The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Peter Gee on 22.07. 2019 14:02
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I wonder if every A7/A10 rockerbox was made on Friday afternoon? I have an old one that has at least .0003" gap in places, as tested on plate-glass but that has seen wars so I tried a mint NOS one I have in stock.
Well, that was better. It only had some .00015~.0002" gaps". And they say the rockerbox is a leak spot!
Anyone else have these experiences?
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I have not carried out that test mainly because I have not had a problem with oil leaks on the rocker box. My present A7 I have owned for about 4 years now and does not leak oil from the rocker box, prior to that I owned an A10 Gold Flash for 16 years and that also never leaked oil from the rocker box gaskets. It could be down to the fact that I now make all my own paper gaskets, I have noticed in the past that some bought rocker gaskets are not a very good fit.
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It is not suprising that some old rocker boxes have distortion given the twisting forces of the valve gear exerts on it and the abuse many will have suffered over the years.
More concerning for the light alloy heads is that the joint faces between head and box have slightly different shapes. Compare the front joint of the inlet side of the box with the head and you will see that it gets quite narrow where the surfaces dont quite match. The try a commercial gasket and you will find that it does not quite match either head or box.
In an attempt to stem the leak at this joint, SRM, at my request, built up the inner edge of the box and machined the surface flat. This has widened the joint surface and made a better match between box and head and improved the sealing of that joint.
A leaking rocker box, particularly on alloy heads, is nothing new. See the interesting bulletin issued by Hap Alzina in the USA in 1959.
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Anyone know the torque figures for the 5/16ths studs and bolts that hold the rocker box on? I have suggested to myself 18 ft lbs all round.....
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Wow. I'd have expected castings of that era to be at least a couple of thou out - maybe twice that. But a few tenths!!! Those old machines our bkes were made on weren't capable of working to half a thou, never mind less.
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I wonder if every A7/A10 rockerbox was made on Friday afternoon? I have an old one that has at least .0003" gap in places, as tested on plate-glass but that has seen wars so I tried a mint NOS one I have in stock.
Well, that was better. It only had some .00015~.0002" gaps". And they say the rockerbox is a leak spot!
Anyone else have these experiences?
I think you might have the decimal point in the wrong place? That aside, thick gaskets can be a major cause of case distorsion. Try surfacing with a sheet of 240 grit wet n dry on your piece of flat glass then use copper gaskets.
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I wonder how out of parallel/warped/ twisted the surface becomes with relation to the rocker shafts...? This would affect the geometry of the rockers to valves....could best be checked on a flatbed mill machine thingy ?
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G'day Dutch. A mill would definitely be the way to go but 1 and a half to 2 thou will come out ok on a surface plate.
If you had to take off 40 thou upwards the geometry may need looking at.
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Yes I was off by a decimal point...they are ,002~.003. Pardon
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Yes I was off by a decimal point...they are ,002~.003. Pardon
As a late-comer to this topic, I immediately went looking for the correction when I read ".0003" gap".
Richard L.
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I found some great notes on the notorious ( for alloy folks anyway) rocker box probs:
http://eborbikes.com/docs/Fitting_notes/Fitting%20note%20for%20Ebor%20copper%20rocker%20box%20base%20gaskets%20for%20BSA%20A7%20and%20A10%20151202.pdf (http://eborbikes.com/docs/Fitting_notes/Fitting%20note%20for%20Ebor%20copper%20rocker%20box%20base%20gaskets%20for%20BSA%20A7%20and%20A10%20151202.pdf)
I will try with my Payen vintage 1960 gaskets ( how thin and compared to the modern compounds..0.020" vs 0.060" for recent sets I received) and Three Bond 1184 but I may well end up with Ebor's copper gaskets. A few runs will prove or not.
IMO engineers blue and a plate glass thruthing plate are essential tools for anyone rebuilding old Brits.
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I think it's just a diseased design personally. All that repeat pressure from the valve gear, plus a head steady on the outside - how can that be expected not to fail over time/usage?
Got about 3000 miles from mine before it started leaking this time. New gasket and goo and all is good again. But at least it is an excuse to do a job that I really love.
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Very correct butch.
My A7 plunger very rarely leaks as the steady is mounted under the head.
Cheers