Hello,
Have an 20HP briggs 2cyl, Model 461707 that I didn't put enough oil in when I changed oil in it,about 4 weeks ago. I run it for about 45 mins then started hearing the motor starting to rattle ,,checked the oil and sure enough I had put 1/2 quart oil and never put the second full quart in:(.This is what answering a phone call and having to leave does for you,,,,how stupid.ME!!:). Anyway, it did run for 4 more mowings fine but is noisy at idle,does not smoke,and has plenty of power.Actually while mowing you don't hear the rattling noise at all and sounds "normal".
Problem. This time when I went to mow,,the blades would barely turn. Looked and the PTO>lower seal is dripping oil steadily. I done some searching and found these motors do have a bushing/seal along with a second outer seal. Has anyone any experience with these motors? I m wondering if I also probably have either one or two rods,,that are probably scored as well,,or just the crankshaft bushings scored out?Trying to decide weather to rip this motor apart or just bite the bullet and buy a new rider. It is a 6 year old MTD/Yard Machine/46"cut. It is still in good shape as my yard only takes about 1.5 hrs to mow.

Thanks,
Barry

The lower seal can be replaced without removing the engine, you would only need to remove the lower drive pulley. The engine could be replaced for less then the price of a new rider.
I was thinking this was a V twin engine but just realized it's an opposed twin. Chances are the noise indicates some internal damage.

I would replace the lower seal and just use it until it starts giving you grief. Then decide whether to replace the entire unit. It could go quite a while before it gives out, or may not make another mowing, but the damage is done.

The bad things about Briggs and stratton engines or mainly any lawmower engine is that there are no connecting rod bearings, just a connecting rod riding on the crankshaft. The rattling sound you heard is probably the top connecting rod becoming loose from heat and wear, theres a good chance the crank is badly scored... All work is going to require you to remove the engine and take it apart.
With the engine out you can gain access to the back cover of the engine, remove the 4 bolts and pop off the cover with a screw driver. Here you can unbolt the connecting rods and see how badly everything is scored. To change the bottom engine seal, however is a little more harder., you must remove the oil sump and then pry out the seal and replace it with a new one.

The bad things about Briggs and stratton engines or mainly any lawmower engine is that there are no connecting rod bearings, just a connecting rod riding on the crankshaft. The rattling sound you heard is probably the top connecting rod becoming loose from heat and wear, theres a good chance the crank is badly scored... All work is going to require you to remove the engine and take it apart.
With the engine out you can gain access to the back cover of the engine, remove the 4 bolts and pop off the cover with a screw driver. Here you can unbolt the connecting rods and see how badly everything is scored. To change the bottom engine seal, however is a little more harder., you must remove the oil sump and then pry out the seal and replace it with a new one.

Justin, most PTO and FWE seals can be replaced without full engine dismantling. We change PTO seals a lot, especially on Kawy's. If one is really into it, you can purchase a seal puller - it looks like a piece of pipe with a narrow taper and fine thread on one end. You screw it into the seal and pop it out. Screwdrivers of course work too, but sometimes they break, or you can damage the seal boss.

A 6-year old mass-merchant rider has lived up to the "durable goods index" and it's projected life. I'd change the seal, keep an eye on the oil and ride it till it no longer works. A short-block is a cost-effective way to keep it going if the rest of it is in top shape.