way. You should also check the tappet clearances.

Some of that looks like sediment in the float bowl, and some of it looks like corrosion. The trick is to clean it up without removing more of the interior surface finish than necessary.

I haven't seen a mistake in the picture. Incidentally I think the in-line fuel filter may be a good idea - I'd rather keep the fuel clean, but you can't always do that in practice. You have the red filter, which is correct for gravity feed.

I ended up putting this engine together. I pulled down the carb, cleaned and reassembled with new gaskets. Sourced a new starter gear. Pulled down the starter as is was seized and got it sorted. Installed the second hand tank and brackets. Installed the second hand muffler. New fuel hose and filter plus a head gasket and it was finished.

Threw it on the Tractor 11 and took it for a run. Changed the oil again just to be sure and then put around 12 hours on it slashing 6 foot lantana, ferns, gum saplings and anything else in its way. What a great old machine. The engine has run perfectly with no smoke or strange sounds. Very happy.

Thanks for the report, Aldo. Obviously you sorted this one out without needing any help, and got a good result.

I'll close this thread. If anyone needs to post to it, just send a PM to a moderator.