BMW K bikes (Bricks)

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51Back to top Go down   New K75 project - Page 2 Empty Re: New K75 project on Thu Dec 24, 2020 10:36 am

kenrams


Silver member
Silver member
As always, thanks for the great info guys. I’ll let you know what I find inside the head. I used a cheap endoscope and it looks like the valve is seated OK but has a little bit “missing”. I forgot to mention that when I took the cover off a small brass colored 10mm bolt fell out! Yep, it was one of the camshaft bolts. So who knows other joys I will encounter but I will completely strip the engine as I really know nothing about the history of this engine and also winter is just starting here🥶.

@roach374 The bike does sit a bit lower at the front and you can see that from the angle of the horizontal lines on the engine cases. This, in my opinion, really helps transform the K75 from a hefty tourer to a racy looking cafe (plus a ton of other work😥) As you might be able to see from the pictures, I have also mounted the clip ons above the trip tree which means the bike sits even lower. Mounting the bars here gives me a bit more turn lock clearance to the tank and it feels ergonomically correct for me when I sit on the bike.
I’ve not ridden this one yet, but I have modified the stance on previous bikes and I have always liked the way they feel. However, remember I am not a professional.

You also have to decide where to mount the Retrorides suspension as that will also affect the stance, as you suggest. I have not seen any exact measurement information for placement of the bracket so I just eyeballed it by tack welding the mount before final welding, to get the swing arm angle correct (in my opinion). Note also that the bracket has to be mounted maybe 3/4 inch towards the center line of the bike to make sure the  arm and suspension unit tracks parallel to the center line. This is part of the reason that you need to put the triangular bracket near the frame mount.

The real proof will come with testing of course, but that is a ways off. Rainy day here so I’m going back to work in the garage to get the head off and see what is happening to the valve train.

    

52Back to top Go down   New K75 project - Page 2 Empty Re: New K75 project on Fri Dec 25, 2020 4:10 pm

roach374

roach374
active member
active member
@kenrams wrote:You also have to decide where to mount the Retrorides suspension as that will also affect the stance, as you suggest. I have not seen any exact measurement information for placement of the bracket so I just eyeballed it by tack welding the mount before final welding, to get the swing arm angle correct (in my opinion). Note also that the bracket has to be mounted maybe 3/4 inch towards the center line of the bike to make sure the  arm and suspension unit tracks parallel to the center line. This is part of the reason that you need to put the triangular bracket near the frame mount.

Here's the resource I'm looking at for the mounting point: 
New K75 project - Page 2 Bmw-k710

That 109mm between the mounting point and the (I think?) rearsets mount is supposed to be the "sweet spot" (according to RR). Your mileage may vary.

    

53Back to top Go down   New K75 project - Page 2 Empty Re: New K75 project on Fri Dec 25, 2020 4:41 pm

kenrams

kenrams
Silver member
Silver member
The Mount point affects what angle the swing arm settles with respect to the flat road surface and the picture shows this as maybe an acute angle of roughly 10 degrees. If you, for example place the mount higher up the frame this angle will increase - ask me how I know🥴. I think this angle should be the same as the factory bike. Also, I found it really tricky getting this just right as I work by myself. You need to completely build the suspension and connect the mounting bracket to it first. Then offer it up to the frame to get the correct angle and mark the weld point. Remove the bracket, clamp it to the frame as accurately as you can and lay down a decent tack weld. Test fit the suspension again and if you are happy weld it up properly. I probably took 3 tries at it as I was working alone. Took me a few hours but this part is super crucial, so I suggest getting a friend to help and it will go smoother. 
One thing I should mention is that you are adding a very thick ( maybe 5/16” ) bracket and that will take a lot of heat to get good penetration. That very heat will almost certainly cause distortion of the frame at the Mount point if you have the frame removed from the engine. So, if you can, leave the frame attached to the engine for as long as you can for the weld. 
One other thing to note is that the geometry of the K75 is slightly different to the K100 pictured. So maybe all the measurements are not the same. On my bike I have the crank cover lines tilting forward slightly as I like what it does to the stance.


__________________________________________________
1981 R100 cafe- 121,000 miles
1985 K100 cafe - 55,000 miles
1987 K75C - 44,000 miles
    

54Back to top Go down   New K75 project - Page 2 Empty Re: New K75 project on Fri Dec 25, 2020 4:53 pm

roach374

roach374
active member
active member
@kenrams wrote:Took me a few hours but this part is super crucial, so I suggest getting a friend to help and it will go smoother. 

What would you have done differently, with a friend?

@kenrams wrote:One thing I should mention is that you are adding a very thick ( maybe 5/16” ) bracket and that will take a lot of heat to get good penetration. That very heat will almost certainly cause distortion of the frame at the Mount point if you have the frame removed from the engine. So, if you can, leave the frame attached to the engine for as long as you can for the weld.

You do need to take the frame of the engine at some point though, to get at the other side of that triangle / bracket, right? (Just checking...)

Thanks for all this info, super interested in following along with your build!

    

55Back to top Go down   New K75 project - Page 2 Empty Re: New K75 project on Fri Dec 25, 2020 5:06 pm

kenrams

kenrams
Silver member
Silver member
An extra pair of hands can hold the rear end up to the correct angle while you are getting happy with placing the bracket and marking. I tacked it in the wrong place a couple of times and I think if someone was helping I would have nailed it first time. 

Yep, you will need to remove the frame to get at the rear of the bracket, but do as much welding as you can with it connected. Your going to have to lay down a number of beads as you want this mount solidly welded, so that will mean more heat to be dispersed. Just take your time as this is a crucial weld and needs to be super strong all over.


__________________________________________________
1981 R100 cafe- 121,000 miles
1985 K100 cafe - 55,000 miles
1987 K75C - 44,000 miles
    

56Back to top Go down   New K75 project - Page 2 Empty Re: New K75 project on Tue Dec 29, 2020 5:49 pm

kenrams

kenrams
Silver member
Silver member
OK, once the sprockets came off it was plain sailing. I have the head off now and happily have no chipped valve with associated damage. looks to me like it is just the valve seat that needs lapping. As you can see from the photo below, I used a trick I learned from the might Interweb. I shone a bright torch into the exhaust port and can clearly see the issue of light escaping from the inlet valve seat on #3. Exhaust #1 & #3 are also poor.. I'll lap all the valve seats until I see no light leaks and put the engine back together. 
New K75 project - Page 2 Img-2311


__________________________________________________
1981 R100 cafe- 121,000 miles
1985 K100 cafe - 55,000 miles
1987 K75C - 44,000 miles
    

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