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41 Posts
Discussion Starter #1
Hello all,

Been slowly moving my project forward and after removing the body off my frame I noticed that the frame is pitted and I have one spot where rust has eaten through the metal.

My plan was to box this frame and make new and stronger cross members for my diesel conversion.

If I were to cut around this piece of the frame and put new metal into that area, is there some technique I can use to make sure I don't get stress cracks?

P.S. Can't afford to find another frame and have already cleaned it and started to paint it with POR15 on the outside of the c channel.


Super Moderator
26,007 Posts
yo W,

Frame Service; Drilling Precautions, Straightening, Reinforcing, Member Replacement, etc. by Ford @
"Drilling Precautions

CAUTION: Do not drill holes in the frame flanges. This will reduce the strength of frame (5005).
If a hole must be drilled in the frame, make sure that it meets all of the following requirements:
1.The hole is located in the upper half of the frame.
2.The edge of the drilled hole and the edge of the nearest hole are at least 25mm (1 inch) apart.
3.The edge of the drilled hole is at least 25mm (1 inch) from the edge of the flange.
4.The drilled hole is not adjacent to any other existing brackets or components of frame

Welding Precautions
CAUTION: Disconnect the battery ground cable (14301) before using any electric welding equipment.
All welding on frame must be done with electric welding equipment, and the heat should be kept in a small area to prevent change in hardness of the metal. Do not use gas welding equipment. A double reinforcement must be added to frames where heat or weld is applied to the area to be repaired. The welds are to run lengthwise along the reinforcement when a reinforcement is to be welded to the frame side rail.
Frame Strength Identification
F-Series, F-Super Duty Chassis Cab and Bronco all use a 36,000 psi steel frame.
Frame Straightening
Misalignment of frame can be corrected by straightening the out-of-line parts or by replacing the crossmembers, braces, or brackets if they are badly damaged.
Straightening should be attempted on frames that fail to meet specifications of the diagonal checking method or where damage is visually apparent.
However, to prevent internal stresses in the metal, frame straightening should be limited to parts that are not severely bent. If heat is needed to straighten a frame member, keep the temperature below 649°C (1200°F) (a dull red glow). Excessive heat may weaken the metal in the frame members and cause permanent damage.

Frame Reinforcing
After a bent frame member has been straightened, inspect the member closely for cracks. If any cracks show, the frame member should be reinforced or replaced.
Reinforcements should be made from angle or flat stock of the same material and thickness as the frame member being reinforced, and should extend a minimum of 152.40mm (6 inches) to either side of the crack. Ideally, the reinforcement should be cut from the corresponding area of a similar frame.

Weld Attachment
To ensure a quality repair, adhere to the following procedure if it is necessary to weld reinforcements to the frame.
1.Wire brush the area around the crack to remove the paint, grease, mud, etc., and to expose the crack completely and ensure good weld adhesion.
2.To stop the crack from spreading, drill a 6.35mm (1/4-inch) hole at a point 12mm (0.50 inch) beyond the root of the crack.
3.Grind out the full length of the crack to the hole to form a V-shaped slot with the base of the V-slot contacting the reinforcement.
4.The base of the V-slot should have at least a 1.52mm (0.06-inch) opening to ensure weld penetration to the reinforcement when welding the crack.
5.Drill clearance holes in the reinforcements to clear rivet heads and bolt heads or nuts where necessary.
6.In the event that repair is required on more than one frame surface (i.e., a flange crack that extends into the web), two pieces of flat stock (one for each surface) should be utilized and welded together where they join. The web reinforcement should be a minimum of 76.20mm (3.0 inches) high and have a 63.50mm (2.5-inch) radius at each of the two corners.
7.Completely clean the surface of frame under and around the reinforcements.
8.Clamp the reinforcements securely to the frame prior to welding.
9.Weld the reinforcement all around after welding the crack V-slot.
10.The flange edge weld should be ground smooth after all pit holes have been filled by the weld.
11.If a damaged bolted-on frame bracket is to be replaced, the new bolts, washers, and nuts should be of the same specifications and bolt torques as the original parts.
12.In cases where it is necessary to remove rivets, replace them with Property Class 9.8 metric (Grade 8) nuts, bolts and washers of the next larger size (i.e., for 3/8-inch diameter rivets use 7/16-inch bolts, for 7/16-inch diameter rivets use 1/2-inch bolts). This requires line drilling of the holes to the same diameter as the new bolt (i.e., either 0.437 diameter or 0.500 diameter).

Frame Member Replacement
If a damaged frame member is to be replaced, new bolts, Property Class 9.8 metric (Grade 8) fasteners and rivets required for replacement of parts should be of the same specifications as the original bolts or rivets. In cases where it is necessary to substitute a bolt for a rivet, use the next larger size bolt."


Frame Strength, 96 F-Series, F-Super Duty Chassis Cab and Bronco all use a 36,000 psi steel frame. from 1996 F-150, F-250, F-350, F-Super Duty Chassis Cab and Bronco Workshop Manual; The F-Series and Bronco frames are made of all steel channel with replaceable steel crossmembers..." read more
Source: by miesk5 at

41 Posts
Discussion Starter #5
Thanks for the replies!

I am thinking either a 7.3 IDI turbo, 6BT or 4BT...Which ever makes the most sense at the time money wise but I prefer the 4BT for the weight.

Above is a link to show how this person has boxed his frame. I too will want to add a cross-member but also replace the ones already existing to strengthen the frame.

My frame is completely stripped down.

I hate how much flex I have in the frame just moving it by hand and want to strengthen it.

I am thinking I would do pretty close to the same things as in the pictures I linked but with square tubing instead of round for the cross-members.

Premium Member
1,916 Posts
Keep in mind that the reason the ladder frame design used on your bronco was designed to flex in order to minimize stress points. The frame is more than strong enough to handle the diesel conversion. Unless you're doing a desert racing/rock climbing rig, boxing the frame is more for show than anything.

As far as repairing a rust hole, you'll run into problems with changing the temper of the metal around the repair. Small holes, up to maybe quarter sized when cleaned up, can be done, but you have to be careful not to heat up too much of the metal and not cool it with water at all. Just let the welds air cool and do maybe an inch of weld at a time. I've heard it called stitch welding if that helps. Then you go back over and fill in the gaps once the first welds are cooled. Not as clean as a straight weld would be, but you're less likely to break any welds this way.

518 Posts
Only way i would box the frame is if i had a full road cage fully tied in to the body.On the diesel around me i can almost buy 2 6bt for the price of a 4bt brings lately.On the 7.3 i have heard you can use a international version if you change the oil pan on it.
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