Hydroboost / Master Cylinder Mounting Hole Bolt Size

[FoMoCoBronCo]

Could someone help me out with the bolt size/thread pitch on the 2 mounting holes for a hydroboost unit where the master cyclinder attaches to the hydroboost? 3/8" and M10 are too small. 1/2" and M12 are too big. It really appears to be a 7/16, but I've tried coarse (14) and fine (20) threads to no avail. I've tried from both sides. The threads appear to be in great shape and not cross threaded. Was there perhaps another thread pitch for 7/16" bolts that GM used other than 14 or 20?

The only thing I haven't tried is an M11, but i would be surprised if it was metric, given that the mounting bolts on the other side of the hydroboost are standard 3/8"

 

[dmoses42]

Found a conversion table on the web, says 7/16 converts to 11.13 mm

 

[m_m70]

11mm is not a common thread. Not sure if they're even made. I would get a 7/16 tap (looks like 20 threads per inch) and see if you cant get that to start and clean up.

here's a chart that i like to keep handy.

 

[m_m70]

I think this is your answer:
I bought a HB unit from Matt also and the one I received came with 2 special metric bolts to mount the MC.
They're 10mm and about 2.5 or 3 in long (it's home and I'm at work so guessing at length). What makes them special is they have a short reverse thread just under the bolt head to mount into the HB casting. The rest of the bolt have the threads cut normal to accept the 10mm nut. I believe they're grade 8 bolts, too.

So, Special stud with two different threads (one reversed). See pic below....

 

[toddz69]

I think this is your answer:
I bought a HB unit from Matt also and the one I received came with 2 special metric bolts to mount the MC.
They're 10mm and about 2.5 or 3 in long (it's home and I'm at work so guessing at length). What makes them special is they have a short reverse thread just under the bolt head to mount into the HB casting. The rest of the bolt have the threads cut normal to accept the 10mm nut. I believe they're grade 8 bolts, too.

So, Special stud with two different threads (one reversed). See pic below....

View attachment 915403

^^Bingo! They have a short section of left-handed thread on them so you can thread the nuts on them and they won't back out.

To the OP - if you need a pair, I might have a set in my stock at home. Just let me know.

Todd Z.

 

[FoMoCoBronCo]

^^Bingo! They have a short section of left-handed thread on them so you can thread the nuts on them and they won't back out.

To the OP - if you need a pair, I might have a set in my stock at home. Just let me know.

Todd Z.

Thanks Todd! It looks like the hydroboost unit I got was missing the two reverse thread studs picture above.

Instead of waiting to find those, I just drilled out the holes and tapped them for 1/2-13 threads and threw some stainless steel bolts in there- worked like a charm!