..the pirate jack rear kit I’m assuming has an adaptor flange or redrilling... haven’t installed it yet...
as far as a spacer for the back spacing, I don’t think we can run spacers over here, I think they’re illegal to run..so if I got 18 x 8 with 5.25 back space(I do see a lot of posts about 4.5 backspacing? ) and run the 245 40 18 I should be right ?
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I'm not familiar with the Pirate Jack rear disk kit beyond what's shown in the pictures. The axle difference is important, BBP axles have a different flange offset than SBP axles. That changes the track width, which will change your backspacing requirements.
With BBP axles the ideal backspace for maxing out your tire width in the back isn't 4.5" on a Dart, especially with 18's because there's less tire overhanging the rim. But, before you buy anything my suggestion would be to measure your own car. You don't need to spend money on a Percy Wheelright, all you need is a tape measure and a carpenter square. Or even just a straight edge and a tape measure if you measure carefully.
Use the straight edge across the wheel mounting surface on the axle, like the straight edge is the rim (ie, don't lay it across the hub, lay it across the flat where the rim mounts). Then measure back to the springs, making sure you keep the tape perpendicular to the straight edge and the spring. That's your wheel mounting surface (wms) to spring measurement. Do the same for the wms to quarter flange. Your straight edge has to be long enough that when you make the straight edge vertical you can keep the tape perpendicular to the straight edge and the quarter lip. That's your wms to quarter. Measure BOTH sides. With the way the body tolerances on these cars work, they probably won't be identical, so you will likely have one side that's a little tighter for clearance than the other.
Then it's just math. On my '71 Dart with an A-body 8 3/4 and BBP axles I have a WMS to spring of 6", and a WMS to quarter of 5". So for example lets say I used an 18x8 with a 245/40/18. An 18x8" is actually 9" wide outside lip to outside lip, most 245/40/18's have a section width of 9.8", let's call it 10" to be safe. So a 1/2" of tire overhang on each side. So 10" of tire at the section width (widest part), 11" of space from the spring to the quarter. I usually start from the spring, so we want a 1/2" of clearance to the spring. So 6" - .5" is 5.5". Next we have a 1/2" of tire overhang. So now we've got 5", which is the backspace we need. Then I check the other side. If the backspace it 5", then the front space is 4". 4" + 1/2" tire +1/2" clearance is 5". Perfect, centered. 18x8 with 5".
But, that assumes a full 1/2" of overhang, which there won't be with an 245/40/18 on an 18x8", the measuring rim is an 8.5 so there will be a little squeeze, and the tire was 9.8" to begin with. It also assumed a full 1/2" of clearance to the spring, which you don't need with an 245/40/18. Personally I'd rather set the clearance tight to the spring and have the extra to the quarter, because that's where it's more likely to rub . So on my car I'd say the ideal is more like 5.25". That'll put me around 3/8" clearance after the tire overhang correction. But that's for drums, so, if I wanted to add disks I'd add another 3/16" or so. Which is 5-7/16", which I'd call 5.5" and run a small spacer if I needed to, because I know that's actually the tight side of my car and the other side actually has 6 1/8" to the spring so it has more room to begin with.
But that's
my car. The '67-'69 cars are, in general at least, usually a little tighter to the quarters than the '70+ cars. I don't know what the deal is with spacers over there, but I wouldn't be surprised as the laws for classics are pretty ridiculous in Australia. That makes your measurements more important, I usually just fudge to the side of a little more backspace because a 1/8" to 3/16" spacer is no big deal and I run longer than stock wheel studs anyway. And it's easier to run a small spacer than re-work the quarter lip if I'm a little off.
