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St-120 Briggs And Stratton Engine Help Needed 12 Hp I/c

measuring clearances

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#1 Nascar3244888 OFFLINE  



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Posted January 01, 2014 - 11:50 AM

Finally got started on some of my projects that I have been putting off.  In case no one knows this, I am a big fan of the ST Series with the Hydro Transmissions.  They are great little tractors.  Anyways, I have an St-120 that needs an engine, and I decided to go with one Ive had laying around for a few months which is a 14.5 OHV B&S.  The engine that was in it from the factory was ran low on oil the way it seems because it smokes bad and knocks as well.  As I wait to find a muffler that will work on my Ohv engine I decided to take the old 12 apart and try and take some measurements of the clearances to try and get an idea of what was causing the knocking but also to just do it to try and get some experience on how to actually read and understand what Im actually looking at.  


Briggs and Stratton 12hp I/C:  281707-0127-01-87061011


Above you see the engine numbers of which Im working with.  I do have a B&S L-Head Design Book foor which I will show some of the specs they list in there book.  I dont really understand what Im looking at other than the fact that I have measured The rod journal and the connecting rod itself and I would like to share them with you guys and see what your take is on them.  According to the book the rod itself has a reject size( which I assume means no good ) of 1.252.  I am getting a reading of anywhere from 1.245-1.252 in different spots.....What does this mean??? Also the crank journal has a reject size of 1.247 and getting a reading of anywhere from 1.251-1.258 at different spots....what does this mean???  I really am trying to understand the readings and what is good or bad.  As far as the condition of the rod & crank where the go together: there are no discolorations, no scoring, both are nice and smooth. The bore is scored badly and you can feel the scoring with your finger nail, it does have a lip as well at the top.  I also found that the side of the piston that faces the valves to have a crack on both sides( Pto & Flywheel Side ) near where the skirt is. Im guessing the engine isnt worth fixing./ salvaging but if it could be I wouldnt mind fixing it...

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#2 EricFromPa OFFLINE  


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Posted January 01, 2014 - 07:41 PM

Most of the time when you have a knocking Vertical L head briggs it's going to be the Synchro Balancer bearings or the Dogbone that's making all the noise.If you could have the crank balanced and completely remove that huge hunk of iron that engine would be worth saving.


Here's a decent video on L-Head briggs synchro balancers.



If you decide to put the 14.5hp in it,you can use the stock 12hp muffler on it.Just cut the header off on the 14.5 just before the 1st bend and use that for your Flange to weld up an exhaust.They use a standard 1" flange on all Briggs OHV Singles.


Make sure that you replace the Float needle seat if it's sat unused for any amount of time.They have a nasty habit of leaking and filling the entire block up with fuel.Also check the gaps on the valves.Exhaust should be .003-.005 and intake should be .005-.008.If not set correctly the compression release will not work and you'll burn up the starter.If the rockers are loose then more than likely the head gasket is on it's way out.The older 14.5hp head gaskets don't blow out as often as the 15-19.5hp OHV singles.They seem to have more material between the push rod galley and the jug.



Head gaskets are $9-$18 each depending where you get it.Carb kit is $8-$30 depending if your engine has the Walbro LMT or Nikki carb.

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#3 bolex OFFLINE  

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Posted January 02, 2014 - 06:20 PM

bolt the connecting rod together and recheck the bore diameter any variation in size means that the crank end of the con rod has eggshaped which means it's out of round and should be replaced . if the rod and piston are both shot i would replace them , have the cylinder bored to a standard oversize and the crank turned to a standard undersize and replace the rod and piston with like sized components and the engine should run like new with maybe just a bit more power , happy motoring :thumbs: