1958 A-120 4x4 Top Speed?


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Post Thu Apr 18, 2019 8:41 am

1958 A-120 4x4 Top Speed?

Hey IH pickup folks -

I have my A-120 4x4 running and stopping and rewired... :) It is progress.

I am getting about 40mph in 4th gear and wondering what is typical.

I have the dual stick transfer case controls set: left stick all the way forward (HI) and right stick all the way back (out).

It shifts easily, but still sounds a bit loud around the transmission and transfer case.

Line Ticket says I have the following:
RA10 Sgl Rear Axel (4.77)
FA15 front axel (4.55)
T-15 Transmission
T/140 transfer case.

Any thoughts or suggestions?
thanks in advance - Ben

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Post Thu Apr 18, 2019 9:58 am

Re: 1958 A-120 4x4 Top Speed?

Ben, others will probably speak up in the near future, but those axle numbers indicate to me that it is not a "long in the legs department" set of axle numbers. It sounds to me like it was more of a work horse truck when bought, designed for slow steady speeds than for running down the highway for town. Just my uneducated thoughts. I know we would all love to see pictures of your truck.

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Post Thu Apr 18, 2019 10:28 am

Re: 1958 A-120 4x4 Top Speed?

Here are some pics from December - So far I have upgraded to an alternator, changed the 4 tires I have the 17.5 inch regular steel rims (not split). I had to re-upholster the seat as it was a giant mouse nest, I removed the gas tank and had that boiled out, same with the radiator and heater core. I had the water pump re-built and I re-wired the whole truck...new muffler and tail pipe since these pics.
She is a beast!
1958 A-120 4x4 - 1.jpg

1958 A-120 4x4 - 2.jpg

1958 A-120 4x4 - 3.jpg


More pics coming :)

Now that the snow piles have melted I can continue to work on it and of course use it as intended.

ben
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1958 A-120 4x4 - 4.jpg

AZD

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Post Thu Apr 18, 2019 12:22 pm

Re: 1958 A-120 4x4 Top Speed?

Nice truck, I really like it.

I recognize the tires as STA Super Traxion type. According to their website the diameter is 31.4 inches for the 17.5. With your rear ratio of 4.77 and redline of around 3500 (not sure what engine you have, but ballpark for the SD/BD) then you’ll top out at 70 MPH… on paper. Chances are it’ll feel like a runaway freight train before you get there.

BTW, at 40 MPH with those gears and tires you’re only at 2050 rpm. Guarantee it feels like a lot more though.

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Post Thu Apr 18, 2019 3:12 pm

Re: 1958 A-120 4x4 Top Speed?

Love your truck!!!! Super cool and a bit rare, too.
Just my 2 cents: AZD is right. your max will probably be 70 mph; but, you wouldn't want to stay there.
a constant- top speed, would be around 55, maybe 60. That would limit the stress on the engine.
I lost my engine due to a rod failure, at freeway speed (50 mph). The rod was probably compromised
already; but, the extra engine speed didn't help (I have an R 120 with 4.77 gears)

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Post Thu Apr 18, 2019 6:05 pm

Re: 1958 A-120 4x4 Top Speed?

With that gear ratio your engine should turn 3063 revolutions in one mile. If you can achieve 60 miles per hour or one mile per minute the engine will be at 3063rpm.
You need to calculate the circumference of tire by using 3.1415 X the diameter of 31.4 inches. Your tire circumference should be 98.643 inches or close to 8 feet. One mile is 5280. Divide 5280 by 8 feet ( 98.643"). You should get 3063 revolutions in a mile. One mile in a minute is 3063rpm.
40 mph is 2/3 of 60 mph. Divide 3063 by 3 and multiply that new number by 2 and you get an rpm very close to that given by another poster.
If the engine in your truck is in fair condition it should last at 3000 rpm. Less rpm is less stress and possibly a longer engine life.
I would advise using a 15/40 oil. This oil is recommended and works well with many Diesel flat tappet lifter applications, like those in your engine,
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Post Thu Apr 18, 2019 6:33 pm

Re: 1958 A-120 4x4 Top Speed?

Ever since the days I drove Porsche and tuned Porsche engines, I have used a machinist dial gauge to set valve clearances. The natural wearing of the contact points of the valve stem and the rocker arm form a wear pattern that may not be able to be measured and adjusted correctly with a flat feeler gauge. The feeler gauge will measure across the high points and this my not be the clearance that needs adjusting on a well worn engine. A machinist dial gauge mounted to the cylinder head and the stem of the gauge touching the rocker arm will give an accurate measurement. The rocker arm clearance can be adjusted to "correct" with the dial gauge in contact with the back of the rocker. The stem of the dial gauge should be inline with the valve stem for a correct reading.
A person on a budget does not have to buy a multi hundred dollar STARRET dial gauge. Harbour Freight should have something inexpensive with enough accuracy for this job. If the gauge does not come with a magnetic base, spend a few bucks and get one.
A magnetic base will work on an IHC engine. The Porsche engine was an aluminium alloy and I machined a bolt-on adapter to hold the mag-base.
You will never be sorry to have spent the money when your Old Binder changes from a bunch of clatter to an even ticking. I have done this more than a few times.
https://www.harborfreight.com/1-in-trav ... 63521.html
https://www.harborfreight.com/Multi-Pos ... 63663.html
I would rather have tools I do not need than to need tools I do not have
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Post Fri Apr 19, 2019 8:38 am

Re: 1958 A-120 4x4 Top Speed?

Hi folks and thank you for the perspectives and information.

Yes those are the Super Traxions - virtually the only tire I could find for the 17.5 inch rims :)

I will rig up a tachometer and get a better sense for things. It sure sounds like I am winding the BD240 way up just getting it to 40mph. I have this nagging feeling that something is holding it back, or it is not properly disengaging the front axle. I can clearly feel and hear a difference using the High/Neutral/Low shifter - while the throw on the In/Out lever seems awfully short.

I will take it to heart that it will sound like a runaway freight train as I approach 55mph. I don't have any need or desire to go 70 but it's nice to know I should be functional at 55 for a stretch or two. Heck I might pick up a little speed if I lightened that rear bumper!

thanks too for the valve adjustment info, oil recommendations and overall appreciation for my truck.
I will upload some more pics this weekend.
ben

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Post Fri Apr 19, 2019 10:25 am

Re: 1958 A-120 4x4 Top Speed?

If it feels like it's being held back, as you say, there may well be something going on with the transfer case and/or front axle. You noted that the ratios are slightly different between the two, 4.55 vs. 4.88. When both are engaged the front axle is being overdriven by about 7.2% compared to the rear. (Or the rear is being underdriven by about 6.8%, however you want to look at it.) That may not make much difference crawling on dirt, snow, or mud. But on pavement with increased speeds it probably matters more. At 40 MPH your front axle is trying to go 43 MPH. So between the two there's going to be some tire scrubbing AND increased wear, plus more rolling resistance.

BTW, remember GM's full-time 4x4 setup from the late 70s or early 80s? Those things were tire eaters even with matched ratios. My grandfather had one for a while.

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Post Sat Apr 20, 2019 11:56 am

Re: 1958 A-120 4x4 Top Speed?

I do remember those full time 4x4's! That was right about when I graduated from high school and went off to college. So I will be mindful about the axle differences to the extent I can choose to drive on dirt...Thanks AZD...and I need to make sure my front axle is disengaging.
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