MCfan

Brenden, you are absolutely correct. I wasn't sure which rally wheels you currently have/use. If they are stock first gen Monte rallys (RPO ZJ7), they are 15x7 with +0.30" (+7.65 mm) offset. However, if not stock Monte rallys, they might well be stock Corvette rallys which are usually 15x8 with 4.25" backspace (-.25" or -6 mm offset). Of course, they could also be any of the custom made rallys (I have both 15x7 and 15x8 rallys from WheelVintiques, both with custom offsets of +12mm). Note that when discussing and comparing wheel/tire combos it is imperative to only use OFFSET measurements, NOT backspace! All wheel manufacturers specify offset measurements/dimensions because offset is completely independent of wheel/rim width! Offset is measured from the centerline of the wheel, regardless of width, to the mounting surface of the wheel on the hub. If that surface is outboard of the centerline, the offset is positive (+); if inboard of the centerline, it is negative (-). Offset measurements are always expressed in millimeters (mm) to avoid confusion with backspace measurements which are always expressed in inches (in). Note also that most car enthusiasts discuss and compare wheels by referring to their BACKSPACE measurement. However, that can be dangerous as backspace for any given wheel is totally dependent on wheel/rim width. Backspace is measured from the mounting surface of the wheel to the inside rim lip/edge. Therefore, you can only compare wheels by their backspace measurement if they have the same rim width. The backspace measurement that corresponds to a zero (0) offset for any wheel always = (rim width +1)/2. For example: A 7" wide wheel with a 4" backspace has zero (0) offset (mounting surface aligns with the centerline of the wheel) while an 8" wide wheel with a 4" backspace has a -12mm offset (mounting surface is .5" inboard of the centerline) which pushes the wheel tire combo .5" further outboard. Study the table given a couple screens down in the Wheel Offset Calculator tool to see the relationship between Offset and Backspace for any Rim Width. The relationship is Offset = Backspace - (Rim Width+1)/2 You can easily measure the backspace of any rim by laying it face down, then laying a straightedge across the inside rim lip and measuring down to the mounting surface, however, you MUST also know the rim width to know its actual offset measurement (see equation above). Rim width is easy to measure on a naked rim but more difficult to determine if you have a tire mounted.

Brenden, Like you, I'm a Mechanical Engineer so I dig this mechanical fit stuff. 🙂 I used the Wheel Offset Calculator at tiresize.com to compare your proposed set up of 255/60-15 tires on 15x8" wheels with 4.25" backspace to your existing setup of 235/70-15 tires on the same wheels. Here's what I found: First, you can use the table several pages down in the Wheel Offset Calculator to determine the offset for your wheels which is -6mm. This is confirmed by subtracting the backspace of any 8" wide rim at zero offset which is (8+1)/2 = 4.5" from your wheel's measured backspace of 4.25". So, 4.25 " - 4.5" = -.25' which converts to -6mm offset. On the front: Since the 255/60-15 tire is 10" wide and 27" tall (versus 9.3" wide and 28" tall for the 235/70-15 tires) your inside and outside sidewall clearances will be .35" less. I think you can probably get away with that if nothing is rubbing now. On the rear: The reduction of your existing inside and outside clearances will be the same (.35"). HOWEVER, the sidewall clearance with your wheel well lip will only be around .44". I have confirmed this by comparing your proposed setup (using the Wheel Offset Calculator) to my existing rear setup where I have measured my sidewall clearance at .75". I suggest you carefully measure the sidewall clearance to your rear wheel well lip of your current setup at the narrowest point and decide if you can afford to lose another .35" of clearance there. Don't forget to allow for some body shift during cornering or hard acceleration/braking. Wider tires on the rear of any first gen Monte really need to be mounted on wheels with about +12mm offset (5" BS on an 8" wide wheel) to move the combo inboard (away from the quarter panel) where there is more room. Wheels with -6mm offset will actually move your combo outboard about .75" more than is ideal. I think your proposed combo will probably fit on the rear, also, but you'll need to decide if the small outside clearance is livable. Good luck.

Dennis, here are three '70 Build Sheets I have - one Van Nuys SS, one Baltimore SS and one Baltimore non SS. Hope they help in some way. Good luck.

Thanks, Bryan! So good to see that the USPS did not drive a truck over it as they sometimes apparently do! Also, good to know the clutch fan was not damaged as that was my main objective. You may not believe the condition of packages (Amazon, eBay and OEMs) that get delivered to our fairly remote summer home in MN. Crushed, ripped open, taped up boxes with damaged or missing contents are common across all three major carriers. 😬

Hi, Rich, I just sent you the detailed conversion instructions for a '70 via email. There are sight variations in the '72 and possibly '71 fuse panels but most of the instructions will work for all years. I know some members have installed that kit in '72s so maybe they will chime in if that is what you need.

Congratulations, Tom!! Six Saturdays in every week from now on! Enjoy!

Great! You're welcome!

The '70 Monte Carlo Assembly Manual shows power coming from the ACC port on the fuse panel and going through the switch and back to the blower. I can't tell from the '70 Wiring Diagram Manual if the power wire runs independently for that RPO C50 or if it is included in any cabin or rear lighting harness.

Hey, Mike, As you have already learned, the Z-bar frame bracket is unique to the Monte. Here is a link to an older thread on 4-speed conversions that confirms that fact: https://www.firstgenmc.com/forums/index.php?/topic/13767-my-4-speed-conversion/ Unfortunately, I don't believe that thread identifies a source for the correct Monte bracket but maybe you can take a closer look. I would guess that a search for an original bracket might be long and difficult as so few manual transmissions were installed in first gen Montes. Good luck.

Ditto

The https://tiresize.com/ site has outstanding online tools for comparing tire sizes and calculating wheel offsets. If you scroll down a couple of pages in their excellent Wheel Offset Calculator tool, you will find a very useful chart that gives you a backspace figure for any given wheel width and offset. To find the backspace for any wheel at zero offset simply add 1 to the wheel width and divide the sum by 2. Next you add the desired offset (usually given in millimeters to avoid confusion with backspace which is given in inches). Positive offset moves the wheel inboard. Negative offset moves the wheel outboard. Lots of trial and error experience with wheel/tire combos on first gen Montes shows that the best wheel offsets (independent of wheel width) are +6 mm for the front and +12 mm for the rear. Those offsets will keep any wheel/tire combo centered within the available wheelwell space. If you want to use the same wheel width on all four, a good compromise is an offset of +8mm (or .30") which is exactly what the factory spec is for the 15 x7 rally wheels. So, for a 15x8 wheel, zero offset is at 4.5" backspace (8+1)/2 = 4.5 and then adding a +12 mm offset (or +1/2") increases the required backspace to 5". For most wider tires, having more backspace (i.e. 5.5" on an 8" wide wheel) is better than having too little if you want to maintain clearance between the sidewall and the wheelwell lip of your quarter panel. However, too much backspace can also get you in trouble with rubbing on tight turns when used on the front. If you are planning to mount a 275x60-15 or larger tire on the rear using a 15x8 wheel with only 4.5" backspace, you will have little, if any, outboard sidewall clearance (unless you don't mind trimming the wheelwell lip). I believe that's why others have already advised using a 5" backspace on a 15x8" wheel.

Hi, Richard, Someone may have done exactly what you want to do and will chime in. In the meantime, several of us have simply converted our "idiot light" dashes to the optional stock full gauge dash with a tach replacing the fuel gauge to the left of the speedo. The clock is retained in its original position and the fuel gauge joins the other smaller gauges for amps and temperature. Oil pressure is still indicated with a red warning light in this configuration. There is a guy on eBay (sonnie24) that sells a complete kit for this conversion. I did this conversion on my '70 Monte several years ago with good success. I also took that opportunity to upgrade all of the dash lighting to LEDs. If you are interested in that approach, I can send you a detailed instruction set to follow. Below are a couple of Before and After photos of my dash.

Welcome to the club and forums, Peter! You have a classic black beauty there! Love the old school touches of Cragar SS style wheels and what appears to be a shorty lakes pipe or exhaust cutout. Bet she moves as good as she looks with that stroker motivation! Always happy to see and learn more about your ride. Enjoy the club!

Love that exterior and interior color combo - exactly like the new '70 I bought right out of college! Mine was a 402/TH400 - I wasn't even aware of the SS option, nor did I know a 4-speed could be ordered with the 402. I've owned and greatly enjoyed both since but still have fond memories of my first gold/white/gold '70.

No, Kevin, I'll be at my summer home in MN by then but if anyone is going from SW FL, I might be able to get it to them if they have room to bring it to you. If not, I could check into shipping options.

This is probably a long shot … but you never know. I am ready to get rid of the original spare tire and wheel that came with my ’70 402 4-speed car. On the Build Sheet it was RPO PU8 G78x15 Dual Stripe White Wall on a 15x6 black steel wheel. It was definitely used at one time. Tread wear is even and remaining tread depth is .20”. It has retained air pressure after all these years and the wheel is in good shape, also, but I’m sure it has little to no functional use as the rubber is far too old to trust on the road. Before I throw it out, I was wondering if anyone had any interest in it for display or other sentimental purpose? Or maybe someone might want it for the original 15x6 steel wheel? I would be happy to gift it to any FGMCC dues-paying member that wants to pick it up in SW FL or pay shipping. The combo weighs 47.5 pounds. I have more detailed photos if needed. Shoot me a PM if you are interested or have questions. Thanks.

If this statement is true (and it may be), our Technical Info article "Is it a real SS?" needs to be corrected in at least two places. "The 1970 Monte Carlo brochure reads as follows for the SS454 model: Monte Carlo SS454 includes a 360-hp Turbo-Jet 454 V8, Automatic Level Control, G70 x 15 wide-oval white stripe tires with 7”-wide rim wheels, heavy duty chassis components and SS exterior identification." and ... 2. 15X7 Rims In 1970 the PH1 - 15" x 7" rims with wheel covers were standard with the SS454 package. Other wheel covers and rally wheels were optional. In 1971 the ZJ7 - 7" x 15" Rally wheels were part of the SS454 package. The Build Sheet for the '70 SS I owned shows the standard PH1 (15x7) wheels with PA3 Special Wheel Covers. Another Build Sheet for a '70 SS that I have seen shows the optional RPO ZJ7 15x7 Rally Wheels which is also consistent with our current article above. If there is factory documentation to the contrary, we should probably consider updating our article.

For 1970 15 x 6" wheels were standard but RPO PH1 15 X 7" could be ordered (included in RPO Z20 SS package)

Shocking news ... so sorry to hear it, Steve! Will be thinking of you in this time of loss and sorrow.

Dave, thanks for coming forward with all of that interesting historical information on this unusual Monte! Rob has confirmed that the one just sold by BJ is the same one you owned previously. Assuming you can believe its prior owner to you, the source of the black tail panel and SS rear bumper was apparently neither the factory nor an original dealer installation. My observation from many descriptions of finds of "rare/unusual" Montes on this site is that they often have a history of multiple owners who incorporated their personal preferences, just as you did - a completely natural thing to do. I plead ignorance to what a GM.9 Executive Order was or how radically it could be configured. About four years ago, one of our members bought a gold/white '70 SS that was allegedly built for a GM executive and it had nearly every option available (at that time) from the factory, but claims of non-standard configurations coming from the factory are highly suspect, IMO. Even the only known '70 SS with an LS6 (currently owned by Tim Pestinger) was a Dick Harrell conversion, not a factory exception. In any case, I certainly agree with you on the enjoyment of driving a 402/4 speed first gen Monte. I get to drive mine at least 3 times a week while I am in SW FL during the winter months. It's not for sale ... 🙂

Forget the inconsiderate woman ... now's your chance to own it, Rob! Good luck!

Dan, unless your BB Monte has a manual transmission, I wonder if those spare headers are for use with automatics only? The #7 tube must exit straight out (or slightly forward) and down to clear the Z-bar that operates the clutch. I searched high and low for a pair of full length headers for my BB 4-speed Monte about four years ago. Finding none, I went with Patriot shorty headers. If those particular ARH headers will work with a manual transmission, they might be very attractive indeed.

Rob. I did notice the name and figured it was your father. I purchased my first Monte at Pappas Chevrolet in KC, MO in July of 1970 and sold it in 1974 to get a bigger car for my small but growing family. I have searched in vain for any original documentation of my first Monte or even a good photo - nothing so far.

Great find, Rob! Very cool! Hang on to it at least as long as you own the car.

So, did anyone help him? We collectively have done that numerous times. Maybe he should become a member (assuming he isn't already)?