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Allisgrandson

blown 12 hp out of a B12 rod advice needed

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Allisgrandson

So, I blew the motor in a B12. It's my own fault. I put a muffler from a Geo Metro on it, but I never got around to posting picture of it. Well, the muffler created too much back pressure and burned off all of the oil. That's my theory, at least. The engine ran great for a year, never burned oil prior to me putting the Geo Metro muffler on. Anyway, the rod shattered. Multiple pieces.

shrapnel resize 2.jpg

The piston and rings look good, though they have a little baked on oil.

piston resize 2.jpg

The bore looks good, though I did find a small, 1/8th inch long nick in the bore.

bore resize.jpg

The crank worries me a little bit. There is a gouge on the crank in the area where the rod connects to the crank. Can this damage be filed and sanded down without much of an issue?

crank damage resize.jpg

Do I need to hone the bore where the nick is? Can I hone it?Can I replace the rod with a rod from a 10 hp motor or are they of a different length?Is the crank salvageable? Do I need to remove the crank to replace the rod and piston? How do I get the rings compressed to fit in the bore again?I look forward to any and all assistance with this! I'd love to get the 12hp running again. It was the first motor I was able to get running on my own, with help form all of you of course!Thanks,

57e05ae507008_shrapnelresize2.jpg.ca9be70a96d4be3b901cd07a9fd928e8.jpg

57e05ae51cfa2_pistonresize2.jpg.468333f0ee211e90b85804ceeb830b1a.jpg

57e05ae5a6997_boreresize.jpg.c73391aca2ea1f64947500d5eddb767c.jpg

57e05ae5c1737_crankdamageresize.jpg.cc68e35c6bae000aef09e1ef33d4d32c.jpg

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Brettw

The crank may clean up with muriatic acid and some #400-600 wet emory paper. Careful working with the stuff though, and lots of ventilation.

Without miking the cylinder, it is hard to say whether you can just hone it out. However, if I were to hone it, I owuld put in a new set of rings. First you have to determine if it is a standard piston and bore currently, or 10-20 over. It should be marked with a 10 or 20 (or even 30) on top of the piston if oversized.

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Allisgrandson

The 14HP will fit. I'd buy it at $50, but I don't want to pay for shipping. I have a 16hp in the tractor currently, but I'd like to rebuild the 12hp and keep the whole tractor original. The 16hp is going in a tractor with a hi-lo and hydrolift, If I can get around to it!

Can I hone the cylinder with a hone that fits in a drill and then put a micrometer on it to make sure it is in spec and still round?

Also, is the rod in the 12 HP the same as the 10 HP?

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Chris727

The rods are not interchangeable. Especially with that scratch, the crank might be junk unless you want to have it welded up and ground back to spec. The cranks can be turned to .020 undersized but Briggs has discontinued that rod, though they can sometimes be found aftermarket or NOS on eBay. I don't know if .020 would be enough to remove that scratch. The rings for the 12hp are NLA, all sizes. You could have a machinist bore it to 16hp bore specs and use a 16hp STD piston.

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huffy
quote:Originally posted by Chris727

The rings for the 12hp are NLA, all sizes. You could have a machinist bore it to 16hp bore specs and use a 16hp STD piston.


id="quote">
id="quote">That's what I had to do to rebuild the two 12hp's I had. Could not find .10 or .20 rings and other parts for a 12hp, so I had to have it bored out to 16hp and bought a kit off eBay with 16hp piston, rings, new rod, seals, etc. Frankly, though, unless you just want to rebuild the engine for funzies, it would be much cheaper just to find a replacement motor. Btw, to answer your one question. to get the piston and ring assembly back in you need a ring compressor.

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theniteowl

As Brettw said, the crank may be able to be cleaned up.

It looks to me like what you see on the crank is a build up of aluminum with a small patch not filled in rather than a scratch in the extremely hard surface of the crank.

Muriatic acid will disolve the aluminum without damaging the crank but the stuff can be dangerous to work with so you have to take precautions.

Any metal the fumes touch will rust pretty quickly also.

With proper precautions taken though the muriatic acid will clean the aluminum off that crank pretty fast. Then you would have to measure the crank to make sure it is still within spec.

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MrSteele

A muffler off an engine that size likely did not cause this problem, would not have had enough back pressure for the lawnmower engine to even notice, but likely ran very quietly. I have a Pinto muffler on a ZZ Briggs engine, has been there for years, and I used the tractor it is on for breaking ground and cultivating my garden. The pic of the oil sump pan shows an engine that did not have enough oil in it. The aluminum rod parts are visible where they landed after the break up. Before using any small engine, always check the oil, and check again when adding gas. How much oil did you drain out before tearing down?

As for the build, clean the rod journal, then check for roundness and size with a micrometer. If still in spec, it can be reused. The cylinder looks like a light hone will clean it up. Can't see in the pic, but how much ring groove is at the top of the bore? That can be removed with a ridge reamer. Be careful with the reamer, it's nickname used to be a 'short boring bar'. You can do too much with it. If you are going to use the block without boring, the ridge can break the top ring if too bad. If you can catch a finger nail on the ridge, it should be removed. Do not try to do this with a hone, you will score the cylinder in the ring location, ruining the bore.

Rods are available on ebay, or possibly from a good small engine parts shop.

Look at your piston to see whether it is standard (no marking on top), .010, .020. or .030 embossed in the top. I have been able to find .030 rings through my small engine part store. If the piston is .030, and the bore out of spec, it will have to be sleeved back to standard at a machine shop. A costly operation to keep the original engine. I have a 16 in that condition, now, worn .009 in a .030 bore. You can hone with a drill mounted hone. Release almost all the spring pressure on the hone. All you really want to do is break the glaze in the bore, not bore the cylinder. Use a light oil for lube on the hone stones.

enjoy the fun of the build!

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Allisgrandson

Thanks for the advice so far!

What does removing the crank entail for the 12HP? I this this is the syncro balanced motor, as it has the bulge and gears at one end. Does it remove easily or is there a trick to it.

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