Popular Post D2Denny Posted January 30, 2023 Popular Post Share Posted January 30, 2023 My G-75 came from the factory with a RADL 515 drive axle and CRD91 carrier assembly. The ratio is 6.45. Even with the TRTL720 trans and tall rubber, it maxes out at 60 MPH against the governor. The other ratios available were 4.39, 5.07, and 5.46. I checked with all the truck junk yards within 50 miles of North Central Ohio and no luck on a possible replacement with a better ratio. I checked with several of truck gear unit rebuilders to possible re-ratio it, but no parts are available for this carrier anymore. Anyone know of any possible sources of a replacement CRD91carrier in Ohio or half way into the surrounding states? Replacing the entire drive axle assembly with a later model top loader to get a better ratio is not an option at this time. 3 Quote Link to comment Share on other sites More sharing options...
mattb73lt Posted January 30, 2023 Share Posted January 30, 2023 Is that a top loader axle? I was thinking of you swapped to a RAD508 (or similar axle) front loader and the went to a CRD117 carrier, it would give you a much wider selection of ratios to choose from. I run CRD117's in both my trucks with a 4.62 ratio and a TRTL720. One has 22.5 rubber and the other 24.5 rubber. Granted, I only had limited test run time on the truck with the 24.5's, but I was turning 1650 RPM's and was going 73 MPH in 5th Hi-split. Just some food for thought. Quote Link to comment Share on other sites More sharing options...
Mean Green Posted January 30, 2023 Share Posted January 30, 2023 I have a 4.62 ratio single axle front loader out of a b model if it will fit 1 Quote If your going to be a bear be a grizzly Link to comment Share on other sites More sharing options...
D2Denny Posted January 30, 2023 Author Share Posted January 30, 2023 The CRD91 carrier is a top loader. With the short wheelbase and length of the triplex trans, the driveline is only 36" long. A front loader carrier would drop the diff input shaft about 1 foot. This would make for an untenable driveline angle. Thats the reason Mack used this rear axle assembly on the original build. Quote Link to comment Share on other sites More sharing options...
mattb73lt Posted January 30, 2023 Share Posted January 30, 2023 Damn, that is short! Would there not be a later model top load carrier the would fit that axle? Like the rear carrier out of a set of tandem axles. I think that's a CRD92. Quote Link to comment Share on other sites More sharing options...
mattb73lt Posted January 30, 2023 Share Posted January 30, 2023 I'm off. CRD92 is the front carrier. CRD93 might work. Just saw some prices, ouch! Quote Link to comment Share on other sites More sharing options...
D2Denny Posted January 31, 2023 Author Share Posted January 31, 2023 Matt: Thx for the reply but I shot that angle a while ago. The 91 series has a totally different mount bolt pattern and size than the 92 and 93 series. Also, the axle shaft spline diameter is larger on the 91 series. Quote Link to comment Share on other sites More sharing options...
67RModel Posted January 31, 2023 Share Posted January 31, 2023 Did you Try Adelman's in Canton, OH? Quote Link to comment Share on other sites More sharing options...
67RModel Posted January 31, 2023 Share Posted January 31, 2023 It might seem like a far fetched notion but couldn't you just remove and tear down your carrier and have new ring gears made? I know there is probably a little more to it since its double reduction but you have the pattern for the new gears from your existing 6.45 gear set. Its just a matter of reworking the tooth count on the new gears to whatever ratio you are going trying to achieve. This is very easily calculated based on your tire size, overdrive ratio, and what rpm you desire the engine to run at a given road speed. I know there are several companies that will "reverse engineer" helical cut gears (any gear really) for you to pretty exacting tolerances if you have something to start from. I doubt you would have to remortgage your farm to get this done. I'm sure it wouldn't be cheap but by the time you find an axle out of a junk truck (assuming you can find one in the ratio you want), haul it home, and go through it before you drop it into your axle your probably around the same money? Just throwing that idea in the ring as something to consider. 1 Quote Link to comment Share on other sites More sharing options...
D2Denny Posted February 1, 2023 Author Share Posted February 1, 2023 67RModel: I have called Adelman's some time ago. The parts man had never heard of a CRD91, let alone having one on the yard. As for having a new helical ring gear and pinion designed, machined from scratch, and properly heat treated, I really can't see this being cost effective. However if you could provide me with the name of one or two firms that you know of that provide this service, I'll give them a call on the feasibility and cost of this endeavor. Dennis Quote Link to comment Share on other sites More sharing options...
67RModel Posted February 1, 2023 Share Posted February 1, 2023 Dennis, Try these two outfits. One is in Pennsylvania and one in Illinois. There may be others as well. https://www.rushgears.com/ http://www.circlegear.com/index.html Quote Link to comment Share on other sites More sharing options...
terry Posted February 1, 2023 Share Posted February 1, 2023 Can't you take a newer mack rear housing with a desired ratio and cut the mounting brackets off and weld on the needed brackets. terry Quote Link to comment Share on other sites More sharing options...
Freightrain Posted February 1, 2023 Share Posted February 1, 2023 I think that is where Matt posted above about the cost. I got real lucky buying my air ride cut off for $500. Most yards are way over that for just a housing. Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment Share on other sites More sharing options...
D2Denny Posted February 1, 2023 Author Share Posted February 1, 2023 67Rmodel Thanks for the response. I have reached out to both firms with a request for a feasibility assessment and ballpark cost. We'll see what response I get. In any case I wouldn't take this route until next winter as I doubt that this could be accomplished before show season. As to replacing the entire rear assembly as some have suggested, (I hate to be a wise guy) but please refer to the last sentence in my original post. I didn't want to list the litany of reasons for not doing a complete axle swap at the time. I have already gone through the wheel ends in regards to brakes and seals. I want to retain the 24.5 Dayton hubs and rims It has to be a top loader for driveline requirements with an acceptable ratio Chances of finding a "drop in" complete axle assembly to fit my requirements are about as slim as finding just a carrier assembly. I understand upgrading the entire rear unit is certainly possible and within my skill set, but the time, effort, and cost is not justified for a truck that might be driven 500 miles a year. If i need to go a distance with this truck it will go on the trailer and I'll go down the highway at 75MPH in air conditioned comfort dragging it behind my Autocar. I have to believe there is a CRD91 carrier out there somewhere with an acceptable ratio. Quote Link to comment Share on other sites More sharing options...
67RModel Posted February 1, 2023 Share Posted February 1, 2023 On a side note: how rare is a G-75? I don't have any knowledge of the model or production numbers. In fact I think this is the only one I have seen on the forum in my 3 years here.....at least that I recall. Quote Link to comment Share on other sites More sharing options...
67RModel Posted February 1, 2023 Share Posted February 1, 2023 Denny, Also let me know if you are interesting in the below lead. I visited this place in October and he had a bunch of stuff. It is out of your search radius though... Quote Link to comment Share on other sites More sharing options...
D2Denny Posted February 2, 2023 Author Share Posted February 2, 2023 G model Macks were produced 4 years 1959 - 1962. A total of 2181 were built with 318 being G-75's 67Rmodel: If you could provide a name or contact # for the Va. place I'll give him a call. If its in the Wytheville Va. region I could turn it in a long day. Thx Dennis Quote Link to comment Share on other sites More sharing options...
mrsmackpaul Posted February 2, 2023 Share Posted February 2, 2023 What about a joey box ? Think you blokes call them brownie boxes or something like that Anyway I reckon something should be do able here I do like the idea of a fresh set of gears gettin made, you can get any ratio you want then Paul Quote Link to comment Share on other sites More sharing options...
Brocky Posted February 2, 2023 Share Posted February 2, 2023 A Joey box from Australia is known as an auxiliary Transmission up here.. Which sound like a good idea.. Put a big overdrive box behind yours???? Quote Brocky Link to comment Share on other sites More sharing options...
D2Denny Posted February 2, 2023 Author Share Posted February 2, 2023 OK. As previously mentioned, I have a 36" driveline. That is the distance between the Transmission output yoke and Diff input yoke. Obviously, the auxiliary boys know something I don't. Can you direct me to an auxiliary box to use that is so super short in length, that I can fit it in the allotted space (36") along with its input and output shafts, the required 4 u-joints, and a slip yoke, and still maintain adequate articulation to allow for rear axle suspension movement? From my recollection a Spicer 7041 is close to 24" long from its front companion flange to the center of the rear yoke. That would only leave me 12 inches to install the two connecting drive shafts and required slip yoke. Quote Link to comment Share on other sites More sharing options...
Mark T Posted February 2, 2023 Share Posted February 2, 2023 Why not just a newer housing and rear ? My buddy here had single axle coal trucks with them. It was like the back rear out of an air ride with 46s . And yes it was a double reduction drop in. Sounds like you're not loaded with options unless a 65 year old rear just happens to materialize for sale. Quote Link to comment Share on other sites More sharing options...
mrsmackpaul Posted February 3, 2023 Share Posted February 3, 2023 Warn overdrives used to be fitted to a lot of big 4x4's back in the day I realize your truck is a lot bigger than a big 4x4 But if you only run about bob tail I reckon your weight might be well within range Not saying it would be simple to adapt but it would be close coupled and maybe only 6 inches to the transmission It won't be simple no matter what you do So doing some googling and from what I can gather you have a Triplex box Google tells me these were in either single over drive or double over drive Which one do you have ? Double overdrive gives a pretty hand overdrive Maybe also be a option, dunno enough about these transmissions and their numbering Paul Quote Link to comment Share on other sites More sharing options...
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