OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Will a 3.5HP Briggs flywheel fit a 3HP?

I pulled the flywheel on an old 3HP Briggs engine today and found a hairline crack running right across one side. It also has a few broken fins. I have some old 3.5HP engines with good flywheels. Can anyone tell me if the flywheel off a 3.5HP will fit a 3HP? If the answer is "no", that will save me the bother of pulling another flywheel. The coils are the same size and the flywheels look to be the same diameter, but I'm wondering if the key and magnets are in the same location? Also, will the same key fit? The shafts look the same, but I wonder if they are tapered on exactly the same angle?

Pretty sure they mix and match ok

Thanks Norm, I'll pull one of the flywheels tomorrow and see if it matches. I find most Briggs stuff can be mixed and matched to some extent. I've put 3.75HP carbys on 3HP engines and vice versa in the past and they have all worked fine.

Yep, timing would be the same. Should have no problems with the swap.

Put it all together yesterday. It started first pull and runs great.

Only problem was with the old engines I bought from the tip. After I cleaned the oil and dirt off the flywheels, I found all three had similar hairline cracks in them. In all cases it is a fine crack around one fin which goes down as far as the magnet. There are no signs of cracking on the inside of these flywheels. I decided to use the best one. After running the engine for about half an hour I pulled off the cowl and inspected the crack. It was still the same and had not grown any further or opened up at all. It is of course potentially dangerous. Considering this is essentially a display engine which will never be used for any heavy work, I think it will be okay for the time being. I will of course replace it if I can get hold of a good flywheel. Generally cracks are only really bad if they extend through the centre of the flywheel, close to or across the shaft. This one is confined to a fin on the very outer edge and only on one side. It could survive for decades in this condition - it may have even been there from day one - but it could of course fail at any time. Considering I bought three old Rovers from the tip and all of them had hairline cracks concealed under half an inch of grime, I wonder how many people out there are mowing their lawns daily with similar cracks in the flywheels? Most people never inspect them and would be blissfully unaware. If I had not thoroughly cleaned down these flywheels I would have been completely unaware of any problem.

Those with poor eyesight may also be at a disadvantage. I recall an elderly relative was fixing his mower one time. I picked up the flywheel and said "Looks like you've got a cracked flywheel?" It was news to him. It had a hairline crack running right across the middle!

Can't say I have heard of too many people being injured by exploding Briggs flywheels, the cowl will prevent most bits from escaping