OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines 4hp Briggs and Stratton Quattro

Which one did I blow up that time? I think there are 3 or 4 there, the old gmc went off with a pretty good bang sayonara Chinese Briggs.....

yep it was the gmc one! mine makes exactly the same sound. To date I have never found one that bad.

Any suggestions on the not starting would be appreciated! I am sensing compresion given grumpy's advise but I'll wait and see what he thinks unless you have a thought?

I'll be interested in Joe's comments too, Michael. It seems though that your engine has little compression, and a major clunk internally. It sounds like a partly broken (one side of the big-end bearing only) connecting rod, which sometimes happens to Briggs engines that are run without oil, though I think it is more common for both sides of the big-end bearing to break. However from previous threads, especially Joe's posts, about early Chinese Briggs engines, I think the combination of this seeming to be an early production Chongqing engine, and it having a major internal clunk, suggests there is a very good chance that at least one of the connecting rod bolts was not tightened in the factory, and what you have now is a badly damaged connecting rod. If the crankpin is OK, it is possible that replacing the rod would get this engine back into operation.

My guess as to why there is very little compression, is that the loose, mangled big-end results in the piston not travelling far down the bore on the intake stroke, so the compression ratio might only be 3 to one, instead of 6 to one. That is not enough compression pressure for the mixture to ignite.

You might find it interesting to open the crankcase and find out what is actually going on, and whether my guess is correct. Joe is the expert on what you might find in there.

The lightweight blade of a gmc mower gives the false impression that they have compression, I concurr wit hthe thoughts of maybe the conrod bolts never being torqued enough at the factory, and the very obvious fact that they are very pourous castings.

I would say the problem with the compression would be a HUGE ring gap, I have pulled some down before with a (no bullcrap) 5-10mm ring gap, they will act like they are trying to fire up but they are very weak. ill see if I can digup a photo of at least one bad ring gap case I have seen...

this was a 3 year old briggs and stratton with poor compression and just a tad smokey, however I have seen worse.

I agree the not starting, and very little compression, could be just ring gaps Joe, but what about the internal clunks? Peter reported that they sound like your bad Briggs with the loose connecting rod.

Personally I like to do the oil-through-the-plughole trick to check for ring leakage before opening a crankcase.

Grumpy and Joe,
Thanks for the comments - It makes for an interesting thread. I havent had a chance to pull the engine down yet as I've been doing a few other jobs.
I am interested in what you mentioned Grumpy about putting a teaspoon on oil down the plug hole to see if the compression increases. If I read that right, id like to try it.

I will pull the engine down when I get some spare time so I can learn from it.

Funny thing is I got given another GMC which has a 375 Briggs that actually has a snapped rod. It must of went off with a loud bang. What I didnt understand though was the engine had plenty of oil (I am thinking however the owner must have added some oil after it went bang) Nevertheless I'll eventually pull it down to ensure I get all the usable parts off them both.

The oil-in-the-plughole trick is a standard old-time mechanic's test, Peter. It works on the principle that engine oil is fairly viscous, especially when you first put it into the engine (before it warms up, if the engine is warm). It therefore takes a while to be forced past the piston rings by compression pressure, when you pull the starter. This means you can do a compression test, then put in the spoonful of oil and immediately to another compression test. If the pressure increases by more than say 10-15 psi with the oil in there, the rings are leaking. If it is more like 40 psi, which it commonly is with worn out rings, you reach for the spanners.

Just remember not do do something I did a couple of years ago. I was doing the oil trick on an OHV Honda. Those engines have 8:1 compression and therefore take a bit of rotating, especially if you are doing the test with the decompressor disabled, as I was. I therefore used a very large two-handed back-geared electric drill to spin the engine by the flywheel nut. After I got the second reading (the one with oil in the cylinder) my mind must have wandered because I removed the compression gauge from the plughole before I let go the trigger of the electric drill. I was wearing a brand new ODK-logo white T shirt at the time ....

Thanks Grumpy I did some checking on the engine and I am now pretty sure the rings are stuffed.
On a separate note since we were talking about loose rod bolts I thought I�d add a couple of pictures. It is very interesting to note the bolt connecting the rob was loose, supports Joe�s comments.
The engine had plenty of oil when it came into my possession. I was pretty sure it was stuffed and the photos supported my findings.
I have completely rebuilt a number of two strokes however I haven�t split a case on the briggs engines before so I�d appreciate some of your comments around each of the parts and key things to consider about repairing the engine. I have watched a few videos and it doesn�t seem that complicated but I have no doubt there are some special things I should consider like usual.
At worst out of this exercise I have a perfectly straight crankshaft that might come in handy for a future project.
Oh and I haven�t got the piston out of the bore yet.

Funny point - I have never snapped a boss - they must be made of rubbish

While it is commonplace for a side valve Briggs to break its connecting rod, it is nearly always for one of three reasons:
1. Engine run without oil.
2. Engine run with governor disabled, resulting in gross overspeed.
3. Engine incorrectly assembled.

The state of the crankpin is the most likely issue as to whether the crankshaft can be saved. So far as the rest of the engine is concerned, my recollection is that it is an early Chongqing Briggs with a broken connecting rod. There seems little point in regarding it as anything but scrap aluminium. And I don't know what the broken piece is, in that sixth picture.

Thanks Grumpy.
A key point to note was the connecting rod bolt was loose and not snapped. It actually fell out of the rod end which made me immediately think of the engine not being assessbled correctly.
The broken piece in the 6th picture is a piece of the boss and nothing else.
1) The governor was installed properly before I stripped the engine
2) The engine had plenty of oil. Well it could have been filled up after the rod snapped, Who knows some owners don�t care of their machines.
I thought it might not be suitable for repair because of the minimal cost of a replacement block on ebay.
I was hoping you could provide some advice about getting the timing correct if I was in fact going to repair the engine and any other tricks about re-assembling the bottom ends. I�ll no doubt one day be repairing a bigger more worthwhile engine so your advice will come in handy. Ive watched some video�s and they are all slightly different in the way things are explained.

The timing is not much of an issue. The ignition timing will inevitably be correct if the flywheel key is intact and in place correctly. The valve timing requires that you put the camshaft in the correct rotational position when you reinstall it. Briggs engines have timing marks on both timing gears: the one on the crankshaft, and the one on the camshaft. You position the crankshaft so that the dot on its gear is at its nearest point to the camshaft, then you slide the camshaft in so that its dot will end up at its closest possible point to the crankshaft gear's dot. Note that the gears are helical, so the camshaft will rotate a bit as the gear slides into mesh: you have to start with the dot about one tooth below the crankshaft one, because the camshaft will rotate clockwise about one tooth-width as it slides into place. It's a bit like leading a duck slightly when you point a shotgun at it. (Many Hondas don't have a dot on the crankshaft gear, because it is not on a key, it is just pressed on. You use a slightly different method with those - you put the crankshaft on Top Dead Center, then slide the camshaft in so that its dot is as close as possible to the crankshaft gear. I prefer to put a centerpunch dot on the crankshaft gear before I lift out the camshaft, so I don't have the hassle.)

Installing the piston and rod is a bit more complicated, since you have to worry about half a dozen things while you are doing it. This would be a rather long post if I went into all of them.

Grumpy thank�s for your explanation on setting the valve timing. Upon disassembly it was relatively easy to line everything back up as it should be. When find an engine that needs a full re- build and we agree it to be a worthwhile process this post will be good to refer back to as a guide. Your centre punch idea makes the most sense, so I will chalk that up in the memory bank when and if a Honda is on the operating table (my shed bench)
On a separate note I checked the ring gaps of the last 2 dismantled side valve briggs engines and they were both massive. They were not however lined up as Joe thought they might have been.
I have watched a few video�s on correctly installing the piston and rod during a full rebuild and it doesn�t look too complicated however I would appreciate some pointers around the importance of installing the piston and rod correctly before I attempt the process in future.
Thanks
Peter

Hi Peter, there are many old Briggs engines out there with worn-out rings, it is probably the most common wear problem. You'd think the aluminium bore would be a bigger problem, but it has a much bigger surface area than the rings do. I suggest you measure the bore diameter if possible, just 1 cm down from the top, to see if it is worn before spending money on new rings.

There are some especially critical points regarding re-installing the piston and rings. These are not the only things to be careful about, as you will have noted from the video you've seen, but they are extremely easy to mess up. First, do not take the piston off the connecting rod, or you will have problems putting it back on the right way around. Second, put the piston in the right way around: the gudgeon pin is offset, it is not on the centerline of the piston, and if you put it in backwards you will wear one side of the bore excessively. Third, put the connecting rod cap on the right way around. It was bored at the factory with the cap installed, and if you put the cap on backwards the two halves of the big end bearing will not match (there will be an offset in the big end bore), which will cause early failure. Fourth, oil the crankpin before you assemble the big end bearing. Fifth, clean the bottom of the rod and the top of the bearing cap and ensure there is no dirt in the joint between them, then assemble them by hand, don't pull them together by doing up the bolts. Then tighten the big end bolts to the right torque, don't just "do them up". On such a low speed, low stress engine, just doing them up is probably fine after you've used a torque wrench enough times to be accustomed to what the right torque feels like, but with a small bolt in an aluminium rod, until you get the feel of it, it is very easy to get it wrong in one direction or the other. Remember, tighten the two bolts in stages, just snugging both of them first, then bringing them up to torque alternately, a couple of Nm at a time.

There are fifty other things you have to get right, I've just focused on the ones people are prone to getting wrong.