Progressive spring but keep air assist

[anthony8488]

I bought a 82 cb750sc a few months back and when I bought it the left fork was seeping  that turned into leaking and now the right fork is leaking so its time to replace seals. So most everyone suggest swapping springs to upgrade 30+ years of springs.

So could I put progressive spring in and still use the 9-12psi the factory spring was suppose to have rather then junk the air system? I already have a bike pump so the low pressure is not an issue I'm just not sure if progressive with air plays well or if I need to buy the caps to remove the air system. (With progressive rear also of'course)

I see the older bikes take 5.5oz of fluid is this the same for the 82?

Also when I do searches for cb750sc I notice a lot of cb750k or cb750c queries would these parts be similar to what I need or do I need to find specific cb750sc parts?

[shinyribs]

Alot of parts are the same between the different models,but not all of them of course. If the part in question looks identical to what you have then it's a swap,otherwise the changed things entirely. Like drum brakes vs disc's. Are there any parts in particular you are curious about right now?

Progressive springs are nice. I put a set in my '76 and it made a very,very noticeable difference. Definitely worth the money IMO. As far as the air assist: I don't see why not. When you order springs for your particular model the manufacturer may or may not have special instructions/settings regarding the air assist. Otherwise,I'd say just play around with the air pressure and see what feels good to you after you do the swap.

No need to swap to non-air caps if you decide not to run air in the system. Just make sure them little caps over the valves are sealing well ( good gaskets in them) and rock on.

Progressive springs will have a tighter wound coil than some stock springs which effectively displace a certain amount of fork oil. Chances are you might need to run about 1/2 oz less oil than the FSM (Factory Service Manual) calls for to prevent possibly blowing out a fork seal on severe compression.

If you havent downloaded the FSM for your bike off the homepage here I'd highly recommend it. It's a very quick download. Probably 1-2 minutes.

Good luck!

[anthony8488]

Thanks for all the advice, it makes sense that Honda would use the same parts across models just wanted to make sure.

Only parts for the moment are parts for rebuilding the forks for now and then also the rear struts and springs.
Looking at the front again today I noticed the rubber grommet on the brake calipers were wearing so will need to replace those too while I have it all apart.

I did download the manual and have it on my iPad so I can reference it while I work.  

[anthony8488]

So i finally got around to doing this project, some notes as far as my bike went.

The fork caps do have a good amount of pressure on them (air was bled out) but nothing bad, i loosened them and then with a rag over the top i twisted them off by hand, the pop off feeling was similar to opening like Pillsbury rolls when you peel the label back.

My bike per the service manual required around 12 oz per fork (they vary between each forks, 12 was a number in range on both).

When removing the lower section of the fork there is a bearing that is pretty tight so i had to use the lower like a slide hammer to separate them. Having a seal driver/substitute is definitely ideal for reassembling, i used the two old seals to drive the bearing back down then pulled the old ones off and installed the new seal.

The progressive springs came with an paper reference guide that stated what size to cut the spring spacers to, looked like there were a few different options for different years but in my case for my 82 CB750SC it was 1.5" spacers. With that there was no tension on the fork caps when i re-installed them. ***There is a note also with the springs that stated progressive springs do not require air to function correctly, but if you were to add air to do so in small (1-2psi) increments as the pressure would change handling. In case your sticker is gone or you swapped bars factory springs with fluid and air was not to exceed approx 12psi.

The rest was pretty cut and dry and the advice from all the SOHC fork threads was enough to get the job done. Also I used a piece of 2x4 (so it wasn't metal on metal) and a car jack to lift the front off the ground, while the bike was on the center stand. I bought the rears shocks with progressive springs also, the rubber grommet on top was the only "hard" part, then two bolts and done.

[sgtslag]

I'm a little surprised there was no tension on the fork caps with the spacers in place -- normally there is some pressure.  I would suggest you contact Progressive to ask about that:  the pressure keeps everything in place; it also prevents too much play in the front end, as you do not want there to be any free-fall before the springs are engaged.  I upgraded two different bikes with Progressive Fork Springs:  '79 CB750K, and a '93 Voyager XII heavy touring bike.  Both had tension in the fork caps with the new Springs in place, as well as the OEM Springs.

On my Voyager, I mistakenly put the metal, OEM spacers back in place, along with some metal screen, oil filters -- suspension was extremely tight, and dangerous to ride.  Later learned they were supposed to be removed, and after I did, everything has been exceptional.  I would be concerned that you are in the other ditch, with no tension on the caps.  Best to be sure, before you ride too much -- an unstable, or too much slack, in the front forks, could be quite dangerous.

Be sure to keep the paper received with the Progressive Springs, as you will likely need to reference it when you change the fluid in a year, or two.  By the way, Dexron ATF fluid clocks in at ~8 Wght. oil for the forks.  If you want to dial it in, you can experiment with heavier/lighter Fork Oils, but it will take time, hassle, and money.  The Dexron ATF is a good, middle of the road viscosity.  The Dexron VI is a full synthetic, it will last twice as long as the III, which is conventional.  My M/C Technician textbook says to change the Fork Oil annually, but that may be overkill.  Be sure to change it more than every 20 years, though!  Cheers!

[anthony8488]

I just shot an email over to the folks at progressive suspensions to double check the spacer req, I thought it was off too but read the attachment a few times and then online they have the same note to see line 18 which says to cut supplied spacer to length. I haven't rode it more then up and down the street in the neighbor hood yet as I'm sorting out stalling/throttle issues now. I will update this post with what I find. I used Lucas brand ATF this go around but now that I know how easy it is to drain and refill yearly changes will definitely happen.