The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: BagONails on 29.01. 2025 07:16
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Hey Fellas, I find myself at a loss today. The rebuild saga continues...
I ordered new push rods from Draganfly which duly turned up very smartly and I assembled things as far as initial setting of tappet clearances to then notice exhaust rockers sitting too close to the rocker box (0.010" clear, covered in a previous post)
Stripped down again and measured the new push rods carefully with my big vernier caliper. I found the inlets OK but the exhaust rods were undersize, referred back to Drags description which showed 8-17/32" Inlet and 9-17/32" Exhaust.
I got back to Drags and they confirmed after a quick check at their end and refunded my money with apologies.
I would have been happy to wait until they got new stock in but they couldn't tell me when that would be so I accepted the refund and duly ordered another set from an Australian source which have now turned up (parts made in England) and of course first thing I did was measure them only to find Inlets at 8-17/32" OK and Exhausts 9-14/32" *ex*
These exhaust push-rods are even shorter than the Draganfly parts... 3/32" is .094" or 2.38mm which is significant when magnified by the rocker ratio.
I guess I'd like to get an idea if anyone has experienced this problem or even start a survey to confirm what the length really should be? I don't know what BSA's original design intent was but my bike seems to need the longer size i.e. in line with Draganfly's quoted length of 9-17/32"
My originals measure 8-1/2" and 9-1/2" but they are all slightly bent so I can't be sure they weren't slightly longer before. I'm going to have to take this up now with the Aussie supplier but it would be very useful if we could do a bit of comparison on here to get a consensus of what size these darn things should actually be. If anyone has any loose A10 push rods on their bench I would be very interested to know what length they are *????*
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Hello to down under *smile*
You can make your own.
https://www.a7a10.net/forum/index.php?topic=9604.0
Klaus
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Hi Ian,
I sent original BSA and SRM lengths to your previous posting
John
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Hello Ian,
the three Sets from Downstairs are :
1. In 8.071
Ex 8.976
2. In 8.08
Ex 9.98
3. In 8.31
Ex 9.488
Cheers
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Hi O'Nails
I have originals at 8 1/2" and 9 1/2" and after market at 8 15/32 and 9 13/32. The important thing though, which originally alerted you to this is the rocker geometry, and that is also affected by follower length and valve recession. Klause's article points the way, I made up adjustable pushrods to achieve the ideal length suitable for the tappet to be in a straight line with the valve stem at a little over half lift. A rod tapped to fit the tappet will help indicate this, the reason to go past half lift is that is where the spring pressure increases.
I then averaged the lengths so they could be interchanged, I was also going to try fitting elephant foot tappets but that required shortening the valve stems and I did not do that in the end.
So plenty to consider
Chris
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i am certain i had a problem with rocker geometry on betsy beeza to do with rod lengths but i can't find it, i remember it being the exhaust tappet as well that was troubling me i even took piccies of it and posted on here.
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Just measured mine at 8 1/2 and 9 17/32, these are original pushrods . The motor is similar to yours, RR with a new 357 cam and followers plus the 1106 twin carb head , no issues with clearances. Hope this helps.
Don
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Thanks for your responses guys, I'm clearly not on my own in this particular minefield which is comforting...
I'm still uncertain about what the original sizes for A10 Push Rods were but it looks most likely to be:
Inlet 8-1/2" / Ex 9-17/32" at least for the Alloy head models and I'm pretty sure these would work for me if I can find a set. Well I have the inlet pair so it will be a case of going back to Draganfly to see when they are likely to get new stock of 9-17/32" parts in or follow in Klaus's footsteps and make my own...
Klaus there is a Germen fella here, Ralf, who is an ex Formula One guy and has run a VW air-cooled engine specialist shop for many years in Adelaide. I might contact him to see what he's got laying around. I'm thinking do a dry build and make adjustable P/rods up so I can tune the optimum length which I think is to allow the tappet adjuster to be in line with the valve stem just past the half open point on the valve as referenced above by Radlan2 and from Musky in other threads I've since found on here.
To try and pull things together on this I have listed below some of the threads which I've found that contain useful info in relation to this question. Not that any of them are definitive as there seems to be a divergence of opinion and quite a range of varying lengths being found out there:
Threads found :
Peter in Aus - Engine Push Rod Lengths
https://www.a7a10.net/forum/index.php?topic=16194.msg138337#msg138337
Klaus - Engine Improvements
https://www.a7a10.net/forum/index.php?topic=9604.msg69737#msg69737
Scotty – A10 Push Rods
https://www.a7a10.net/forum/index.php?topic=14066.msg116304#msg116304
Bergs - Cylinder Head - thread hijack!
https://www.a7a10.net/forum/index.php?topic=5047.msg34347#msg34347
Keep it coming though guys the more measurements we get the better I reckon!
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From this thread…
https://www.a7a10.net/forum/index.php?topic=13931.0
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Goodness me! Why couldn't find that post straight off? I grabbed some neatly labelled bits only to find the gremlins had been in the box and mixed up the rods and labels.
Just sorted them out, given them a good talking to and a dose of WD.
Swarfy.
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BON you are a star you found it and i couldn't , thick me *beer* *wave*