B25 Valves

[2LZ]

Hey folks.....Been all over the web. I have an early 1967 BSA B25 Starfire, meaning that the valve eccentric adjuster flat spots point "9 and 3" instead of "3 and 9". My bike is a "before C25 2050", before they reversed the eccentrics and changed the tappet cover to the new one or whatever the fine folks at BSA decided to do.

I consulted my Rupert Unit Single, Vol 1, and my Haynes, and I'm at a loss of how my eccentrics ended up lining up, after hours of trying to get them within the "grey areas". The intake simply would not have much joy in that area. I did the "spin it in and back it out 180'," method, etc....

I know over the last 56 years, others have laid their hands on my baby, so it's part of the game. Long story short, I needed a measurement instead of following Mr. Ratio's advise directly. I set them tighter than the recommended factory settings at .004 and .005. I've read this should suffice.

The intake however would not cooperate to have the flat spot inside the 9:00 range. It ended up being this....and it's perfect. The push rods spin freely.
Question: Can a later eccentric (backwards?) be fitted in an early rocker box?

[Spaceman]

First off - the early (-2050) and later cylinder heads and rocker spindles are not interchangeable. The two rocker spindles you have should be identical - if you thought you might have a later one fitted, I am sure you would see that by comparing them - as said, the eccentric direction on one would be reversed.

In principle, if you start with the inlet rocker spindle flat at 9 o'clock then you should get the full range of tappet adjustment by rotating it between 6 o'clock and 12 o'clock. It would be interesting to know how much the inlet clearance varies between these 2 limits. Rotating the spindle through 180 deg should not change the level of clearance adjustment and it is therefore difficult to see why the inlet flat ends up where it is?

[2LZ]

:
First off - the early (-2050) and later cylinder heads and rocker spindles are not interchangeable. The two rocker spindles you have should be identical - if you thought you might have a later one fitted, I am sure you would see that by comparing them - as said, the eccentric direction on one would be reversed.

In principle, if you start with the inlet rocker spindle flat at 9 o'clock then you should get the full range of tappet adjustment by rotating it between 6 o'clock and 12 o'clock. It would be interesting to know how much the inlet clearance varies between these 2 limits. Rotating the spindle through 180 deg should not change the level of clearance adjustment and it is therefore difficult to see why the inlet flat ends up where it is?

Spaceman, thank you for the response. Please forgive my error in the earlier post. Not meaning 180' (late night math typo), but I spun the spindle clockwise to the stop, then backed it out 360', then set the inlet spindle to the 9:00 position. I did the same with the exhaust spindle (tightened to the stop, the 360' back-out, then set at 3:00) and the exhaust spindle set to .005 right within it's grey range in the manual, no problems.

Anything below the 10:00 position on the inlet spindle was stiff and pushing the valve. Where the inlet spindle is set in the pic (about 1:30) is where it's exactly .004 and the push rod spins freely. That's why I was wondering if someone may have put a later spindle in the rocker box on the inlet side. I'm going to set it per Rupert Ratio's method and see where it ends up. Mind you, I did the above adjustment at TDC compression stroke, not RR's method. I'll check back after I reset again.

[Spaceman]

Your exhaust adjustment looks to be ok but the behaviour of your inlet spindle looks to be a bit odd!

What would help to clarify what is going on is to work out the direction of each spindle lobe (the direction of maximum eccentricity). For the later spindles (2050+), I believe the lobe direction is the same as the direction of each flat - if your spindles are the same, then in your pic, the inlet lobe will be at 1:30 and the exhaust lobe will be at 3 o'clock. By looking at how each rocker moves as you turn the spindle, you should be able to work out where each lobe is. Essentially, if you turn each spindle until the rocker is at its highest position this will mean the lobe is pointing vertically upwards. I suspect the corresponding flat will also be at 12 o'clock?

The way the B25 cylinder head is designed is that you have to keep within the appropriate (grey) adjustment range. If you don't, the extra sideways displacement of the rocker might cause problems with the engine and a greater amount of wear in the valve gear.

[2LZ]

Thank you, Spaceman. Here is what I (we) worked out today. Being no Greenhorn to mechanics and motorcycles, that does not mean I am beyond being an occasional bonehead. So......I invited my 74 year old, master machinist neighbor over to read over the Haynes, and of course Rupert Ratio Vol. 1. Then we tackled the bike. He has a full understanding of the difference between the early 1967 rocker box and the ones built after January 1968 regarding proper clocking of the spindles

We performed the valve adjustment with both the "TDC" method, and also Mr. Ratio's method. Bottom line is, as you've stated the exhaust came right in to .005 in the proper grey area. The inlet, on the other hand, refused to cooperate on setting it with either technique. The grey area between 6:00 and 12:00 (9:00 being the mid point), kept the rocker binding. Not on the valve stem, but the rocker was misaligned enough for the underside of the rocker arm to push down the edge of the spring keeper to open the valve. Even with the valve opening, I could still slip a feeler between the rocker end and valve stem. We also unscrewed the spindle 360', as Mr. Ratio suggested and got the same result.

After resetting the spindle to the original position, once we past 12:00 into the 3:00 side of the rotation (Approx 2:00 as the pic shows), we were able to perfectly set the inlet at .004. This is why I think either someone replaced the inlet spindle with a post January 1968 version....or something has been machined enough (valve seat or rocker face) to swing the adjustment that far around the rotation. I guess the only way to really tell is to pull the rocker box and look......

[Spaceman]

If I understand you correctly, the pad on the inlet rocker arm is somehow pressing down on the valve collar rather than on the end of the valve stem? What is therefore happening when you rotate the flat past 12 o'clock, I assume, is that it it is moving the rocker far enough forward to stop the contact with the collar?

I'm struggling a little to picture exactly what is happening. With the inlet rocker flat at 9 o'clock, is the bottom of the rocker pad not in the middle of the valve stem? Or is it displaced far enough to be basically just be contacting the edge of the valve stem? If so, it would be interesting to know whether this is the forward edge or the back edge?

I don't really know what the difference is between the earlier rocker spindle and the later one. The lobe on your inlet spindle appears to be aligned with the flat which I believe is the same for the later spindle. All I can think of is that the operating arm on the later rocker may be slightly longer than the earlier one. If the pad on your inlet rocker is contacting the back edge of the valve stem (as seen from the front of the bike), it could well be that you have the wrong rocker fitted?

[2LZ]

Here's a (very lousy) hand drawing of what's going on. Exaggerated but you get the idea. It's almost like I have two exhaust spindles the way the inlet spindle is cammed. Hopefully you can open the pdf.

[Spaceman]

Yikes - I can now see why you are having a problem! I will have a look at the inlet side of one of my (OIF) B25's tomorrow to see how it compares. It's not obvious what's going on!

[Spaceman]

I would have thought the geometry would have been essentially the same (except for being mirrored) for both the inlet and exhaust rockers. If the inlet side looks like the top drawings with its flat at 9 o'clock, how does the exhaust side compare with its flat at 3 o'clock - in other words, what is different?

The only thing I can think of that might cause this problem for the inlet, is if its spring collar is closer to the top of the valve stem than for the exhaust valve or the inlet valve seat has been machined in such a way that the valve stem is now higher in the cylinder head than the exhaust.

If I can get to it tomorrow, I will take a look at the geometry on my OIF B25 to see what the inlet and exhaust rocker geometry looks like and, in particular, how much clearance there is between the body of each rocker and its spring collar.

[2LZ]

Much appreciate Spaceman. The exhaust rocker/valve relationship acts exactly as it should at 3:00 and set right at .005".

I noticed on the nut side of the exhaust spindle it looks as though someone cut a screwdriver slit in it with a Dremel, but the inlet spindle does not have that slit. Was it changed out at one time in the past 57 years???....and if it was replaced, was the correct (pre-2050) spindle used? The inlet spindle acts completely natural in the 3:00 position, like its post-2050. I wonder if these are marked with any part#'s? Taking it apart may be the only real way to tell.

[Spaceman]

Just stepping back slightly, you originally indicated that your engine number was earlier than C25-2050. This means you would have one of the first <2000 x C25 engines produced?

Having just read the BSA service sheet describing the revised cylinder head design with reversed spindles, a test of whether you have the earlier C25 version is whether the rib above the push rods is present. If you insert your finger through the push rod inspection hole, for an earlier rocker box, you should feel a traverse rib projecting down from the roof formed by the rocker cover above the push rods. If it is a modified rocker box, the roof will be smooth (ish).

The other change to the earlier rocker box assembly was to remove the spring thrust washer on the LHS. If you have had the rocker box apart, it would be good to confirm whether the spring washer was there or not. If you have a later rocker box fitted then clearly all you will need to do is to reverse the direction of the spindle flats. If you really do have a very early engine then it might be necessary to take off the rocker box and examine in more detail what you have from the numbers cast on to the various components.

[Spaceman]

Some further thoughts:

I'd assumed that the geometry for the inlet and exhaust rocker mechanisms would have been essentially symmetrical although mirrored. However, the reason given in the Service Sheet for changing the C25 rocker box assembly was to improve the symmetry. Given that the change was made after only 1950 x C25 engines, this would seem to indicate there was quite a significant problem with the earlier design that had to be put right part way through the 1967 model season. 

It's possible the rocker box of the earlier engines was changed under warranty and that's what you now have. It may be that the problem was the one you are experiencing with interference from the valve collar with the rocker body and the solution was to move both rocker spindles closer together. To facilitate this, it may well have been necessary to remove the pushrod tunnel roof rib and, perhaps, to also change the length of each rocker arm.

At this stage, it would be good to find that the pushrod tunnel roof rib has been removed on your engine and you have a modified rocker box assembly.

[JulianS]

This is the parts bulletin.

[2LZ]

Thanks for the info guys. I'm not sure all that changed in the early rocker boxes other than cutting out the casting bridge and changing the spindles(?), or how they clock. If the location of the spindles and the rockers were also changed, I haven't been able to find any info on that.

I just checked and my rocker box has not been modified regarding having the bridge cut out (see pic). Also, mine if definitely pre-2050, and the numbers match. (frame has a B). I've also included a pic of the nut side of the spindles, so you can see the exhaust spindle has a slot for a screwdriver, and the inlet does not.

[Spaceman]

I was hoping there would be a simple solution to your problem

What we don't know is what problem the early C25 suffered from that caused BSA to change the design of the rocker box assembly. I assume they had to solve this problem by moving the rocker spindles further away from the valve stems by reversing their lobe direction and I assume, because the rib got in the way, they had to machine it off. As part of the re-design, they also got rid of the spring thrust washers which, presumably, were also involved in the C25 problem. Although the rocker design was changed, this may simply have been to increase their width to account for the missing thrust washer.

In order to try to understand what the original problem was, I'll have a close look at my OIF B25 tomorrow and see, in particular, what happens when I rotate the spindles through 180 deg. If their is a problem with valve collar clearance, it should show up on my B25. I'll also measure the position of the ends of the rocker spindles to compare with yours although I doubt that they would have changed.