valve adjustment: torque question

[pdx_r100s]

I'm adjusting the valves for the first time on this new-to-me 78 R100s. I've read several posts on the topic and watched a few videos.

Here are a few questions:

1) my "OT" mark does not look like any of the ones I've seen in videos or pictures. I move past the F mark, which is clearly visible through the hole, but rather than seeing "OT" through the opening in the same place the "F" mark is, I only see a "T" written on the side of the gear next to the teeth (i.e., it isn't on the same surface as the "F" mark, it is 90 degrees to the side, which means I have to look more from the rear of the bike in order to see it). It doesn't say "OT," only "T". Is this still the correct mark? When I have it in place (there is a small hole punched next to the T on the surface that is visible when looking directly into the hole) there is a little wiggle at the valves. They both appear too tight.

bmw t mark.jpg (1.09 MiB) Viewed 2216 times

2) there is some play in the rocker. I've read about adjusting this. It sounds like I need to slightly loosen the nuts, tighten the blocks together, and then retorque to 25 ft lbs in a cross pattern. However, different accounts seem to say that I can just retorque the four nuts holding the rockers, or that I have to retorque all six nuts holding the head on. I did not remove the spark plugs and wasn't going to if it isn't necessary. I will need to remove the plugs if I am to retorque the two other head nuts. Is this necessary if I'm retorquing the 4 that hold the rocker blocks?

3) Is the correct valve clearance for this bike .008 ex / .006 in, or .008 ex / .004 in? I have read both values in various places.

thanks.

[Kurt in S.A.]

My thoughts:

1) You can find OT for yourself. Be sure you are looking at the end of the compression stroke. You will need to remove the spark plugs. Put a plastic straw or the like in the hole and watch the straw move out, stop moving for a bit, then back in as you rotate the engine. In the middle of those spots is OT on the compression stroke. See if that matches up with your "T".

2) See 1)...I think you'll end up removing the spark plugs anyway. When you retorque, do it in stages...don't go to 25 all at once.

3) My preference is 0.15mm intake an 0.20mm exhaust (or 0.006" or 0.008"). It is a "metric" bike afterall!

Kurt in S.A.

[pdx_r100s]

thanks, Kurt. I did end up taking out the plugs, and that method helped to verify that T = OT on my bike. I used the 0.15mm/0.2mm spec. Hopefully it is all set now.

Just out of curiosity, how much do folks back off the nuts when re-torquing?

[Kurt in S.A.]

The torque is a sliding torque. It's not really necessary to fully loosen and probably 1/4 turn towards loose and then a steady pull to torque should be good. I find there's no real need to retorque the head bolts every time that I check the valves. I "cheat" a bit by using the torque wrench and just see if it takes load without moving. I figure that if it doesn't move at all, then it can't be that far off. YMMV

Kurt in S.A.

[Rob Frankham]

2) there is some play in the rocker. I've read about adjusting this. It sounds like I need to slightly loosen the nuts, tighten the blocks together, and then retorque to 25 ft lbs in a cross pattern. However, different accounts seem to say that I can just retorque the four nuts holding the rockers, or that I have to retorque all six nuts holding the head on. I did not remove the spark plugs and wasn't going to if it isn't necessary. I will need to remove the plugs if I am to retorque the two other head nuts. Is this necessary if I'm retorquing the 4 that hold the rocker blocks?

thanks.

THere are at least three different versions of rocker gear used on the 247 series engine. In later models (IIRC from '78 but I could be wrong) the rocker blocks can't be moved together by any sufficient degree because they are solidly located in the head. The final design uses shims to adjust the axial play and the same shims can be used for the penultimate type although this isn't recomended by BMW. It's not unusual to find the that the later types of valve gear have been used to replace earlier types.

Finally, if you are able to adjust the axial play on the earliest types of gear, there is a special tool which sets holds the setting while tightening the studs. It can be done without but be careful... any stiffness in the gear may cause the valves to float at speed with disasterous consequences.

Rob

[melville]

You probably have a lightened flywheel and the original OT mark was machined away. It was very kind of the machinist to remark the flywheel. They don't always do that.

[gspd]

Why bother with the OT mark when adjusting valves?
When the valves or on the overlap (rocking) on one side, adjust the other side.
As much as 90 degrees off from OT in either direction will still be on the heel of the cam (valves fully closed) on the side you are adjusting.

[pdx_r100s]

Rob—
the long Snowbum valve discussion says that the gear wasn’t used until 85, and that the design of the 78 rocker still allows some adjustment:

it is possible on /5 and /6 machines, especially up to 1975 (and slightly on /7 and into 1984), to adjust the rocker blocks by squeezing them towards each other for near zero end-play (just oil film movement being possible from fingers pressure up and down on the rocker arms).

https://bmwmotorcycletech.info/setvalves.htm

I found this to be true on my bike. I loosened the rocker arm nuts, squeezed, and retightened, and it took up the small amount play.

[gspd]

Even the late model shimmed type rocker blocks allow for a small amount of vertical play adjustment.

[Rob Frankham]

Rob—
the long Snowbum valve discussion says that the gear wasn’t used until 85, and that the design of the 78 rocker still allows some adjustment:

Both of my '81 machines have 'located' pillar blocks which only allow a minimal adjustment other than with shims. The Haynes manual shows a '80 engine with 'located' pillar blocks. Difficult to tell when they first came in because the 1985 version was supplied as a replacement for all later models once it had been produced. I think it was part of the '78 engine upgrade.

The change in '85 was to a slightly different system which allows a certain amount of play and uses a plastic washer (Presumably mainly to lessen the valve train noise).

Both of these setups can only really be adjusted with shims.

Rob