7.25 drum to drum measurement

[Tuesday]

Hey All - I have ordered a new rear end from Quick Performance and they need a measurement that I don't have. I measured flange to flange and center to center of the leaf springs but they are requesting the measurement for the outside of drum brake to outside of the other drum.

The car is a 1973 Scamp - the 7.25 is stock and all my measurements have pretty much been 'exactly stock'. For various reasons I am unable to get the car up and wheels off to get this measurement but am confident a stock measurement will be correct - does anybody have this to hand or can point me in the right direction to find it?

 

[72bluNblu]

Hey All - I have ordered a new rear end from Quick Performance and they need a measurement that I don't have. I measured flange to flange and center to center of the leaf springs but they are requesting the measurement for the outside of drum brake to outside of the other drum.

The car is a 1973 Scamp - the 7.25 is stock and all my measurements have pretty much been 'exactly stock'. For various reasons I am unable to get the car up and wheels off to get this measurement but am confident a stock measurement will be correct - does anybody have this to hand or can point me in the right direction to find it?

If it’s a BBP 7.25 the drum to drum should be 56.75”

There are a few 7.25/8.25 measurements here

An accurate 8 3/4" rear axle width list

Also note, the 7.25 is narrower than the A-body 8 3/4 when considering wheel backspace.

 

[Dana67Dart]

I just measured the thickness of a new drum.

0.136" thick each.

If you measured axle flange to axle flange add 0.272" (0.136 + 0.136) to your measurement.

I measured my loose 67 7 1/4 and came up with 57 outside of axle flange to outside of axle flange.

There was a slight deflection over the center of the diff.

That would make it all in 57.272"


The posts say 57 1/8"

Which is a little under an 1/8" less than I measured.

I bet if you measured 100 rears you would get 100 different measurements.

 

[Tuesday]

If it’s a BBP 7.25 the drum to drum should be 56.75”

There are a few 7.25/8.25 measurements here

An accurate 8 3/4" rear axle width list

Also note, the 7.25 is narrower than the A-body 8 3/4 when considering wheel backspace.

Thank you, as ever, lots of good advice. Am thinking i need to measure in any case....but.....

Two things:

1. Am not sure if i have a BBP, i think it might be SBP, as this is the stock 1973 axel. Am not sure why Quick Performance need the measurement but think best i get it up on a ramp and double check but going by your comments on the backspace this got me thinking! Am presuming it might be better to provide the 8 3/4 drum to drum measurement - see photos and point 2 for further reasons?
   • If that does make sense - @72bluNblu I didn't see a measurement for a 1973 on your shared post link.
2. So leading on from this, i have measured the outside of the wheel to the edge of the arch - the rear sits a long way in (second photo), so thinking i'd be better giving them the 8 3/4 measurements in any matter - as the rear wheel sits nearly 1.5" inside the arch.

What do you think - imagine i could come out a little wider on the rear?

 

[bcschief]

I seem to recall all A bodies are 43 inches center to center on the spring perches.

 

[Tuesday]

I seem to recall all A bodies are 43 inches center to center on the spring perches.

Spot on - I have those - it’s the measurement outside drum to outside drum I need but am wondering if I can go a little wider than the existing 7.25.

 

[72bluNblu]

I just measured the thickness of a new drum.

0.136" thick each.

If you measured axle flange to axle flange add 0.272" (0.136 + 0.136) to your measurement.

I measured my loose 67 7 1/4 and came up with 57 outside of axle flange to outside of axle flange.

There was a slight deflection over the center of the diff.

That would make it all in 57.272"


The posts say 57 1/8"

Which is a little under an 1/8" less than I measured.

I bet if you measured 100 rears you would get 100 different measurements.

Oh boy.

A ‘67 7.25” may not be the same width as a ‘73 7.25. The BBP 7.25’s are not the same width as the earlier SBP 7.25’s.

The 57 1/8” measurement is the SBP 8 3/4 width, which definitely is NOT the same as any of the A-body 7.25’s.

Using a tape measure and going over the center hump will make your measurement longer than the actual width, potentially by a significant amount. None of the A-body 7.25’s are wider than a SBP 8 3/4, so the 57.2” measurement is totally inaccurate.

And as for “measuring 100 and getting 100 different measurements”, that has not been my experience. I’ve measured dozens of Mopar 8 3/4’s, and all them were less than 1/8” from the specs in that list I posted when measured correctly. Their width is somewhat critical to the function, and unlike body tolerances they were held pretty tight.

Thank you, as ever, lots of good advice. Am thinking i need to measure in any case....but.....

Two things:

1. Am not sure if i have a BBP, i think it might be SBP, as this is the stock 1973 axel. Am not sure why Quick Performance need the measurement but think best i get it up on a ramp and double check but going by your comments on the backspace this got me thinking! Am presuming it might be better to provide the 8 3/4 drum to drum measurement - see photos and point 2 for further reasons?
   • If that does make sense - @72bluNblu I didn't see a measurement for a 1973 on your shared post link.
2. So leading on from this, i have measured the outside of the wheel to the edge of the arch - the rear sits a long way in (second photo), so thinking i'd be better giving them the 8 3/4 measurements in any matter - as the rear wheel sits nearly 1.5" inside the arch.

What do you think - imagine i could come out a little wider on the rear?

View attachment 1716172297

View attachment 1716172298

Your ‘73 could be either SBP or LBP, depending on whether or not it came with factory front disk brakes. In 1973 the disk brake cars went BBP, and all factory V8 cars got front disks in ‘73. A /6 car could be drum/drum, and those care remained SBP. So, just knowing that it’s a ‘73 with a 7.25” doesn’t confirm which bolt pattern it is.

You can definitely use an axle wider than the A-body 7.25” regardless of its type, but your clearance to the quarters also depends on your wheels backspacing. And if you’re going from SBP to BBP that could be changing!

 

[Tuesday]

Oh boy.

A ‘67 7.25” may not be the same width as a ‘73 7.25. The BBP 7.25’s are not the same width as the earlier SBP 7.25’s.

The 57 1/8” measurement is the SBP 8 3/4 width, which definitely is NOT the same as any of the A-body 7.25’s.

Using a tape measure and going over the center hump will make your measurement longer than the actual width, potentially by a significant amount. None of the A-body 7.25’s are wider than a SBP 8 3/4, so the 57.2” measurement is totally inaccurate.

And as for “measuring 100 and getting 100 different measurements”, that has not been my experience. I’ve measured dozens of Mopar 8 3/4’s, and all them were less than 1/8” from the specs in that list I posted when measured correctly. Their width is somewhat critical to the function, and unlike body tolerances they were held pretty tight.



Your ‘73 could be either SBP or LBP, depending on whether or not it came with factory front disk brakes. In 1973 the disk brake cars went BBP, and all factory V8 cars got front disks in ‘73. A /6 car could be drum/drum, and those care remained SBP. So, just knowing that it’s a ‘73 with a 7.25” doesn’t confirm which bolt pattern it is.

You can definitely use an axle wider than the A-body 7.25” regardless of its type, but your clearance to the quarters also depends on your wheels backspacing. And if you’re going from SBP to BBP that could be changing!

Thank you, again. I have called my shop, and booked her in, as they are specialists in chassis dynamics and will measure everything, including space to the quarters and advise on tyre widths and we can address the axel width. I think this is too important for me to make a guesstimate - especially if we widen the axel.

@72bluNblu - out of interest, what are your preferred wheel and tyre specs (if you're not using stock wheels). I have a 450+hp 360 Magnum going in, set-up for daily driving but with more power low down - dynamically, on the street, the aim is punching in and out of corners and some autocross down the line, once a Tremec goes in - any of your excellent advice always appreciated.

 

[michiganpat]

my 1976 small bolt pattern 7.25" measured 57 1/8" drum face to drum face. they want that measurement since that's where the wheels mount and determines axle shaft length and where the wheel sits in the wheelwell. housing flange to housing flange doesn't mean much without knowing what brakes you're using (1.5" wide drums, 2" wide drums, 2.5" wide drums, discs off of a jeep patriot, etc). I was actually quite suprised when I measured my duster that with the stock width, even though it has that hippo on stilts look with the 215/70R14 tires on small bolt pattern ralleys (which measure .5" positive offset, or 3.75" backspace for 5.5" wide wheels), the tires were about perfectly centered in the wheelwell, 2" spacing to the inner wheelwell and 2" spacing to the lip of the wheelwell opening.

so what width you want to do will determine what offset wheels you plan to use and how big of a tire you want to go. 0 offset rims and maximizing tire size, I'd want the width probably 56" or so. +30mm offset (late 90's bullit rims), you'd probably want something more like 58.5" wide....

 

[72bluNblu]

Thank you, again. I have called my shop, and booked her in, as they are specialists in chassis dynamics and will measure everything, including space to the quarters and advise on tyre widths and we can address the axel width. I think this is too important for me to make a guesstimate - especially if we widen the axel.

@72bluNblu - out of interest, what are your preferred wheel and tyre specs (if you're not using stock wheels). I have a 450+hp 360 Magnum going in, set-up for daily driving but with more power low down - dynamically, on the street, the aim is punching in and out of corners and some autocross down the line, once a Tremec goes in - any of your excellent advice always appreciated.

I run 18x9's and 18x10's with 275/35/18's and 295/40/18's on my Duster. But making that work and taking advantage of the Falken Azeni's with a 200 tread wear takes more than just the right wheel specs. Obviously it takes that too, but literally the only stock suspension parts I run are the spindles.

It sounds like you're building a "pro touring" style car, and depending on what other suspension work you wanted to do you could run 18x9's with 275/35/18's all the way around if you did a 1/2" offset on the rear springs. Which would be pretty simple if you're already having a rear axle built.

my 1976 small bolt pattern 7.25" measured 57 1/8" drum face to drum face. they want that measurement since that's where the wheels mount and determines axle shaft length and where the wheel sits in the wheelwell. housing flange to housing flange doesn't mean much without knowing what brakes you're using (1.5" wide drums, 2" wide drums, 2.5" wide drums, discs off of a jeep patriot, etc). I was actually quite suprised when I measured my duster that with the stock width, even though it has that hippo on stilts look with the 215/70R14 tires on small bolt pattern ralleys (which measure .5" positive offset, or 3.75" backspace for 5.5" wide wheels), the tires were about perfectly centered in the wheelwell, 2" spacing to the inner wheelwell and 2" spacing to the lip of the wheelwell opening.

so what width you want to do will determine what offset wheels you plan to use and how big of a tire you want to go. 0 offset rims and maximizing tire size, I'd want the width probably 56" or so. +30mm offset (late 90's bullit rims), you'd probably want something more like 58.5" wide....

I'm guessing you measured that on the car with a tape measure? If anything they'd be the same or narrower than the BBP 7.25's, which themselves are narrower than the SBP 8 3/4. I haven't personally measured one of the '73+ SBP 7.25's, only the BBP version, but based on working with other members here on their backspacing all of the 7.25's are narrower than the 8 3/4's. The backspace that works on an 8 3/4 puts the tires right on the springs with a 7.25 of either bolt pattern variety.

 

[michiganpat]

yup, I bolted some pieces of wood with the lugs to the drum and stretched the tape across it and subtracted the thickness of the wood.

and I suppose in all honesty, the tires weren't perfectly centered. the measurement from the tire to the inner lip of the quarter on the drivers side was 1.875", and from the tire to the inner wheelwell, 2.125". the passenger side the measurements were reversed (it appears the axle is shifted about 1/8" towards the drivers side).

 

[Tuesday]

I run 18x9's and 18x10's with 275/35/18's and 295/40/18's on my Duster. But making that work and taking advantage of the Falken Azeni's with a 200 tread wear takes more than just the right wheel specs. Obviously it takes that too, but literally the only stock suspension parts I run are the spindles.

View attachment 1716173322

It sounds like you're building a "pro touring" style car, and depending on what other suspension work you wanted to do you could run 18x9's with 275/35/18's all the way around if you did a 1/2" offset on the rear springs. Which would be pretty simple if you're already having a rear axle built.



I'm guessing you measured that on the car with a tape measure? If anything they'd be the same or narrower than the BBP 7.25's, which themselves are narrower than the SBP 8 3/4. I haven't personally measured one of the '73+ SBP 7.25's, only the BBP version, but based on working with other members here on their backspacing all of the 7.25's are narrower than the 8 3/4's. The backspace that works on an 8 3/4 puts the tires right on the springs with a 7.25 of either bolt pattern variety.

Sorry - work got in the way of life. I’ve ordered the rear end - coming after the Christmas holidays. Absolutely love the look and stance of your Duster!! Spot on with the pro touring observation - exactly what I’m looking for - I love corners - so want something that rides well but is punchy braking into and out of turns (but also works as my daily driver). I’m in 100% agreement on your wheel and tire specs - just need to find a set I like of the look of, built for driving is top priority but want that old school look in a satin black finish.

Re measuring: We measured very carefully- 3 times - with a decent tape measure and drum to drum we got 55 3/8”.

I’ll be moving to wheels and rear end installation in January - in the meantime, this little beauty (360 Magnum EFI build) goes in tomorrow - very excited…

Thanks again @72bluNblu and the forabodies community - my Scamp would not be the same (or as enjoyable) without everybody’s real world and practical advice!

 

[Tuesday]

my 1976 small bolt pattern 7.25" measured 57 1/8" drum face to drum face. they want that measurement since that's where the wheels mount and determines axle shaft length and where the wheel sits in the wheelwell. housing flange to housing flange doesn't mean much without knowing what brakes you're using (1.5" wide drums, 2" wide drums, 2.5" wide drums, discs off of a jeep patriot, etc). I was actually quite suprised when I measured my duster that with the stock width, even though it has that hippo on stilts look with the 215/70R14 tires on small bolt pattern ralleys (which measure .5" positive offset, or 3.75" backspace for 5.5" wide wheels), the tires were about perfectly centered in the wheelwell, 2" spacing to the inner wheelwell and 2" spacing to the lip of the wheelwell opening.

so what width you want to do will determine what offset wheels you plan to use and how big of a tire you want to go. 0 offset rims and maximizing tire size, I'd want the width probably 56" or so. +30mm offset (late 90's bullit rims), you'd probably want something more like 58.5" wide....

Sorry I got to this so late. You’re right, that measurement is crucial. I managed to make it to shop and up on ramp to measure. We measured 3 times and got 55 3/8” . My shop are excellent at chassis set-up - we had a long chat and agreed we’d go with the drum to drum measurement we had and would look at offsets and spacing once we have it on (which will be mid to late January). All being well we’ll be golden!

 

[michiganpat]

Sorry I got to this so late. You’re right, that measurement is crucial. I managed to make it to shop and up on ramp to measure. We measured 3 times and got 55 3/8” . My shop are excellent at chassis set-up - we had a long chat and agreed we’d go with the drum to drum measurement we had and would look at offsets and spacing once we have it on (which will be mid to late January). All being well we’ll be golden!

might get a bit interesting....for a wide rim in front, to fit a decent tire, you're going to need probably 19-30mm positive offset with an 8" rim. (5 1/4-5 5/8" backspace). with that narrow of a tire, you're probably going to need 0 offset to -15mm offset to maximize your tire/wheel size. something more like 58-59" wide if you wanted to keep the same offset front to rear probably would have worked better. at least that's what the math tells me after I measured my duster.