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Block to G/Box Drilled & Tapped to 5/16"
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Author:  MiniBob [ Thu Mar 12, 2015 3:15 pm ]
Post subject:  Block to G/Box Drilled & Tapped to 5/16"
Seeking any second opinions again please.
Have had the block drilled and tapped to take 5/16 bolts for block to g/box.
Should I make the holes in the 'box 5/16" or 8mm?

5/16" = 0.313" = 0.79375mm

So a little amount of movement at 8mm or virtually none at 5/16"
Does it matter??
Author:  Babes [ Thu Mar 12, 2015 4:04 pm ]
Post subject:  Re: Block to G/Box Drilled & Tapped to 5/16"
Mine were done to 5/16, dont know if it matters to be honest.
Author:  kiwiinwgtn [ Thu Mar 12, 2015 5:27 pm ]
Post subject:  Re: Block to G/Box Drilled & Tapped to 5/16"
I cannot see how it can matter unless the bolt holes are not square and the bolts are tight. You will find that your 5/16 bolts will all ways be undersize they always are.

Do a dummy assembly of box to block if you are concerned
Author:  drmini in aust [ Thu Mar 12, 2015 5:45 pm ]
Post subject:  Re: Block to G/Box Drilled & Tapped to 5/16"
Original bolt holes were at least 1/64" bigger than the 1/4" bolts.
I would drill them 8.5mm for 5/16" bolts.

My 2c..
This folklore of putting 5/16" bolts in to replace the 1/4" ones arose back when all the blocks were thin flange. I reckon it's a waste of time doing it with thick flange blocks, particularly for street use.
A 1380 thin flanged (1100S block) stroker I built a while back (with stock 1/4" bolts) still has NO leaks there.
Gasket sealants have improved in the last 30 years.
Don't believe everything you read in Vizard's book. :wink:

[edit] My car's Mk2S box was drilled out when I got it (had a thin flange S motor on it with 5/16" bolts). It now has a 1360 (1300auto block) on it, but with 1/4" bolts in. It doesn't leak there, either. 8)
Author:  mini_mad_matt [ Thu Mar 12, 2015 5:56 pm ]
Post subject:  Re: Block to G/Box Drilled & Tapped to 5/16"
It shouldn't really matter if there is a bit of play there because you should have 2 dowels to locate the gearbox and block.
Author:  Harley [ Thu Mar 12, 2015 6:28 pm ]
Post subject:  Re: Block to G/Box Drilled & Tapped to 5/16"
If you drilled it 5/16 the finished hole will be bigger than 8mm anyway.
Author:  boomini [ Fri Mar 13, 2015 5:20 am ]
Post subject:  Re: Block to G/Box Drilled & Tapped to 5/16"
Use a decent sealant. I use loctite 518. Its pricey yes at $50 a tube but works brilliantly. I use it on all oil joints when I build engines. Never had a leak yet..
Author:  MiniBob [ Fri Mar 13, 2015 8:19 am ]
Post subject:  Re: Block to G/Box Drilled & Tapped to 5/16"
Thanks for all the details, much appreciated. Taken it all onboard 8)
Will start with a 5/16 bit and go from there.
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