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  • Rebuilding and increasing torque capacity of 71-C

    214 Skier
    • Year: 1985 Brand: Malibu Model / Trim: Skier Cost: $500-$999 Engine Type: 5.7

    I'm hoping this thread will cover off two or maybe three things;

    1: How to rebuild a Velvet Drive 71c

    2: One way to increase the torque capacity of a Velvet Drive 71c

    3: Maybe help those who are having issues with their Velvet Drive diagnose the issue.

    Before I get started, a bit of background. I'm in the process of building a new engine for my old Skier, the torque output of this new engine will be far higher then the original Ford 351 that was in the boat. I'm sure some of you are thinking why doesn't he just buy a 72C and slap it in there? I have a few reasons why I don't want to do that. Size and weight, the 72c is approximately 1" longer and 10 lbs heavier then a 71c the weight isn't a huge deal but its something, the extra length just makes the packaging a bit of a tight fit between the coupler and the packing nut. Secondly the cost of a new 72c tranny either re-built or new is pretty expensive, and I wouldn't really know what I'm getting if it was used. There are some other small differences between the 71c and 72c but the main one is that the 72c has a forward clutch pack with more plates in it.

    There isn't a whole lot of good information online about these trannies as far as modifying them. There are a few companies down in Florida such as BAM marine that sell really stout Velvet Drives that the Off Shore boats use they sell modified Velvet Drives but they don't really sell the parts to do it yourself, only the final product. The typical modification that they do (my simplified interpretation) is increasing the number of clutch plates in the forward clutch and replacing the paper or bronze friction plates with graphite which increases the torque capacity. There's obviously way more to a tranny that can handle 1500 ft lbs, but you get my drift. I found a place called Stern Drive Connection located in Ontario, Canada, which is good for me, that sells some parts.

    I spoke with them over the phone for a while they, were really helpful, and were willing to help me out with parts.

    So, Velvet Drives can have either paper or bronze friction plates from the factory, I don't know for sure, but I assume as the torque ratings don't change both the bronze and paper plates provide the same amount of capacity from the factory. One advantage to the bronze plates vs paper or graphite for that matter, in single engine boats, is that if the tranny starts slipping and it has bronze plates, the heat will cause the plates to warp and while the forward clutch will effectively be locked, at least you will have forward drive and wont get stranded. The graphite plates have the highest co-efficient of friction of any of the plates, but like the paper plates, if they slip and get burnt or worn out you will loose forward drive. Given all this I wanted to continue using bronze plates if I could.

    More to follow.

    So before I decided to do this thread I had already taken the tranny to bits and cleaned it all up, so to kinda back up to the start I just mocked it up so I could take pics of the disassembly process, don't pay a ton of attention to all the bolts and snap rings missing in the pics, I'll do my best to explain it along the way. The only slightly unusual tool I needed along the way was an 1-1/2" socket, my sets only whet up to 36mm and 1-1/4", everything else is just basic tools. I should also say I couldn't be bothered to look up exactly what Velvet Drive calls each part, so I'm just kinda making my own educated names up as I go.

    I found it easy to just stand the tranny up on the drive flange, it seemed pretty stable this way. Drain all the fluid first by removing the return line from the cooler that feeds into the "pan" of the trans.

    The transmission is capable of accepting input in either direction, it all depends on how the pump is installed, so make sure you mark the pump so that it goes back in the right way, I just used a chisel and make two marks.

    Once you remove the four bolts that hold the pump on mark the gears, it's always best to put the gears back in the same spot as they lap themselves over time.

    Once you get the pump off the reverse clutch cylinder/front plate is next to come off, its held on by four 12pt bolts, the studs that hold the trans to the bell housing help too.





    Once you pop the reverse clutch cylinder / front plate off flip it upside down and using air pressure applied to the cylinder pop the piston out, you shouldn't need a ton of pressure, just apply it gently and put your free hand on top to provide some resistance.

    You can now see the reverse clutch frictions and steels, there are several springs which keep the clutch disengaged and three pins that stop the steels from spinning. You can see in the pic some of the springs, there are 12 of them and one of the three locator pins that sit in grooves machined in the case.





    You can see the 12 springs and three pins in place here, there are corresponding dimples and notches in the middle and top steel plates. The two friction plates engage on the forward clutch drum.

    You can see the dimples and notches in the top steel and the notches in the middle steel.

    There is this pesky little thrust washer, or maybe bushing that needs to go on the top of the forward clutch drum, I forgot it when I was initially mocking it up for the pics.

    Once all that reverse stuff is removed just pull the forward clutch assembly out of the tranny, easy peasy lemon squeezy.





    So once that forward clutch assembly is out you can see the sun gears attached to the carrier and a bushing sitting there. Thats all we can do from the front of the tranny, gotta flip it over now and start from the other end.

    The drive flange nut is a 1-1/2" nut, and its freaking tight, so your gonna need an impact to get it off.

    Then its just a question of taking off the six bolts that hold the rear bearing retainer in place and popping that off.

    I then just used my favourite Snap-On plastic dead blow hammer and knocked the out-put shaft out of the bearing. The rear bearing then just popped out of the case.

    The drive assembly is now out of the case.





    Now we just need to finish taking out all the parts still in the case, which isn't really much.

    That piece of nylon in there is a shield of some sorts that keeps the fluid in the bottom of the case separate from the gears, I'm assuming they don't want it getting all frothed up by the spiny bits. It just pops out.

    The tube that runs across the bottom of the trans is the pick-up / return tube. The tube at the front of the pic that has the screen on it goes to the inlet side of the pump. There is a tube inside of that tube that gets threaded in from the rear of the case that is the return from the cooler. These tubes are important, especially if your running heavy ballast and the boat is running nose up, with out them the pump can get starved of oil, the pressure will drop, and your clutches will slip. Not all Velvet Drives come with these tubes.

    To get those tubes out just unscrew the big nut on the back of the case, I think they are using a 45* fitting on this cooler set-up, I'm gonna be changing all the plumbing to standard AN stuff with -8 line. Once the tubes are out thats almost it for the case, just the valve left.

    The first thing to come off to take the valve out is the selector lever, super simple, just one nut, then the lever pops off, it is indexed by a "D" shaped cut out.





    Once you take the nut off, the selector lever should just wiggle off, be careful though as there is a spring loaded ball behind it that engages detents in the selector lever for forward, neutral and reverse.

    The neutral safety switch threads through the back of the the valve retainer plate and interacts with a disc with detents in it that sits on the back of the valve and rotates with it as the selector lever is moved. With the cover removed you can see the disc with the dimples, and then the tooth on the disc that engages the selector valve.





    The selector valve just pops out of its bore with a light tap. Inside the valve is the pressure regulator, as best I can figure the oil goes from the pump to the selector valve, then through the valve to whichever hydraulic circuit is selected, the oil that is not needed to apply the clutch is diverted by the pressure regulator through the fluid cooler and then back into the bottom of the case.

    To disassemble the valve I just used my drill press to put a bit of pressure on the spring retainer plate then used a screw driver to fenangle the snap ring out and it all came apart. The inside just consists of a piston and a spring.

    The case is bare now, there are two pressure test plugs, remove those and your good to start cleaning everything.

    I'll get to the disassembly of the forward clutch in the next few days.




    So now that the forward clutch assembly and input shaft are out of the case we gotta take apart this part. There are two snap rings that hold the front bearing on the shaft and the shaft in the whole assembly. Remove the top one and the shaft can be pressed or knocked out of the drum, then remove the larger one and the bearing can be removed from the forward clutch cylinder.

    Then there is a large snap ring that holds the forward clutch cylinder in the forward clutch drum, this pops out pretty easy.

    If you flip the drum over you can see the bottom steal plate sitting right where the screw driver is pointing. In some trannies there will be a snap ring there, this snap ring is available in different thicknesses and is what is used to adjust the free play in the forward clutch. My tranny does not have this part.





    Once the snap ring on top of the forward clutch cylinder is out you can the push the cylinder out of the drum using a press or a little bit of persuasion. Once it pops out you will see the forward clutch piston and the piston return spring, the conical piece of metal.

    There are three holes on the inside of the forward clutch cylinder, this is where the fluid comes in to apply the clutch, just use two fingers to cover two of the holes and use compressed air just like you did for the reverse clutch to pop the piston out of the cylinder.

    Now if you look into the top of the drum down at the clutch pack you will see another snap ring, it is there to hold the cylinder in place, between this ring and the top one that you already removed. They are slightly different sizes, the bigger one goes on top, the thicknesses are different so It'd be hard to screw up where they went.








    Just pull all the guts out of the drum, you'll have two thick steel plates, one for the top, one for the bottom, the one with the kinda ridge on it goes on the top, while the one with the recessed portion goes on the bottom. The recess allows clearance for the planet carrier.

    Thats it the whole trans is now blown apart, clean everything really well, check the bearings, look for unusual wear on any surfaces. The clutch surfaces should be shinny with no signs of bluing or heat checking, the friction plates should look like mine do. It should be fairly obvious if you have any damage to any of the clutch packs.

    Next I'll be getting into the modifications to add one more friction plate, and installing a larger forward clutch piston for more clamping force.



    I've finally got it all back together, it went back together no problem.

    The seal kit is fairly simple, it comes with gaskets for v-drive and gear reduction transmissions too so there will be some extra bits if your just doing a direct drive.

    So first thing that needs to go in is the oil pickup/return tube, pretty simple, just slide the assembly in, making sure it is engaged in the hole that feeds the pump, a little teflon on the large nut should do her just fine.

    Then the plastic windage tray goes in next, just make sure its seated on the cast in nubs in the case.

    Next I replaced the four cast iron rings on the output shaft, be careful when sliding the rings over the output shaft, they are cast iron and brittle, don't expand them any more then you have to.

    Then place the rear out put shaft bearing in the back of the case with the groove in the outer race facing up.





    Once the rings are on and the bearing is in, use ATF and lube up the output shaft where the rings are, then slide it into the case being careful to be sure that the rings don't get snagged on the case and damaged. Then just put the rear bearing retainer on with a new gasket, the torque on the bolts is 50 ft-lbs. It's easier to pop the new seal in before you put the bearing retainer on.

    Once its in, you can put the output flange on with some grease on the splines. Torque the 1-1/2" nut to 200 ft lbs.





    Because of the machining that I had to do I started on the assembly of the shift valve. It is a pretty simple device, the fluid comes in the from the pump where the screw driver is, then gets routed to the clutch pack requested, with the remaining fluid being diverted through the cooler and then back to the sump. You can see the pressure regulator valve piston in the slot.

    Just lube up the piston, slide it in with the open end facing out, the spring goes in there, then the spring retainer goes in next, I used my drill press to compress the spring and put in the snap ring. Then install the rubber o-ring on the end of the valve, lube it up and tap it into the case.

    The next piece to go on is the neutral safety switch actuator disc, this disk has a tang on it that engages a grove in the valve and make the disc rotate with the valve. Then there are detents in the valve that actuate the safety switch.





    Once the disc in in place, put the new gasket in place and put the neutral safety switch cover on, its easier to put the cover on, then screw the switch in.

    Then install the pressure test port plug, 8mm allen.

    There's a pic of the stock forward clutch pack with five frictions and 4 steels.




    So I measured the stock pack of 4 steels at 0.277" and 5 frictions 0.306", which gives me a came a total clutch thickness of 0.583. Adding the one extra bronze friction gives me a total of 0.375" for the frictions, however with the thinner steels, now 5 of them, measuring 0.243" saving me 0.034", it's not looking too bad.





    Then mock up the forward clutch. Some transmissions will have a snap ring on the bottom, you can get this ring in different thicknesses to adjust the clearance in the clutch. Mine doesn't have this ring. So once its all in, then whole clutch assembled as it would be in the final trans measure the clearance. I was told to look for 0.025", which is slightly tighter then factory, the prop might drag a bit in neutral until everything seats itself. In my case I had to remove 0.010" to get my clearance to 0.025"


    So to get the extra 0.010" clearance I clamped the bottom plate in a lathe using the inside surface and took material off the ridge where the screwdriver is pointing, its hardly any material so won't effect the clearance between the other moving parts.

    Then I started on the cylinder, the original forward piston is 4.565" in diameter, with a 2.244" hole in the middle giving me a total of 12.41 sq inches. The new piston is 4.971" in diameter, same centre hole diameter so I end up with 15.5 sq inches, which is close to 25% more surface area, or 25% more clamping force. I machined the new bore of the cylinder to 4.986 giving me 0.015" clearance and polished it with some sandpaper. I basically just machined 0.25" or so off the bore of the cylinder.

    Then its just a question of putting the seals in, one o-ring for the centre, and an umbrella style seal for the outer, lubing it all up and sliding the new piston into the cylinder. That dark metal ring is where the release spring rides on that disengages the clutch.




    The clutch goes back together just like before. Put the snap ring in, below the cylinder, then the assembled cylinder with spring, the cup of the spring should be facing the piston. Then tap it all together and put the top snap ring in place, hopefully for the last time. Check the clearance one more time to make sure you've got it set up right.




    Now that the forward clutch is complete we can work on the input shaft. Just like on the output shaft you gotta replace the cast iron rings, this shaft only has two. Once their on, lube it up and slide it into the forward clutch, you will have to giggle it about to get all the little teeth on the friction disks to line up with the hub on the shaft, it'll be pretty obvious once its in all the way.

    Then put the bearing back, my just slid into place with a few light taps of a hammer, then the two snap rings that hold the shaft and bearing in place in the forward clutch drum.

    And then finally lube the bronze bushing and drop it in place one the planet carrier.




    Now just put a bit of lube on the gear set and drop the forward clutch assembly into the case, it will bottom out with a nice thunk.

    Next install the 12 little springs into the holes in the case and the three pins that lock the reverse clutch plates to the case.

    Then you can put on the top reverse clutch plate, it will only go on one way, the notch that doesn't interact with a pin goes to the top of the transmission. Once it's seated properly it should be sitting pretty darn close to flush with the top of the case.




    Now we can lube up the black plastic bushing and drop in place on the top of the forward clutch cylinder.

    Then add the seals to the reverse clutch piston, lube it up and gently wiggle it into the reverse clutch cylinder. I then used some ATF to wet the gasket surface to hold the gasket in place and put some ATF on the needle bearing that supports the input shaft and just dropped the reverse clutch cylinder in place on the top of the case.

    The reverse clutch cylinder is just held in place with four 12pt bolts at this point. When the trans is bolted to the bell housing some of those bolts hold the cylinder on too.

    Then we can install the rubber sealing ring for the pump, along with the woodruff key and drive gear for the pump. When your putting the gears back on the input shaft and into the pump housing make sure you put them in the same way they came out, they'll have already worn themselves in this way.




    Then install the input shaft seal on the outside of the pump housing, and lube the pump gear and drop it into the pump housing making sure once again that it is sitting in the pump the way it originally was.

    Now, just flip the pump assembly over, line up the mark that you made when you disassembled the pump, for the rotation direction, and drop the pump housing on. Be careful not to damage the seal on the input shaft as your sliding it over. Four bolts and your done.




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    Dang it this is a great post. Ive only done clutch pack replacements and seals on the few Ive had apart. Thanks for all this info!!  I use Harbor Marine out of Everett WA and they have been great for trans and Vdrive parts. 

    Edited by freeheel4life
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