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12.5 briggs ?


midnightrider's Avatar
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03-05-05, 08:26 AM   #1  
12.5 briggs ?

i have a 12.5 horse briggs that would knock on high rpm tore the motor apart the crank has no play on the rod the wrist pin has no play on the rod. but the piston has a little bit of play on the wrist pin would that cause the knock.if so do i just replace the piston and gaskets thanks

 
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03-06-05, 12:52 AM   #2  
12.5 briggs

Hello midnightrider,we have many people on the forum who in well versed and qualified to
answer any of your questions pertaining to small aircooled gesoline engines....but,12.5 briggs just isn't enough infomation to go on to give you an answer that has the accuracy you're looking for.

Is your engine one cylender or two?over head valve or an 'L' shaped engine(with a valve train)?Is it aluminum or cast?And please supply us with the model#,type# & code# of your engine then we will be able to help you a lot better and faster.

 
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03-06-05, 01:30 AM   #3  
Hello Midnightrider!

Assuming this is a 280000 series engine, chances are you have a bad balancer bushing surface. Probably on the bore closest to the top of the engine. This is a common source for high rpm knocks on this engine. That said, a wrist pin should be snug in the piston with no play. If there is noticeable play, I would replace the piston. That will definitely cause a knock, although I don't think I ever saw a 12.5 briggs with a loose wrist pin, so I don't know how loud or bad the knock would be. I would think a loose wrist pin would knock at all rpms.

Just a thought....Is the play you described side-to-side movement of the pin, or up and down (the hole in the piston is too large for the wrist pin)? Side to side play is fine.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

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03-06-05, 06:55 AM   #4  
the model is289707 type 015401 code9301184e. The play is not side to side.It has just a very slight play up and down. The cylinder is in good shape. And you said it could be a bad balance. Bushing how do i check it.And on the crank there is a gear that runs the cam. It has a little play could that cause a noise. Should i replace the key way in it. Thank you. So your saying the rod and wristpin is probly fine. Or should i replace it all. Thank you for the help

 
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03-06-05, 08:09 AM   #5  
Azis
If the wrist pin has play other than side to side, replace the piston and wrist pin, which I am sure what cheese is saying. You may find while pricing that the rod and rings are also worth replacing while you are this far into it.
Without digging into a manual I dont know about the balancer.

Cheese?

 
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03-06-05, 03:25 PM   #6  
12.5briggs

OK.midnightrider,here is what i was thinkiing.in my 30+years i have never seen a 12.5hp. engine wear the wiest pin or piston enough to knock...so i can't say anything bout that.But i have seen the synchro-balancer mass up.Sounds like a crew of people trying to geet out.Take the the balancer & check it for discololing.(signs of friction and wear).Mike it.The problem that i've run into a lot has been too much play on the mag side the balancer(top end),also check the pin and rod that holds the balancer to the block.On your engine spec.reading is 2.202" or 55.93mm on eccentric(crank end of top piece)and on the other the reading shoult be 2.212" or 56.18mm(beaning end)).Make sure crank will turn free.after replacement of new balancer

balancer part# is 2922423

OOOh make sure too,to get the lubercation hole in rod on mag side in order to give rod excess to plenty of lubercation.

 
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03-06-05, 06:30 PM   #7  
Repair Guy said what I was saying. Never seen the wrist pin with play before, but replace the piston and pin if there is play there ( a piston assembly would come with wrist pin and rings too). The balancer is the large chunk of metal that is connected to the crankshaft. It has 2 large holes where the crankshaft goes through it. Normally, this is the first area for trouble on these engines. The balancer bushing on the top side of the balancer (when installed in the engine, the side closest to the flywheel) usually wears out. It often ruins the journal on the crankshaft where it rides too. This pretty much junks an engine because replacement parts are more than it's worth. Maybe that's not the case here, we hope.


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03-07-05, 11:08 AM   #8  
Have you tached the engine to know what RPM it is running at top no-load? Once the cylinder head was removed how much carbon build-up was there? Here are critical tolerances to check: Bore - 3.4365-3.4375", Piston Pin - .799", Pin Bore ID - .801", Crank Pin Bearing - 1.252", Piston Pin Bearing - .802", Crankpin reject size - 1.247", Crankshaft Bearing reject (both) - 1.376", Counterbalance Bearing area - 2.212", Counterbalance Eccentric - 2.202", Crankshaft Endplay - .002-.023", Cylinder Bearing reject (both) - 1.383",

 
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03-08-05, 05:40 PM   #9  
thanks for the help i ordered a new piston and rings. A new wrist pin and connecting rod.And gasket set and book for it And a new balancer for it thanks for the help

 
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03-09-05, 08:58 AM   #10  
Let us know when the job is complete and the results upon such.

 
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03-14-05, 03:52 PM   #11  
well no good news the guy that was fixing it never checked the crank after i orderd all the parts he started to put it togather and then called to tell me the crank was bad it was wore where the balancer hooks up . can the crank be turned there and put differnt size bearings in the balancer. or do i have to buy a new crank. or is there a place i can get a used crank.what all sizes and models might fit it thank for the help

 
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03-15-05, 01:30 AM   #12  
Man! I think you'd have been better off with a new engine! Anyway, the crank may or may not have removable surfaces where the balancer rides. Since yours are worn, I'd suspect that this was the source of the knock, as mentioned earlier. These jornals can't be purchased from briggs as far as I know, but you might be able to find used ones at a small engine shop. Turning it is not an option because the bearings in the balancer are not removable or replaceable. If the journals are not removeable, then you're in need of an entire crankshaft. Not cheap.


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03-15-05, 03:15 AM   #13  
The crankshaft eccentrics cannot be turned under as there is no undersize bushings for the Syncro-balance. A new crank is the only option, unless you can find a good used one. New part number is 693653 and goes for $155. I will say that this particular crank is used on many engines, all of which are 28c.i. displacement, but to find this particular crank, used, likely will be difficult. You're basically commited to finalizing the repair at this point since you already have alot of money tied up into it.

 
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03-19-05, 08:09 PM   #14  
ok thanks for the help found a good crank put it togather .got the fly wheel on it and put the starter on filled it up with oil turned it over about a 100 times with out the plug it turned real good. put the plug in it turns real good seams to have good compresson my question is there a way to tell if it is oiling good enough thanks for the help

 
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03-20-05, 12:12 AM   #15  
No, there is no oil pressure in that engine. The oil is "slung" around in the engine by the oil slinger on the governor gear.


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03-20-05, 07:17 PM   #16  
I was just wondering if there is a way to tell if it is slinging the oil.The oil slinger it runs of the cam. I lined it up with the governer rod and then put the gasket and base on .The base went right into place and then put all the bolts in it. Does it sound like i did it right. Thanks for the help. And how is it recommended to break them in

 
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03-21-05, 12:37 AM   #17  
Sounds like you're good to go. I crank-em up, let it run at around 3/4 throttle for 15 minutes or so, and then I put it to work. Mow some grass with it and work it out. After 5 to 10 hours of use, change the oil again. (not that you have to use it that much right now, just when you accumulate that many hours on it).


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

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