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I have a 21" 140cc 550ex mower. Nowhere can I find the size of the hex bolt (3/8"-24) head. 5/8" fits - fairly well, but I suspect it is metric. Does anyone know. Does Troy-Bilt generally use metric or U.S. Thanks.
My one observation about the blade on this. mower is that you must be very very careful in aligning all the pieces of the blade assembly. They too easily come out of position when replacing the blade after sharpening.

bolt diam of about 3/8" probably means a head of about 9/16" or 14mm ( close to each other, 14mm would be 'loose' on 9/16".)

Nosmada said:

I find the size of the hex bolt (3/8"-24) head. 5/8" fits - fairly well, but I suspect it is metric.

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This is confusing.
A 3/8 x 24 bolt uses a 9/16 socket on it. How did you know the tread of the bolt ?
5/8 socket will fit sloppy on a 3/8 bolt.
If you know it's 3/8 x 24 it's not metric.

Richie F said:

This is confusing.
A 3/8 x 24 bolt uses a 9/16 socket on it. How did you know the tread of the bolt ?
5/8 socket will fit sloppy on a 3/8 bolt.
If you know it's 3/8 x 24 it's not metric.

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Thanks!
Troy-Bilt identifies the hex bolt as 3/8-24, so I "assumed" (always a mistake!) the diameter is 3/8" with 24 threads to the inch. The head of the bolt is a pretty good fit with a 5/8" socket. A 15mm socket is too small and a 17mm is too big, so I'm guessing it's likely a 16mm socket that's needed (which I don't own presently).

1 Lucky Texan said:

bolt diam of about 3/8" probably means a head of about 9/16" or 14mm ( close to each other, 14mm would be 'loose' on 9/16".)

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Thanks. Please see my reply to Richie F. It would helpful to know if Troy-Bilt still adheres to standard U.S. bolt sizes. If so, I wonder why??

550ex is a Briggs engine, most briggs pusher mower engines and MTD push mowers (MTD owns troybilt) i've seen use a 9/16 head bolt.

/I think you are overthinking it. Most US push mowers use either a 9/16 or 5/8 hex wrench. And if the 5/8 fits then use it. About the only place I find metric blade bolts and nuts is on Kubota and JD.

Take the bolt out, with whatever you have and go to a hardware store.
A lot of them have a board you can test the bolt into.
If it fits 3/8 x 24 it's standard.
Doesn't work ? Try it on the metric board.
The socket head means nothing. Bolt diameter and tread is what counts.

Sometimes if i have trouble with a sloppy fitting bolt head Sae or Metric, i put a wrap or three of electrical tape or duct tape on the bolt head to get a better grip on it and not risk rounding it off.

Sounds like it could be one of the newer crap engines, use a 15 mm. See if that fits better.

16mm and 5/8" are pretty much the same.
Put up a pic of the top of the bolt. It should have some markings on it.

Nosmada said:

Thanks!
Troy-Bilt identifies the hex bolt as 3/8-24, so I "assumed" (always a mistake!) the diameter is 3/8" with 24 threads to the inch. The head of the bolt is a pretty good fit with a 5/8" socket. A 15mm socket is too small and a 17mm is too big, so I'm guessing it's likely a 16mm socket that's needed (which I don't own presently).

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5/8" and 16MM are the same size. They are interchangeable.

ILENGINE said:

/I think you are overthinking it. Most US push mowers use either a 9/16 or 5/8 hex wrench. And if the 5/8 fits then use it. About the only place I find metric blade bolts and nuts is on Kubota and JD.

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Thanks. I do know from experience that every Craftsman mower I've used in the past 10 years has a blade bold that takes a 14mm socket.

StarTech said:

5/8" and 16MM are the same size. They are interchangeable.

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Thanks. So many good replies.

Nosmada said:

Thanks. I do know from experience that every Craftsman mower I've used in the past 10 years has a blade bold that takes a 14mm socket.

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I have never used a 14 mm on a blade bolt in my 23 years of experience. I take that back had a few customers with JD that used 14 mm for the side bolts for the 3 bolt mounts. Never used 14 on craftsman and still don't even on the new ones manufactured this year.