I'm going to be starting my cummins F3 build as soon as I can get my other motor sold and I'm trying to get a plan laid out.

I will be strengthening the frame to handle the cummins 6bt. My plan is to box in the front section and then weld in a crossmember under the engine at the motor mounts. It seems like that would help keep the frame from twisting. Should I maybe put in another crossmember towards the front/back of the motor or both? Any thoughts?

 

We did not want any frame twist and cut all the original riveted cross members out and welded in new ones and boxed the frame from the front cross member all the way to the back. We did leave the front cross member because all the front mounted to it. We do not get any twist and the frame is very ridgid. If you want to go to this much trouble it would probably be worth it.

 

......man...THAT...is one sweet A$$ chassis!!!......

Nice work!...I was cruiZin' thru your gallery...impressive to say the least!

Not to hijack...nice work on the frame man...seriously......I gotta
ask ya about that fuel tank! What did that come out of ???
Looks like it was MADE for that frame...and the seatbelt setup is just
killa man! WHERE did those come from?????

Anyway...VERY nice...very, very nice...congrats...

-

 

mytbtruck,
Nice work on the frame. It looks good. I notice that you aren't running stock suspension. I will be using my stock suspension and have read that the c channel frame twists and flexes to help the suspension on these trucks. It looks like it would be a very strong, solid setup to box mine all the way like yours. I'm worried about the ride becomming even more harsh than it is now. I worry with only boxing the front also because of causing stress points where the boxing ends causing cracks. I'm a "grandpa driver" with my vehicles but I'm also putting a motor in the truck that it was never designed to have in the first place.

Is there a way to keep the frame working the way it was designed and still have the strength to handle a stock cummins? If not I may go the same route and box the whole frame in.

 

I do remember how those old frames twisted and groaned with a load when you went over farm fields that were rough. I think a ridgid frame like mine with the original suspension probably would give you a much stiffer ride. Just my opinion not from experience. I needed a ridgid frame with the front and rear independent suspension, that was the reason for all the modification on my frame.

 

I'm trying to decide how to transition from the boxed front to the c channel rear part of the frame. I'm thinkingthat welding gussets into it may help distribute load and energy. I'm hoping something like that would keep from causing a stress point and causing a crack in the frame. I doubt I will ever use this motor to it's full potential but i will never have a need to upgrade it for more hp or torque.

Any thoughts?

 

My thoughts are that if you are just driving it no matter what you do would be ok, but who can resist really giving it all she has got every once in a while. I can't. Unless you put it on the drag strip I think any mod would be an improvement over the original. Uni-bodys are designed to absorb the energy but our frames just roll with the flow.

 

Thanks for the advice. I'm an aircraft machinist/welder and understand fabricating parts to disperse load but this is the first time working on a frame so your help is very appriciated.

I think I'm going to box in the front of the frame and keep an eye on everything to see how it works out.

Thanks again mytbtruck.

 

I'm just afraid if you don't box the entire frame. You might run into an issue were the boxing plates end. Even with gussets you are going from a firm frame to a flexy frame. So it's going to want to crack in that area.

If it was mine I would box the entire frame. But then I've also got a 91 F-Super Duty( F450 ) sitting in my driveway so I'm use to a rough ride.

 

One of the guys here (I think it was Randy Jack) talked about cutting fishmouths in to the boxing plates you are putting into the frame where the boxing ends.

The mouth would have to be twice as deep as it is tall to be effective.

This would prevent stress risers (and hopefully cracks in the frame) as it makes a smooth transition from boxed to unboxed.

 

If I were to box the whole frame and run F1 springs in the rear with airbags to help increase the load when needed but give the truck a little movement in the rear suspension. The F3 rear suspension doesn't give at all unless it's really loaded down. The front springs are getting replaced since they are so worn out. I will have to do something with those for the added weight of the motor.

Any thought?

I know I keep jumping back and forth on what I want to do with the frame but I'm in the initial planning stages. I want to be ready as much as I can when it's time to get going.

 

My frame is boxed front to rear. Remember that some things attach to the frame, such as running boards and the bed, and you need to plan for that. You'll need to either cut access holes, or weld nuts inside to bolt to. Frames also can change dimension when you box them. I read, after I boxed mine, that frames can spread slightly when boxed. Mine did, and I had to redo my tranny crossmember because it was too narrow.

 

I have found that all the cross members need to be reinforced from the original. If you don't box the entire frame, at least beef-up the crossmembers!

 

If you are a "grandpa driver" there would be no problem with causing a stress point at the end of the boxing. If you are still concerned, carry the boxing to a crossmember, or end it with ~ 60* angle cut on the boxing plate. Fish mouthing or diamond cutting is used for doublers not for boxing. I like to inset my boxing plate ~ 1/8", makes the welding easier and cleaner. You do not need to make a continuous bead, a couple inch long bead every 6" will be just as strong and allow any water to weep out. Be sure to cut access holes in the plate wherever you have a connector attaching something to the outside of the frame. If you are going to bolt something to the frame with a bolt all the way across, be sure to install crush tubes for the bolt to pass thru. I like to use a 2" holesaw to cut 2 holes about 6" apart then connect them with straight cuts to form ovals, put in enough to be able to run wiring, brake lines etc inside the frame for a clean look. Paint the inside of the frame and back side of the boxing plates with weld thru primer before welding then finish paint the inside with a rattle can after welding by attaching a straw (like comes with WD40 or other penetrating oil cans) to the spray nozzle and spraying thru the access openings. Be more concerned with getting full coverage than making it pretty or avoiding runs.

If you really want a super stiff frame install a "K" style central crossmember.

 

Really great info. Thanks.

Where would be the best place to weld in the K member? Should that be midway between the front and back?