Fast Ratio Steering Box For OBS-GMT 400-Here's How I Did It Cheap

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Kawabuggy

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I don't want to get off into a nomenclature argument so if you have an issue with what I'm calling the pieces-DO NOT BOTHER to post. With that out of the way...

I have been driving the truck around now for a while and decided that today was the day to tackle the "twitchy" off-center steering. I had ordered a lower GPM FITTING from Turn One. Since I did not have my pump apart at the time I was ordering we had to guess as to which FITTING I needed. So... the tech told me that the 2.5GPM FITTING was the most common one sold. I purchased the 2.5GPM fitting and it came in this past week. Today I pulled my old FITTING out of the power steering box-hint; suction out as much of the fluid out of the p/s reservoir as you can before you start the project lest you find yourself in a bath of p/s fluid. I have a lift so essentially I was working over-head... Nothing like P/s fluid in your arm pits.. Both of them. Anyway... the Turn One fitting (2.5GPM) the orifice in the middle measures .134". I have a PIN set for measuring orifices and this is the largest pin I could fit through the hole. The factory valve I removed... also a .134". I have a spare p/s pump off another LS here so i pulled that FITTING and measured it at .144" (3.0GPM?). So it looks like I wasted my time ordering the new fitting. While I had it apart I noticed that it had 1 shim under the screened cap on the flow control valve. I measured that shim thickness at approximately .020".. From reading I know that if you shim that cap you can reduce pressure. So.. I found a standard flat washer that is .041" thick and put it under the cap leaving out the factory .020" shim.. Put it all back together and I have achieved exactly what I want.. Going down the road now the wheel has resistance if you attempt to move it either direction left or right of center. I wonder what it would have felt like had I gotten the 2.0GPM FITTING? Who knows.. I'm leaving it alone at this point as I consider this success. When driving straight down the road and you try to slightly correct left, or right, you can now feel resistance and instead of the wheel moving quickly and the truck darting. The wheel just moves more slowly allowing you to correct your aim without over-correcting. Hope you guys are having fun with your projects... Next up for me... I've got all the pieces for "shade tree coil overs" here and I'm putting them together in the next few weeks..
 

1998_K1500_Sub

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When driving straight down the road and you try to slightly correct left, or right, you can now feel resistance and instead of the wheel moving quickly and the truck darting. The wheel just moves more slowly allowing you to correct your aim without over-correcting.
^^^

So now, going down the road, it's responding well.

How does it respond now if you're rolling slowly - 5MPH (or on a lift w/ no weight on the wheels) and you yank the wheel one way or the other, fast, like you're avoiding a skunk?

How does it respond now if you're sitting still, full weight on the front wheels, and you turn the wheel (as though you're trying to parallel park)?

When you talked to Turn One, did they ask you how the steering was performing for you at the time ("twitchy")? If so, did they recommend a particular change?
 
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Kawabuggy

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Great questions.. In talking to Turn One i explained the over-assist I was experiencing just off center of straight ahead and they recommended that I reduce the VOLUME by going to a lower GPM fitting. Turn One does not recommend adding the shims to reduce pressure. When sitting with the wheels on the ground and you turn the wheel left or right it feels firm for the first bit of travel as you start to move away from center and then gets easier to turn the wheel the further you get away from center. It's almost like variable assist-firm when the wheel is between say 9 and 3 on a clock, and much more assist/easier to turn the wheel when you are farther from center/wheel straight up. I have not tried yanking the wheel left or right yet. I put about 200 miles on the truck yesterday as I drove to the deer hunting lease and it seems to be working perfectly. I was pulling a small trailer yesterday and had to back it up to load it, and then back it into the barn to unload it afterwards.. The steering has no hiccups. In tight turns the assist is there and there is no binding, or loss of pressure assist. It feels consistent through the full range from lock to lock. I'm still thinking I should try a lower volume fitting. I now have 2 factory steel fittings that I could TIG the hole closed on and redrill. It only took about 30 minutes yesterday to do the entire job of swapping components and that included locating a suitable shim. If anyone knows the size of the hole in a 2.0 GPM fitting, I'd be interested to hear it. If I have to guess, and I decided to weld/drill one of these factory steel fittings I'll probably go .129" which is .005" under the hole size of a 2.5GPM fitting. I am guessing at that as the difference between a 2.5 and 3.0 appears to be .010".. Anybody care to guess or make a suggestion?
 

1998_K1500_Sub

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Great questions.. In talking to Turn One i explained the over-assist I was experiencing just off center of straight ahead and they recommended that I reduce the VOLUME by going to a lower GPM fitting. Turn One does not recommend adding the shims to reduce pressure.

OK, so you wanted to do what they suggested, but you wound up buying a 2.5gpm fitting... which happened to be the same as what you already had (which you learned upon teardown). So, you tried the shimming the pressure spring. Hey, why not, might as well try...

When sitting with the wheels on the ground and you turn the wheel left or right it feels firm for the first bit of travel as you start to move away from center and then gets easier to turn the wheel the further you get away from center. It's almost like variable assist-firm when the wheel is between say 9 and 3 on a clock, and much more assist/easier to turn the wheel when you are farther from center/wheel straight up.

OK, so it seems you have sufficient pressure to handle that sort of heavy(ier) steering load. It never "runs out of assist", in other words (correct me if I misstated).

I have not tried yanking the wheel left or right yet.

OK. I would suspect that insufficient flow would make it self known in this ^^^ type of test. But since you didn't change the flow rate (you kept the same fitting / same orifice) then I wouldn't really expect a change. All the same, it's good to check.


In tight turns the assist is there and there is no binding, or loss of pressure assist. It feels consistent through the full range from lock to lock.

Encouraging.

I'm still thinking I should try a lower volume fitting.

Doesn't hurt to try, won't learn anything if you don't.

If anyone knows the size of the hole in a 2.0 GPM fitting, I'd be interested

Take a look at the .pdf I attached in one of my earlier posts, "Power Steering Pump Pressure Flows". In it the writer makes these claims:

Opening up the throat of the discharge fitting by 0.016 inch on the diameter will give you approximately 1/2 more gallon per minute. Opening up the hole by twice that amount (0.031 inch on the diameter) will increase flow by about one more gallon per minute at full flow.

0.1144 inch diameter = 2.0 gpm
0.130 inch diameter = 2.5 gpm
0.145 inch diameter = 3 gpm
 
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Kawabuggy

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It never "runs out of assist", in other words (correct me if I misstated).
Correct. There is no point in the full range of steering that it loses power assist, or gets harder than it should.

0.1144 inch diameter = 2.0 gpm
0.130 inch diameter = 2.5 gpm
0.145 inch diameter = 3 gpm
That's interesting because I have measured the 2.5GPM fitting that I received from Turn One at .134" using a pin gauge set. Further, the factory steel fitting I removed also measured exactly .134" venturi size. The factory steel fitting I removed from the spare LS pump I had here was .144" venturi size. If I want to get down to the 2.0GPM I guess I will be TIG welding the hole shut, and then re-drilling to .114", or as close to that size as my drill set allows.
 

1998_K1500_Sub

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If I want to get down to the 2.0GPM I guess I will be TIG welding the hole shut, and then re-drilling to .114", or as close to that size as my drill set allows.
^^^ Yes, narrowing that orifice might do the job, but from what I've read (and seen looking at those fittings) the contour of the sidewalls forms a venturi of sorts. You won't be able to maintain the sidewall's "venturi" profile by simply narrowing the hole... but what do you lose by trying? :waytogo:

I suppose the change you describe have an affect on the flow, but you can't be sure of "how much" affect it'll make.

What's the price for a 2gpm fitting if you shop around? Buying one would give you some confidence while performing this "experiment" with flow / pressure.

I found this, $25, they claim it's 2gpm. I was just casting about looking at prices...

 
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