K75 86 vs 85 parts compatability

I have recently purchased an 86 K75 converted to an "S" model. It has a Works rear shock and Progressive front springs, as well as the "S" fairing and lower engine fairing. I only have one complaint, and it is about the very poor rear drum brake.

I decided to purchase the required parts and convert it to a disk brake. I bought an 85 K100 final drive, wheel, drive shaft, rotor, caliper, master cylinder and associated brake parts.

After reading on this thread that the 85 is completely different, I am concerned that the parts I bought wont work, though they look the same. I can't compare directly for several weeks because the bike and conversion parts are 2000 miles apart at the moment since I am on a cross country trip with My K75(S).

Any thoughts or information would be greatly appreciated.

Jim

I've owned 9 Ks, built 3 back from wrecks. The 85 is different in body panels, even ones that look same mount different. I haven't seen any runtrain problems. I put an '88 K75 rear drive on an '85 K100, it's a boltup. The K100 splines were BAD, its on my K75 spare parts bike just to move it around. Granting you've got the tools, I think you'll find the change straightforward. The only "gotcha" I see is control changeover. From the drum pull rod, to disk master cylinder is another set of parts. Good Luck, you'll like the difference.

CustomSarge said:

I've owned 9 Ks, built 3 back from wrecks. The 85 is different in body panels, even ones that look same mount different. I haven't seen any runtrain problems. I put an '88 K75 rear drive on an '85 K100, it's a boltup. The K100 splines were BAD, its on my K75 spare parts bike just to move it around. Granting you've got the tools, I think you'll find the change straightforward. The only "gotcha" I see is control changeover. From the drum pull rod, to disk master cylinder is another set of parts. Good Luck, you'll like the difference.

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Thanks for the info Sarge, It's good to know the parts should be interchangable. Do you know if an RT rearend is geared significantly differently? I'm a little concerned about having too much low end, and not enough top end. I like to cruise at 80 or so.

I have all of the necessary controls and parts, including the brake pedal, linkage, and hoses. The splines are like new on the replacement shaft. I have a decent set of tools, but is there anything special I will need for the swap?

Thanks again for the information.

Jim

Smooth sailing

Just a good set of metric allen wrenches & other basic wrenches, nothing special. As for ratios, I look them up in my Clymer and/or Haynes manuals. I buy 1 of each whenever I get a model I don't cover already. Either is good, but getting both perspectives act like crosshairs. I seem to remember that the pinion & ring tooth counts are embossed next to the breather hole at the top of the drive housing. Good hunting.

Re: Smooth sailing

CustomSarge said:

Just a good set of metric allen wrenches & other basic wrenches, nothing special. As for ratios, I look them up in my Clymer and/or Haynes manuals. I buy 1 of each whenever I get a model I don't cover already. Either is good, but getting both perspectives act like crosshairs. I seem to remember that the pinion & ring tooth counts are embossed next to the breather hole at the top of the drive housing. Good hunting.

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Great news! I have a decent set of quality tools.

I also have a BMW parts CD that lists every part ever made for any model of car or Bike. Just like they use at the dealers. Mine says that the two have a slightly different ratio, but by the look of it the slight boost to low end power shouldn't detract from top end ability since I don't usuallyride in excess of 100 MPH. Guess I should have looked it up before! LOL

Thanks again,

Jim

Green with envy....

That CD is my equivalent of the Holy Grail. Oft referred to, but never seen & certainly not possessable! If there's a legal way to get one, I'd appreciate the path info.
P.S. A friendly reminder to anti-seize the Hell out of the splines while it's apart.

What is your e-mail address? I'll send you the contact information on how to get it.

Jim

JimVonBaden said:

I have recently purchased an 86 K75 converted to an "S" model. It has a Works rear shock and Progressive front springs, as well as the "S" fairing and lower engine fairing. I only have one complaint, and it is about the very poor rear drum brake.

I decided to purchase the required parts and convert it to a disk brake. I bought an 85 K100 final drive, wheel, drive shaft, rotor, caliper, master cylinder and associated brake parts.

After reading on this thread that the 85 is completely different, I am concerned that the parts I bought wont work, though they look the same. I can't compare directly for several weeks because the bike and conversion parts are 2000 miles apart at the moment since I am on a cross country trip with My K75(S).

Any thoughts or information would be greatly appreciated.

Jim

Click to expand...

I see some other people suggested it would be a bolt-up..

Perhaps not.

The early K bikes ('83-late'85) had 13 teeth on the input shaft and driveshaft splines. The K75 has more and finer splines (I believe 15 teeth - but that's from memory).

If you find a 13 tooth rear-drive - it won't mate up to your current driveshaft (although the later driveshaft will fit just fine).

Update - just re-read your posting.. since you bought both the rear drive and driveshaft - it should bolt up just fine.. but keep in mind - the 13 tooth driveshafts are difficult to impossible to get new now - so if it the splines need rebuilding at some point - you'll have to go to the higher-tooth count driveshaft.

Just one of those got'cha's - I'd suggest not buying any rear-drive that you can't eyeball yourself.

Aside from the difference in the spline count - I'd want to carefully examine any used rear-drive for excessive spline wear. While this can be repaired (as discussed in another message on the forum) - it is an additional cost that you might want to avoid.

I'd suggest looking for an '86 or on rear-drive - then you know the spline count is going to be correct.

BTW - I've never seen the K75 drum brake categorized as "very poor" before. All the tests I've ever seen didn't see a significant difference between the drum and the disk brake.

If your drum brake isn't working as you think it should (if you can't lock it up) - I'd be looking inside it for old hard brake shoes. The friction material does harden up with age. Be lots less $$$ than swapping for a disk (although an "S" with a drum brake doesn't look exactly stock..)

Best,

Hi Don,

Thanks for the info.

You are right, my "new" shaft and rear end have 16 splines. I am not very worried about it since they are in excellent condition. Did I forget to mention that I already purchased them? By the way, aren't you in the BMWBMW club?

Anyhow, I wanted to change the drum to a disc for a couple of reasons. One, the old drum brake doesn't work much at all, and occasionally makes a strange noise. Two, I want to install 3 spoke wheels (have since bought also. Three, I understand that when the rear brake surface (Drum?) wears out you have to replace the wheel. Mine is out of spec. and finally, I want my Psuedo K75(S) to at least look like a K75S. LOL

With all of the previous owner mods, I think it's worth it, and it handles great.

Jim

PS an addendum: Does anyone know what the air pressure range is for a Works shock for a K75?

Am I being a pest yet?

WOW, dodged a spear (maybe)...

I apparently got fortunate with my grafting of an '88 K75 rear drive to the '85 K100. It seemed to have all gone as it should.
The splines meshed as I would have expected. Now, before I turn this bike loose, I'd better check it through.

JimVonBaden said:

Hi Don,

Thanks for the info.

You are right, my "new" shaft and rear end have 16 splines. I am not very worried about it since they are in excellent condition. Did I forget to mention that I already purchased them? By the way, aren't you in the BMWBMW club?

Click to expand...

You didn't forget it - I just didn't read it correctly

BMWBMW - Nope - but you'll see me at their rally in June..

Anyhow, I wanted to change the drum to a disc for a couple of reasons. One, the old drum brake doesn't work much at all, and occasionally makes a strange noise. Two, I want to install 3 spoke wheels (have since bought also. Three, I understand that when the rear brake surface (Drum?) wears out you have to replace the wheel. Mine is out of spec. and finally, I want my Psuedo K75(S) to at least look like a K75S. LOL

With all of the previous owner mods, I think it's worth it, and it handles great.

Jim

PS an addendum: Does anyone know what the air pressure range is for a Works shock for a K75?

Click to expand...

IMPORTANT - do NOT use air to refill a Works (or any other) gas pressurized shock. Use NITROGEN - it's non-flammable. I seem to remember something like 125PSI - but it's been a number of years since I rebuilt a Works shock.. you might look for a shop that does dirty-bike suspension - they'll have the correct regulator and valve to refill the shock.

Am I being a pest yet?

Click to expand...

Nope.

Re: WOW, dodged a spear (maybe)...

CustomSarge said:

I apparently got fortunate with my grafting of an '88 K75 rear drive to the '85 K100. It seemed to have all gone as it should.
The splines meshed as I would have expected. Now, before I turn this bike loose, I'd better check it through.

Click to expand...

Me too since it was an 85 K100RT that I got my doner parts from. Maybe the change took place in late 85 and that is how we dodged the bullet?

Jim

Thanks Don. Very good advice on the shock! I'll find a dirt bike shop and have it filled, afer I find out how much to put in it!

Jim

My recently sold K75 with the factory low seat.

Jim