rod broke on 6.75 hp briggs

Geo-TH,In · Jan 17, 2014

I took my boys engine apart and it looks to me like the rod bearing seized up on crank and the rod snapped off very close to the crank.

I told my boy, that it looked to me like the engine got hot, oil failed, rod bearing got hot, expanded and locked up on crank causing the rod to snap. The crank has minimum scaring. I think I could use emery cloth and polish it up to use again. Don't think he will rebuild it. I found hem another engine. Will keep blown engine for parts.

He says he read that the newer 6.75 hp briggs have bad rods.

So, who do you think is right? A bad rod or what I described above?
Thanks
George

az .peapicker · Jan 17, 2014

Good Morning George
I would say that your probably right .Lack of lubrication caused the connecting rod to seize on the crank & caused the rod to break. What did the cylinder & piston look like?

John in Az.

[email protected]

Mark-Ia · Jan 17, 2014

Lack of oil. Did you know that you could put some muriactic Acid on that crank and it will eat the aluminum off. You might make the crank out-of-round using emery cloth

Geo-TH,In · Jan 17, 2014

I didn't pull the head off, but what I could see, the piston and cylinder wall look good. Just snapped rod off close to crank, the part of rod connected to crank broke two pieces. Bolts holding end cap didn't come off.

Don't think the motor will be fixed. Got another 6.75 hp briggs and put it on the mower he likes, a John Deere push mower.

I can feel a few scratches on just one small part of crank. Many scratches all over the place on rod bearing surface.

I really think he was running motor too fast. The motor only runs one speed. No speed control for carb. So I think the governor may have been set too fast, broken down old oil, little low on oil. Really don't think it was a defective rod. More like bearing failure for some reason.

George

RN · Jan 17, 2014

Ask what kind of oil he had in it when it snapped? The "leftover from car oil change last winter" 5-20 used in the summer is not good for splash lube cushioning of rod bearings- better with Valvoline racing 30 weight or plain 30 weight non detergent IHC low ash. Whatever, the insertless bearing aluminum small engine rods are a minor weak point of cheaper engines, oil changes and checking level helps engine survival and cranks can be salvaged with heads for another engine. Some old hot rodder tricks from the 1950s sometimes help- like a grooved bearing surface in rod upper side or cap or both. RN.

ShadetreeRet · Jan 17, 2014

May be a combination of both,George. We all know that air cooled engines run hot. Many people fail to ever clean the cylinder fins under the shroud and trash accumulation will restrict air flow. If not using a good grade oil and changing on regular basis, can cause problems. I have heard that most small engine manufacturers are cutting corners now, so a combination of heat, improper oiling, and possibly poor quality of the part involved, is likely the cause. BTW, is this a vertical shaft engine?

T.R.K. · Jan 18, 2014

Nope. Not a bad rod. It got low of oil first. Then the rod seized.

You can soak/eat the aluminum off the crank with muriatic acid, but use all the safety precautions.

rod broke on 6.75 hp briggs

Geo-TH,In

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az .peapicker

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Mark-Ia

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Geo-TH,In

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RN

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ShadetreeRet

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T.R.K.

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