350 trans crossmember drop?

[hillshooter]

Hello All,

I have a Danforth IFS in my 51 GMC 150 Longbed and was wondering if anyone knows a ball park of a past build on what the transmission crossmember drop would be? I have a SBC355 and a 350 trans for it. Thanks

[dsraven]

once you have the truck at ride height and leveled side to side with the correct rake angle you have built into the front end install, and the engine/trans placed in there with the engine mounts and using a floor jack or blocking under the tailshaft, you should try to get the engine at about a 3 deg downslope at the rear. just place a cheap angle finder on the oil pan rail where it bolts to the engine block. then the rear end should have a 3 deg upslope to match that when all is said and done. this will decrease the driveline vibration that can happen with mismatched angles.most engines/transmissions are made to have this downslope and the oil pick up tubes, internal oil drain back engineering and fluid levels are built for that as well as carb float bowls etc if applicable. once you have that angle set and the appropriate blocking under the trans you can install the rear trans mount and build the crossmember from there. try to build the crossmember so it is bolted to the web of the frame instead of the flange if you will only have one attachment location on each side. this will make it stronger. think of the original crossmember's shape and go from there. most don't just attach to the frame flange.
going by someone else's drop can set the angles to the wrong degreee due to a difference in engine mounting types etc and so setting the engine higher or lower in the frame when compared to your set up.this is an important thing to keep track of due to the driveline universal joints working the same angles. there is a a good video on youtube just google Drive shaft velocity and pick the youtube video. it shows what happens when things are not right. the same thing goes for the u joints in the steering column. a ujoint is also only really meant to work in a 7 degree variance otherwise wear happens faster and the vibration or driveline harmonics can become unbearable at high speeds which can also cause the driveline to come apart and spit the driveshaft out the side of the vehicle, or worse. I have personally seen a highway truck interaxle driveshaft (between the rear tandem axles) drop the front ujoint which allowed the front of the driveshaft to hit the pavement. it sheared of the pinion shaft of the rear tandem axle like it was butter.that shaft is waaaay bigger than any 1/2 ton pinion. just saying.
pirate 4x4 has a write up on driveline angles as well as a bunch of other good to know stuff. they have a great write up on the ls engine family if you that interests anybody.

[hillshooter]

Originally Posted by dsraven

once you have the truck at ride height and leveled side to side with the correct rake angle you have built into the front end install, and the engine/trans placed in there with the engine mounts and using a floor jack or blocking under the tailshaft, you should try to get the engine at about a 3 deg downslope at the rear. just place a cheap angle finder on the oil pan rail where it bolts to the engine block. then the rear end should have a 3 deg upslope to match that when all is said and done. this will decrease the driveline vibration that can happen with mismatched angles.most engines/transmissions are made to have this downslope and the oil pick up tubes, internal oil drain back engineering and fluid levels are built for that as well as carb float bowls etc if applicable. once you have that angle set and the appropriate blocking under the trans you can install the rear trans mount and build the crossmember from there. try to build the crossmember so it is bolted to the web of the frame instead of the flange if you will only have one attachment location on each side. this will make it stronger. think of the original crossmember's shape and go from there. most don't just attach to the frame flange.
going by someone else's drop can set the angles to the wrong degreee due to a difference in engine mounting types etc and so setting the engine higher or lower in the frame when compared to your set up.this is an important thing to keep track of due to the driveline universal joints working the same angles. there is a a good video on youtube just google Drive shaft velocity and pick the youtube video. it shows what happens when things are not right. the same thing goes for the u joints in the steering column. a ujoint is also only really meant to work in a 7 degree variance otherwise wear happens faster and the vibration or driveline harmonics can become unbearable at high speeds which can also cause the driveline to come apart and spit the driveshaft out the side of the vehicle, or worse. I have personally seen a highway truck interaxle driveshaft (between the rear tandem axles) drop the front ujoint which allowed the front of the driveshaft to hit the pavement. it sheared of the pinion shaft of the rear tandem axle like it was butter.that shaft is waaaay bigger than any 1/2 ton pinion. just saying.
pirate 4x4 has a write up on driveline angles as well as a bunch of other good to know stuff. they have a great write up on the ls engine family if you that interests anybody.

Thanks for the info. Just wasnt sure if someone else has a similar build they have done with the same set up. Looks like another bridge ill cross on down the road.

[dsraven]

ya, I hear you. my truck has been in the garage for 7 years now. work on it between my 2 jobs so not much work getting done with the honey do list added in.
get the frame checked for sag and square.
get it leveled and set at ride height.
get the front end in and assembled so you know where everything will be located.
get the engine and trans in, on the engine mounts out front and place a support block under the trans, out of the way of the cross member area, so you have the 3 deg down at the rear of the engine. use shims under the trans to achieve the proper angle. tapered shims work well, one going in on each side to keep them level. cheap at the hardware store. a selection of washers also works well.
fab up a cross member. you may have to add a shim under the trans mount with the weight of the trans actually on the mount, no biggie. or factor some sag into the cross member when you build it. some square tube makes a good cross member because it can be cut square and easy to mark/cut angles. DOM tubing is also good if you have a bender for round tubing. a muffler shop may be able to fix you up if you want to bend some DOM tubing (DOM-drawn over mandrel, not welded tubing). you could also modify the original cross member if it is close to fitting.
post some pics of what you have to work with. does the front end come with mounts?
when I did my engine mounts I used Camaro engine mounts that bolt to the LS engine block. then I made some tabs that weld to the frame, both sides, then made some connector plates to go from the engine mounts on the motor over to the frame tabs. that way it is easier to get the steering linkage past the mounts because you just make the connector plates fit around the shafting. it can all be unbolted later if I change my mind on power train or want the engine higher or set back some etc. If I remember I will snap a few pics for you after work.