Odd size bolt ? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

teamharvey · 12-23-2004, 07:29 PM

I have a 72 blazer and nee the two hold down bolts for the floor dimmer switch. I have one actually and it seems to be an odd size thread.

Thanks,
Bret

gonebad2 · 12-23-2004, 07:57 PM

They should be 1/4" - 20 with a phillips head.

teamharvey · 12-24-2004, 01:08 AM

I have tried both 1/4 20 and 1/4 24 with no luck. The one the PO did have was wrong as well, the reason it was loose.

OhioDan · 12-24-2004, 04:48 AM

1/4"-20 is National Course thread. 1/4"-24 is National Standard thread. 1/4"-28 is National Fine thread. The only time I've seen National Standard used was on a top cover for a Harley 4 spd. Not a real common size anymore. Take the one you have and go to a nut and bolt house. They should be able to help?

JimKshortstep4x4 · 12-24-2004, 07:41 AM

As gonebad2 stated the screws that were originally on the dimmer switch are 1/4-20 x 1/2" phillips head screws.

Have you tried chasing the threads with a 1/4-20 thread chaser or tap.?

JIm

4thgencreamsicle · 04-24-2021, 03:24 PM

Did you ever figure this one out? I have the same issue on my ‘69 c10.

RustyPile · 04-25-2021, 12:55 AM

Quote:

Originally Posted by 4thgencreamsicle

Did you ever figure this one out? I have the same issue on my ‘69 c10.

After 17 years, I'm sure he did "figure it out".. Regardless of what fasteners you have, the correct fastener is a phillips head 1/4" X 20, 1/2" long... As was stated in that old thread. chase the threads in the hole with a 1/4' x 20 tap before installing the new fasteners...

Accelo · 04-25-2021, 02:27 AM

Sometimes this area gets a lot of rust. This can cause the threads to pull when the original bolt is removed. If they are too loose you have a few options. Drill it out for a well nut. Using a version of a pop rivet gun to install an threaded insert. Or weld it up and re-drill. Not a lot of room to go up a bolt size. Don't forget the bent tab that retains the wire for the dome light that goes on the outside bolt. Cheers.
Rick

4thgencreamsicle · 04-26-2021, 10:56 AM

Thanks guys, didn’t notice the date on this thread. I was just happy to see I’m not the only one. Lol. The factory size on my truck wasn’t standard, learning that about this truck all the way around.like maybe some corners were cut. The hole & threads are actually larger than stated so I’ll go through our odds and ends and see what I come up with. No rust in that area thankfully.

sick472 · 04-26-2021, 11:42 AM

Some thoughts...if the threads are gone beyond repair, you could drill and tap to the next size up. This might be, day I say, need to be a metric bolt to get the next size up without over drilling the weld nuts below the floor. You would just have to see what size hole can be drilled into the weldnuts without getting too close to its edges. Another thought, IIRC, you can access the dimmer holes from under the cab and could just put smaller bolts through the holes from above and use nuts below. Not factory and not as easy to change out, but not a big deal either.

**BigBird05** · 04-26-2021, 12:20 PM

I would do like sick472 suggested get a 1/4 x 20 Philips head bolt and put a nut under the cab and use lock tight on it. That way it would look original.

**Steeveedee** · 04-26-2021, 01:20 PM

And that repair will probably last 50 years, too!

12-23-2004, 07:29 PM	#1
teamharvey Registered User Join Date: Jul 2004 Location: Turlock, Ca Posts: 579	Odd size bolt ? I have a 72 blazer and nee the two hold down bolts for the floor dimmer switch. I have one actually and it seems to be an odd size thread. Thanks, Bret __________________ 71 Custom 20 Deluxe..soon to be SWB 1/2 ton 1 incredible wife 2 great kids

12-23-2004, 07:57 PM	#2
gonebad2 Merciless Butcher Join Date: Oct 2003 Location: CC Nevada Posts: 1,447	They should be 1/4" - 20 with a phillips head. __________________ Your Bought And Sold....I Got Your Soul!

12-24-2004, 01:08 AM	#3
teamharvey Registered User Join Date: Jul 2004 Location: Turlock, Ca Posts: 579	I have tried both 1/4 20 and 1/4 24 with no luck. The one the PO did have was wrong as well, the reason it was loose. __________________ 71 Custom 20 Deluxe..soon to be SWB 1/2 ton 1 incredible wife 2 great kids

12-24-2004, 04:48 AM	#4
OhioDan Registered User Join Date: Apr 2002 Location: New Carlisle, Ohio,USA Posts: 1,404	1/4"-20 is National Course thread. 1/4"-24 is National Standard thread. 1/4"-28 is National Fine thread. The only time I've seen National Standard used was on a top cover for a Harley 4 spd. Not a real common size anymore. Take the one you have and go to a nut and bolt house. They should be able to help? __________________ LIFE AIN'T EASY WHEN YER FAT AND GREASY! GMC= Got More Class! 1970 GMC LWB, Fleetside, 350/4spd, Flowmaster "40's", 2002 GMC 16" Rallies, cowl hood,and added a bunch of options that weren't there when I started.

04-24-2021, 03:24 PM	#6
4thgencreamsicle Registered User Join Date: Jan 2018 Location: Lemoore, CA Posts: 74	Re: Odd size bolt ? Did you ever figure this one out? I have the same issue on my ‘69 c10.

12-24-2004, 07:41 AM	#5
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	As gonebad2 stated the screws that were originally on the dimmer switch are 1/4-20 x 1/2" phillips head screws. Have you tried chasing the threads with a 1/4-20 thread chaser or tap.? JIm

04-25-2021, 02:27 AM	#8
Accelo Registered User Join Date: Mar 2015 Location: washington Posts: 2,161	Re: Odd size bolt ? Sometimes this area gets a lot of rust. This can cause the threads to pull when the original bolt is removed. If they are too loose you have a few options. Drill it out for a well nut. Using a version of a pop rivet gun to install an threaded insert. Or weld it up and re-drill. Not a lot of room to go up a bolt size. Don't forget the bent tab that retains the wire for the dome light that goes on the outside bolt. Cheers. Rick

04-26-2021, 10:56 AM	#9
4thgencreamsicle Registered User Join Date: Jan 2018 Location: Lemoore, CA Posts: 74	Re: Odd size bolt ? Thanks guys, didn’t notice the date on this thread. I was just happy to see I’m not the only one. Lol. The factory size on my truck wasn’t standard, learning that about this truck all the way around.like maybe some corners were cut. The hole & threads are actually larger than stated so I’ll go through our odds and ends and see what I come up with. No rust in that area thankfully.

04-26-2021, 11:42 AM	#10
sick472 Registered User Join Date: Jul 2015 Location: Sedalia Mo. Posts: 1,130	Re: Odd size bolt ? Some thoughts...if the threads are gone beyond repair, you could drill and tap to the next size up. This might be, day I say, need to be a metric bolt to get the next size up without over drilling the weld nuts below the floor. You would just have to see what size hole can be drilled into the weldnuts without getting too close to its edges. Another thought, IIRC, you can access the dimmer holes from under the cab and could just put smaller bolts through the holes from above and use nuts below. Not factory and not as easy to change out, but not a big deal either. __________________ He who is without oil shall throw the first rod. Compressions 8.7:1 1972 C10 1976 C10 (parts truck) 1985 K20

04-26-2021, 12:20 PM	#11
BigBird05 Senior Member Join Date: Aug 2019 Location: Cheraw Posts: 1,305	Re: Odd size bolt ? I would do like sick472 suggested get a 1/4 x 20 Philips head bolt and put a nut under the cab and use lock tight on it. That way it would look original.

04-26-2021, 01:20 PM	#12
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,088	Re: Odd size bolt ? And that repair will probably last 50 years, too! __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA



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