Mark the spring just sits under the oil cap. This one here is about 7/8" long and with a conical shape so that the nut under the cap T handle misses it. Ron
email (option): ronpier@talk21.com
There should be a cap on top of the spring.
I have a handful of these base caps that go directly on the filter but most are missing either the spring and the top cap or just the top cap.
Henk
email (option): ahum@quicknet.nl
My mistake Henk! I never use these filters so I got it wrong. It's how it came with my bike years ago. Ron
email (option): ronpier@talk21.com
Yes it is a pity that a quick check on the oil supply is not possible with the oil cap on.
I found this nice gauzefilter in a prewar oil tank with srewcap which might explain reason behind the (in my eyes rather strange) design of this later filter construction.
Regards, Michiel / Holland
If you have the top cap for the spring missing (the one that goes against the oil cap) you can use a engine core plug.Mine was missing from my ZM20 so went to local engineers & looked through there stock and found one the right size just heated it up and soldered spring into recess 
Dave
I stick will Morris's SAE40 in all my bikes. I usually buy it off Mick Ash at the jumbles, for about £15-£16 for 5 lts. He also sells the gell Cyclone batteries cheap. Ron
email (option): ronpier@talk21.com
I think the oil debate is like the tyre debate (and an M20)...It will run and run..
...Horrors TRM Rock oil sounds OK but its about £35 for 5 litres and arguably may be a bit fancy for an M20. I think I may well use it for the sprinter though....For my road bikes I'm sticking with Morris's..It's worked well over a long period and is reasonably priced.. After a few hundred thousand miles on various BSA singles, some of which were highly tuned and all of which have been ridden fairly hard, I've yet to have a big end go or any failure I could tie down to a lubrication problem. So something must be working right....Ian
email (option): ian@wright52.plus.com
I think Henk has just explained why I use TRM, I don't get metal fillings on the magnetic plug on my Dommie or Commando any more..! Maybe it's because you are using the cheapest oil that you're having this amount of wear.
Although I have to agree Ian, I've argued the case for TRM in the past but really I'm talking about the use of it in different engines than a side valve. As I've only just finished the 16H and I'm changing oil after only a few miles, I've been using Rockoil 40w as TRM is to good to throw away after 35 miles etc. I use it in all my twins and even my Harley, as this is what it was made for. It stands for Twin Racing Mix, so it said on a Ducati forum?
This is also what is used instead of caster oils in speedway bikes and a friend of mine uses it as gearbox oil in his Vello. So a versatile oil with a price tag..!
Cheaper than an engine rebuild though
email (option): horror@blueyonder.co.uk
Hi Horror...I was unaware of this oil until you mentioned it in your post. Since then I have contacted Rock Oil to enquire about it's suitability for 'older' style engines. They said there was no reason why it should not be used in that application and it had various advantages over some 'traditional' oil types. I'm waiting for them to mail me the spec. sheets for it to see if I can glean any more info. from those.
I suspect it is a bit more 'exotic' than what would normally be used in an M20 or similar, but as you say, if it's cutting down wear that has to be a good thing. I'm definitely interested in it for the sprinter which will be thrashed to within an inch of its life..So a situation for which this oil seems to have been made.. ...I'm not too sure about the wisdom of using the cheapest oil available either..It's not something I would do myself...
Oil specifications are not the easiest of things to get your head around...I generally have to read articles at least twice before I really get the details. One thing is certain though...not all oils are the same and full synthetics (they are not all truly synthetic) do seem to have some marked advantages over traditional mineral oils. It seems to be a case of identifying suitable ones and then deciding whether the additional cost is warranted. That may depend to a degree on what type of engine the bike has, how much it is ridden and how hard....Ian
email (option): ian@wright52.plus.com
I'm glad you've contacted them Ian, I'd be very interested in what they have to say.
I have mentioned TRM before when we've had these chats about oil, but people will use what they always have done and it's hard to change. I only have because of disasters.
Just found these posts on the subject
http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1144063&cmd=show
http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1153714&cmd=show
email (option): horror@blueyonder.co.uk
It's always interesting to re read older posts to see if my thinking or opinions have changed over time....Ian
email (option): ian@wright52.plus.com