14 bolt FF vs SF

[66gmcguy]

I'm assuming this has been asked at some point before, but I can't find any info anywhere about it. Anyone know if you have to modify your drive shaft if you switch from a semi floating 14 bolt to a full floating 14 bolt?

[Mathew21]

I found these to threads with some information for you. I hope they help you.

1) https://www.pirate4x4.com/threads/3-...t-swap.474471/






2) Hmmm the second one would not let me link it I will copy and paste it for you.

My 1984 C20 is currently equipped with the 14 bolt semi-float. I'm not happy with the gearing for towing as it has the 305 and 3.41 gears. We've had discussions on other threads about changing the gearing and pretty much decided I'd be best served with 4.56 but a 4.10 would do. I've always wanted to go full floater for the obvious reasons, since I haul a lot with the truck and often take it 100 miles or so for feed hauls.

I've always figured I'd re-gear the existing rear end one of these days and just live with the semi-float, but now it appears I have a bent right rear axle. So basically to turn this rear end into something reasonable for my uses, I'd need to rear gear it $300 if I do it myself, and I'd want to replace both axles, at a cost of about $175 an axle plus shipping. So that totals up to about $650 assuming I don't find any other problems in there and it's still the semi-float.

So I'm taking another more serious look at swapping to the 14B FF, this would have to come out of a wrecking yard. Since I'm in the auto repair business, and there are not many wrecking yards left near me I'd probably order it from LKQ. The good news in this is that I'll be able to order a full floater that has came out of a truck of the same or close year. I've done some research and searches already, but it seems like most threads guys are moving to much newer rear ends or swapping in something they have on hand like van rear end.

So my questions. Truck currently has 14B SF, want to swap to 14B FF.

Will I need a conversion U-joint or is this only needed for 1/2 trucks?
Will I need to shorten or lengthen the driveline?
Will I need the U-bolts for the FF?
Any down sides to switching to FF?


Answer

I swapped a 14b SF in a friends K5 and a 14b FF in my K5 many years ago so not exactly fresh in my mind but from what I recall...

You may need a conversion joint based on what you have and what you buy. I think 14B sf came with two diff sizes; 1350 and the R style, while the 14B came with both a 1350 or 1410.
You should be ok without shortening the driveshaft. I do believe that the FF is slightly longer than the SF but not by enough to bottom out. Neither of the trucks I swapped required a shorter shaft. One originally had a 12B and the other a 10B.
Yes, the axle diameter is larger, so you'll need new U-Bolts.
No downside except for weight. The later ones don't require you to pull the axles to service the brakes if I recall.
The van 14B FF are wider which some people like to match the width of the front axle.
Isn't the Gov-Lock also known as the Gov-Bomb? I have no experience as I used a Detroit in mine.

[hatzie]

The 9.5" 14bolt original equipment u-joint size was either 1350 or GM 3R
The 10.5" 14bolt original equipment u-joint size was either 1350 series or 1410 series

My guess is the 6 bolt 9.5 will have the GM 3R and the 8 bolt will have the 1350.
Likely the higher GVWR 2500 & 1 ton trucks had 1410 Universals. 8600GVWR and heavier rated?

You can get hybrid 1310 to 1410 or 1310-3R universal joints.

If you can get a look at the donor trucks SPID try to avoid the Eaton G80 Gov Loc axles. Not the end of the world as long as you aren't doing burnouts but they have a bad reputation.

If you're willing to move the spring perches and shock mounts you may want to measure the 10.5" GM/AAM 14bolt axles in the 2500HD 6.0L T800 trucks. They had very heavy duty rear disc brakes with good old fashioned two shoe & drum type parking brakes that used the inside of the disc hat as the parking brake drum.
The spring perch positions are not going to match a squarebody frame as is and I'm not positive you can move them out or in to mate em up. You'll need to measure one and do some head scratching. An OEM+ 4 wheel disc brake retrofit would be a nice nice touch compared to the other options out there. If the measurements will let you pull it off.

[66gmcguy]

Originally Posted by hatzie

The 9.5" 14bolt original equipment u-joint size was either 1350 or GM 3R
The 10.5" 14bolt original equipment u-joint size was either 1350 series or 1410 series

My guess is the 6 bolt 9.5 will have the GM 3R and the 8 bolt will have the 1350.
Likely the higher GVWR 2500 & 1 ton trucks had 1410 Universals. 8600GVWR and heavier rated?

You can get hybrid 1310 to 1410 or 1310-3R universal joints.

Ah I see. I figured maybe there was a difference in U joint type. My main concern though is if the lengths of the snout are different between the two. The main reason is because I need a custom driveshaft made when I do my transfer case swap in the next week or so. I would like to swap in a FF 14 bolt at the same time, then take measurements and order the drive shaft, but I can't find one anywhere at the moment. So if I knew for sure that the difference in length between the SF and FF 14 bolt was negligible, I could just swap the transfer case now, order the driveshaft, then find a FF 14 bolt later and swap it in without having to modify the driveshaft again.

Originally Posted by hatzie

If you're willing to move the spring perches and shock mounts you may want to measure the 10.5" GM/AAM 14bolt axles in the 2500HD 6.0L T800 trucks. They had very heavy duty rear disc brakes with good old fashioned two shoe & drum type parking brakes that used the inside of the disc hat as the parking brake drum.
The spring perch positions are not going to match a squarebody frame as is and I'm not positive you can move them out or in to mate em up. You'll need to measure one and do some head scratching. An OEM+ 4 wheel disc brake retrofit would be a nice nice touch compared to the other options out there. If the measurements will let you pull it off.

That would be nice, but I'm trying to keep things simple at the moment. I am looking for a SRW full float 14 bolt with 4.10s from a C20 or K20, so I don't have to modify spring perches or shock mounts.

[SunSoaked]

I swapped my 85 SF (8 bolt) with a 84 (CUCV) FF with no change in pumpkin depth. Drive line length stays the same. That works with my Slip yoke 208 t-case. If you have a fixed output type t-case, the slip-joint style driveshaft will make up for and difference.

[kipps]

Originally Posted by hatzie

....Likely the higher GVWR 2500 & 1 ton trucks had 1410 Universals. 8600GVWR and heavier rated?...

...If you can get a look at the donor trucks SPID try to avoid the Eaton G80 Gov Loc axles. Not the end of the world as long as you aren't doing burnouts but they have a bad reputation...

My 1987 "c6p" v20 with a 8600-gvwr had a 14bsf axle with a 1350 u-joint. A 1975 c20 14bff axle I looked at had the same 1350 u-joint. I did not measure these two axles for relative driveshaft length, though.

For a work-truck, I would intentionally choose the g80 'gov-lock' axle. It's mostly bulletproof in the 14bff axle, and can be useful when used like intended. Don't use that gov-lock for burnouts; find something aftermarket if you're going to be doing that.

The gmt800 14bff axle swap is definitely an option and is frequently done. It's the only good way of getting working parking brakes and disk brakes on a 14bff axle. Some folks will tell you to use Cadillac El-Dorado e-brake calipers with a disk-brake conversion on a squarebody 14bff axle, but those calipers are expensive and almost no-one is satisfied with the performance of the parking brake.

If you do a gmt800 14bff axle swap, look for a 10.5-inch axle out of a 2500hd truck with a 6.0 engine. Avoid the six-lug axle that some light-duty 2500 trucks had, and avoid the bigger 11.5 that was used on the 8.1 and Duramax trucks. The 11.5 is overkill for most of our purposes, adds weight, and lowers the ground clearance.

The gmt800 14bff axle uses a 1410 u-joint. Purchase the driveshaft from the same truck you pull the axle from, have the driveshaft shortened to fit, and use it in the squarebody. The slip yoke on the gmt800 driveshaft will likely be the same exact size as the slip yoke on the 1980's squarebody. The driveshaft I purchased from a 2005 2500hd fit perfectly in a np208 and a np241 transfer case. I just need to get it shortened.

The gmt800 14bff axle has the same exact lug pattern and center hole diameter as the 3/4-ton squarebody axles, up through 2010 or 2011. The spring perches are in the wrong spot on the gmt800 axle. Don't try to move the existing perches. Cut them off, and weld on new ones from offroaddesign or a similar site. Pay close attention to the pinion angle, and be very careful welding on the perches so you don't warp the axle tube.

If you do a four-wheel disk brake conversion, you should swap out the proportioning valve for a disk/disk valve, to increase the braking pressure to the rear. Disk brakes require more line pressure than drum brakes do, so the prop valve upgrade is needed. The quick and dirty method is to swap your existing prop valve underneath the radiator for a 1970's Corvette prop valve. This is available new, and will send equal pressure to the front and rear. The more involved method is to mount a Wilwood adjustable prop valve up beside the master cylinder like GM did on the gmt400 trucks, and run fresh brake lines from that easily-accessible prop valve and all three of the brake hoses.

[hatzie]

A very beautiful thing about the T800 AAM 14bolt rear disc axle is that the G80 option is actually limited slip in those years not a locker.

Officially the lug bolt circle changed from 8 x 6.5" to 8 x 180mm on the 2011 T900 chassis. I think I'd stay away from 2010 wheels and hubs.

Rumor has it that GM went back to drum brakes on the T800 in 2006 or 2007 but I have no direct contact with an AAM 14bolt from those years to verify or debunk said rumor. My 2005 definitely has rear disc brakes but it's also a 2004 build date truck.

I've run a disc/drum MC and combination valve with the stock rear disc setups from the A2 Golf GTI and MK II Scirocco on several 1978-1984 VW Rabbits. Several of us were swapping Rabbits and MK I Sciroccos to rear discs. I was the guinea pig. I was gearing up to change the MC and combination valve but Scott suggested we should take the first brake upgrade to a long safe chunk of unused pavement and test things before changing over the MC and combination valve.
We had Scotts' stock A1 MK II Scirocco to compare things to.
The stock 60:40 bias actually turned out to work very very well. Noticeably shorter braking distances without locking the rear brakes.

It may be worth giving it a try with just the rear disc setup on a lonely stretch of pavement or closed shopping mall parking lot before going to the trouble of swapping over the master and combination valve.