Hello All, I’ve been back to work on my 46 Chevy 3600. Originally planned to keep the truck a long bed, but the more I think about it, I want to shorten it to match a 3100. All sources I’ve found show the 41-46 trucks measured as follows: 3100: 115” wheelbase 3600: 125 1/4” wheelbase Based on this I’d need to shorten the frame 10 1/4”. What’s throwing me off is that when I measure from the center of the rear hub to the front hub I’m getting approximately 123 1/2” (So I’d only remove 8 1/2”) The ch***is isn’t on the ground yet. I’ve swapped/flipped the rearend and had Sid drop the front axle 4”. I need some guidance on how to proceed with removing the correct length from the frame. Appreciate any advice.
You’d need all the weight back on the truck for an accurate measurement of wheel base. But if you want to wing it just remove the 10 1/4” like the book says the difference is. You see with the weight on the truck due to the springs straightening out, the wheel base will grow. Don’t make the cut straight through the rails.
With full weight on the ch***is, the front eye on the front spring will move forward and the rear eye on the rear spring will move to the rear (by design). I believe this is where your measurements are (literally) coming up short. As long as everything is installed to stock specs, I'd trust the GM measurements and cut the 10-1/4". EDIT: @SS327 You beat me by a split second!
Makes sense, I was just looking at it and see that the rear will pivot back and the front forward. I have some rollers I can throw on to prove this out. I know it won’t have the full weight, but should lengthen some. I’ll “Z” the frame and box the rails. You guys are awesome, appreciate the help!
Instead of blindly going by the numbers, I think I’d rather have the cab, bed, and fenders on in the place where they are going to be, then I’d measure and shorten the frame to center the tires in the fender openings. A lot of those old trucks the tire sits too far forward in the fender unless the truck is fully loaded. Even if you don’t get it dead on but close, you can adjust the remainder with offset axle pads.
I see the logic in this. I do not have a short bed/rear fenders yet. I’ll probably source these through aftermarket. Going that route I’d need to build the truck out and shorten the frame at the end. Not sure I want to do that, but I also don’t want the truck to look disproportional either…
124.5 minus 115 = 9.5 that you have to cut out of the frame. The correct length bed will fit that wheelbase. This was posted on the truck forum that I spend a lot of time on a few years ago. DO NOT pay attention to the numbers as the 12 inch cut would now be a 9-1/4 inch cut. do pay attention to how the gent who drew up the design said to shorten the frame with an overlap rather than just cutting and **** welding the frame.
Just to confirm, this is a 46 pickup. The thumbnail in my original post shows 3600 wheelbase at 125 1/4” and a 3100 at an even 115” (10 1/4” difference) You have more experience than I do I’m sure, just want to make sure numbers are right for this series of truck. Appreciate your response/advice..
Is this one of those Chevs that when you set a modern rear axle on stock springs it moves the axle forward in the wheel well a few inches? Cure was to redrill spring pads off center. That might account for the difference in measurements. Dave
Before you cut anything measure your frame diagonally at several points to be absolutely certain it is square and straight. Any existing twist or bend could screw up your final outcome. Cut out whatever you think is right but be prepared to reposition the rearend a little upon final (full weight) ***embly in order to center the wheel in the opening.
Good advice on checking for squareness across the frame. Once I do this and everything is ***embled, what is typically your best option for dialing in the rearend position? Do the leaf springs still perform the same if you need to adjust forward or back off of center slightly?
My 49 is a long. Bought a short ch***is and bed. Decided I dig the long bed Cutting a frame now. Doing a Z cut. **** welded. After the Z is welded and finished off we will build a sleeve to set inside the spliced area. Probably will set about 6 inches behind and ahead of the spliced area. not a good pic but the best I have Pic of the splice area this splice is off center on purpose. It lines up with the center of the section being grafted to it Your section would be centered
if you look at a stock 46 with a box and fenders you will see the rear axle/wheels are a bit forward in the fenders. they are not centered from the factory. i would suggest that you ***emble the body to get the axle where it looks good and not worry so much about the numbers
Something else to think about is the frame is "V" shaped front (narrow) rear (wide) so you need to know the rear frame width on a stock half ton before you go chopping and shortening. Is the goal to make it a stock looking half ton with all stock body parts ?
Yes, my intent is that it will resemble a stock 3100. Because the frame rails run differently after the cab on a 3600, I ***umed I would have to fabricate mounts for the bed. The more input I get from everyone, I see the wisdom in waiting till the ch***is is weighted down and I have the sheet metal to reference. My plan was to shorten the ch***is in the next couple of weeks and then move forward, but I sure would hate to get to the end and realize I’m not happy with where the rearend sits. There’s wisdom in asking questions, appreciate everyone’s input.
I did something like this a few years ago. Two points to consider is the location of the rear wheels to the center of the wheel well of the bed and two the distance between the front of the bed and the rear of the cab. Another thing is to drill two holes in the frame the same distance apart on both sides, so that the holes are located on either side of the frame cut. Measure the distance between the holes before and after the cut and production weld and make sure that they are the same.
Best step ever in frame shorten or stretch. We always made a punch mark an inch on either side of the cut lines and used that as a reference when putting the frame back together. I did big trucks; but the idea is the same. Set the work up so you can stand back a ways and eyeball the straightness and alignment. Eyes sometimes shows up stuff levels and tape measure don't.
My bed is altered in many ways, I shortened the stock 1/2 ton wheelbase, slid the bed forward to eliminate some of the gap between cab and bed, raised the bed up to get the bed rails lined up better with the body line, narrowed the bed 4 inches etc etc. When it came time to place the fenders on the now altered bed sides I must have moved them a thousand times to get the look I wanted, finding center of the teardrop fender was a pure visual game. What helped the most was getting a really tall tire to fill in the fender opening, so you could at least measure smaller gaps front/rear of the tire. After I got the fender where I wanted it I then cut my length off of the back of the bed until it looked right. Yours will be much easier but it'll still take some visual references to gap the bed from cab and center the rear end.
The gap between the cab and the bed is often overlooked. with the front of the bed being usually straight, and the back of the cab usually being curved, an extra 1/2" doesn't sound like much until you step back a ways and see it in real life. Also, if the springs you are using are not the same capacity of the original springs, or the finished truck weight is different from the original design, the difference on the amount the spring plays on the axle movement will be different and the adjustment for the correct wheel base could be different. The dropped front axle could play into your wheel base length and that could have changed it as well.
Check this out (if you have not done so yet): How to shorten a LWB frame - The 1947 - Present Chevrolet & GMC Truck Message Board Network (67-72chevytrucks.com)
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