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1275 bore spacing
https://www.ausmini.com/forums/viewtopic.php?f=2&t=80800		 Page 1 of 2
Author:  simon k [ Thu Mar 21, 2013 2:45 pm ]
Post subject:  1275 bore spacing
does anyone have figures on the bore centre to centre spacing for a 1275?

IE how far from the centre of #1 to the centre of #2, and from the centre of #2 to the centre of #3
Author:  drmini in aust [ Thu Mar 21, 2013 2:55 pm ]
Post subject: 
I have a block here can measure the difference in wall width 1-2, 2-3, 3-4 if that helps.
Vizard's yellow book p415 suggests you move 1&2 .015" North, 3&4 .015" South when doing big bores. If that helps...
Author:  simon k [ Thu Mar 21, 2013 2:57 pm ]
Post subject: 
that'd be handy Kev - would appreciate it :)
Author:  drmini in aust [ Thu Mar 21, 2013 3:06 pm ]
Post subject: 
Bore is 1275 +.040" (1310cc)
spacing-
1-2 =.212"
2-3 = .183"
3-4 = .212"

So Vizard was right ^^^. As there is .030" less spacing on the middle pair.
Why? only BMC knows... :roll:
Author:  simon k [ Thu Mar 21, 2013 3:24 pm ]
Post subject: 
thanks Kev - looks like I'll need to use my head to work out the centre to centre... was hoping to avoid that, there's too much going on up there to think straight as is
Author:  drmini in aust [ Thu Mar 21, 2013 3:25 pm ]
Post subject: 
Bore on this motor is 71.6 near enough.
Author:  simon k [ Thu Mar 21, 2013 3:28 pm ]
Post subject: 
you know... if there was .003" error in your measurement, those two centre bores would be exactly 3" apart....
Author:  simon k [ Thu Mar 21, 2013 3:30 pm ]
Post subject: 
drmini in aust wrote:
So Vizard was right ^^^. As there is .030" less spacing on the middle pair.


so that's where the .030" magic number for offset boring comes from
Author:  justminis [ Thu Mar 21, 2013 5:57 pm ]
Post subject: 
On re-bores over 0.060", ie 1360, 1380, I have the machine shop centre the bores on a head gasket. That way the bores are centred as best can be with the head gasket.
Author:  Kennomini [ Thu Mar 21, 2013 6:59 pm ]
Post subject: 
Ok then what's the centers of a head gasket? wouldn't they be the same as the std block or are the bores in the gasket offset?
Are their different gasket out their for different sized bores?
Author:  drmini in aust [ Thu Mar 21, 2013 7:06 pm ]
Post subject: 
Pretty sure BK450 gaskets are thinner between 2&3. Not by much though.
Still is a good idea to offset bores for 1360cc up. You can just get away with not doing it on a 1380 roady but for 1400 it's a must. And if you are at 1380cc already it's too late to fix it for another rebore.
Author:  TheMiniMan [ Thu Mar 21, 2013 7:28 pm ]
Post subject: 
Years ago when it was very popular to do the 73mm Datsun 1200 piston thing, they had the gaskets made bigger & with the correct offset & size/diam for that conversion

now "all" the 1275 gaskets are made to suit that offset & size
Author:  drmini in aust [ Thu Mar 21, 2013 7:31 pm ]
Post subject: 
I measured a new BK450 the other day, it was 73.6mm diameter.
Author:  Kennomini [ Thu Mar 21, 2013 7:49 pm ]
Post subject: 
Ok so BK450's are different to a std 1275 gasket in the offsets and bore size?
On a side note; I have a 1100S block already bored 0.030thou (1310 correct?) I'm planning on re-boring it and installing 0.095thou (1360) I should make sure they move 1 and 4 towards the outer ends of the block 0.030" each Correct :?:

:EDIT:
Is offset boring the same as stagger boring :?: I always though stagger boring was something like 1 & 3 move forwards (towards the dizzy) and 2 & 4 move backwards (towards the exhaust) :? :?:
Author:  MiniVLT [ Thu Mar 21, 2013 8:19 pm ]
Post subject: 
Std oversizes are .020" is 1293 & .040" 1310 & .060" is 1330, so i'm guessing .030" is 1300. .030" sounds like an odd oversize???

Cheers Shane
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