Hello. I'm using a 1971 inner timing cover on my 1967 C25 engine. The cover came with a crude 3/8" UNC bolt to secure the non return valve ball and spring into the case. The earlier cases used a flush fitting grub screw.
I'm not convinced that the fitted bolt is correct as at 1/2" in length it is quite a bit longer than the equivalent earlier grub screw. The Draganfly website shows this bolt as being part number 21-2044 with a description of 3/8"UNC x 3/8", but also states that they supply a different bolt which has to be shortened. Can anyone confirm that 3/8" is the correct length?
The grub screw fitted to the slightly earlier B25's (21-1898) was 3/8" UNC x 1/4". However, the OIF B25 switched to a 3/8" x 3/8" UNC bolt (21-2044) and there is a comment on the BSA Unit Singles web site that this had to be used with a sealing washer although this is not shown in the parts book. The two screws are not necessarily interchangeable because they used different springs and may have used a different method of sealing the NRV tunnel.
Thanks Spaceman, that's really helpful. The spring that came out of the later timing cover is about 4mm shorter than the spring in the original cover and the new spring that I have received. I have reassembled both valves back into their respective covers. Using a probe to push the NRV ball off of its seat shows that the spring pressure is greater in the older cover than in the newer cover even though the later cover has as 1/2" long bolt instead of the intended 3/8"bolt. I certainly wouldn't shorten the bolt.
I think I'll reuse the shorter spring with a new ball and the 3/8" x 1/2" bolt and a copper washer.
Thanks again for your help.
I'm sure the spring tension is not that critical because the only purpose of the NRV is to stop oil draining from the oil tank when the engine is not running. However, I'm curious as to why BSA changed the sealing plug design for the OIF engines although it was probably on cost saving grounds. Given the later bolt thread was 1/8" longer than the earlier grub screw, I can well understand why the later spring was 1/8" or 3-4 mm shorter. If you use a slightly longer thread, this would be expected to increase the NRV opening oil pressure but, if this is similar to the original (1967) NRV, it should be ok.