Forum bmwbike.com

My 1954 R51/3 has all the hardware (ball joint welded to the rear of the frame, ball joint front engine mounting bolt, etc.) to accept a sidecar. Does this mean this bike had a sidecar from the factory? or were all frames set up this way, sidecar or no sidecar?
If it did have a sidecar I think I'll need to replace the springs as I intend to ride it solo.

Regards,
Dan

All the bikes had three mounting points build into the frame - two tabs on the upper frame to receive an eye bolt, and one ball on the rear lower frame to receive a claw clamp.

But you mention a front engine ball pin mount. That was a replacement part for the straight front engine mount bolt. If your bike has a ball pin front motor mount, then it must have had a sidecar attached at one time.

If your bike had a sidecar, it probably had or still has sidecar springs, which can be distinquished by measuring them:

Front solo 6.0 mm Ø x 284 mm L 31 41 2 054 007
Rear solo 7.0 mm Ø x 272 mm L 33 51 3 054 284
Front sidecar 6.3 mm Ø x 273 mm L 31 41 2 054 009
Rear sidecar 7.6 mm Ø x 271 mm L 33 51 3 054 286

If your bike had a sidecar, it also may have or had a sidecar final drive, which would severely limit the speed of a solo outfit:

35/9 = 3.889 R51/2, R51/3 solo
32/7 = 4.571 R51/2, R51/3 sidecar

Thanks Allan, I'm in the process of dismantling the bike - I'll measure the springs when I get to them. This bike has a 32/9 final drive from, I believe, an R67. And yes, the front engine bolt is a ball pin mount so I suspect in one of its incarnations the bike did have a sidecar - although probably not with that final drive.

Speaking of that front engine bolt - I have been working on removing the thing for days now. I've tried driving it, pushing it with a puller, heating it, turning it, etc. - but it hasn't budged. Am I missing something? Any suggestions?

First, put a jack under the oil pan to take the weight off the motor mount bolt.
Then squirt a penetrating solvent into both ends of the bolt. WD-40 is commonly used, but is more of a dryer and protectorant than a penetrant. Something like Aerokroil as sold by Bench Mark Works is better.
Then use a punch or small drift to tap on the left side of the motor mount bolt, to start it coming out the right side.
I would not try to grab the ball with pliers and pull on it, because it should to remain smooth and unmarked.

Thanks Allan and I've done as you suggest - probably gone through a half can of Liquid Wrench in the process. I've even gone to the extent of drilling a 1/2" hole in the ball so I could insert a bar and rotate the bolt free of whatever it's binding on - can't even turn it with a LOT of force. I'm replacing the bolt (assuming it comes out one day) with stainless steel so I don't care about ruining it - would just like to get my motor free of its frame. I'll keep plugging away at it, just wanted to make sure I hadn't missed a hidden keeper or something somewhere.

Regards,
Dan

How about this:
1. With a brass hammer, tap the left end of the ball pin in until it is flush with the frame.
2. Take a stock motor mount bolt and use it as a drift to continue tapping the ball pin out the right side.

I ended up turning the bike on its side so the oil would run down the length of the bolt. I knew it was doing some good when I saw rust colored oil come out the other end. Anyway, after repeated hammering (or wailing, definitely no tapping involved), oiling, cursing, waiting, etc. it finally gave way yesterday afternoon. Surprising how rusty and stuck the bolt was given that the rear slid right out. So I now have a bare frame, stand by for reassembly questions