The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Radlan2 on 29.12. 2019 20:06
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Hi Folks
In response to joolstacho "how does it go" question on the intro page, first to anyone interested the hard bit is the barrel, this is how I did it.
I wanted to use a small fin A10 barrel for this conversion, reducing the height of the plunger barrel leaves it too thin at the center head bolt. I overcame this by enlarging the hole and tapping it 1/4" BSP, then made a solid screwed cast iron plug to fit, long enough to make contact with the barrel casting lower down, over 1"long.
After suitably chamfering both and making sure when screwed in the new plug was being "forced upwards" the barrel was preheated and the plug was welded in by a mate. After welding I thought it would be easy to skim off the final .005" from the top face, however the welding process resulted in a hard spot at the weld which meant the top face had to be ground. Then using the head as a drill jig, spot through and tap for 3/8 BSF head bolt into the new plug.
The old magazine article about this leaves people to think it could be done in a week end, it took me a couple of months to get the barrel done.
Best Regs
Chris P
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Reckon I was looking the other way and missed something.
To recap, what crank, rods and pistons are you using on this? Sorry to appear non plussed, but a brief confirmation would make things a bit clearer. Looks fascinating.
Swarfy.
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Hi Swarfy
Apologies for not making things more clear. I've had some time on my hands over the Christmas break, joined this wonderful forum, got carried away and kind of blurted out too much info.
The cowboy bike in this post is a project I finished this year. The crank is A10 large journal, the rods are A65 (approx 1/2" shorter than A10) the barrel was shortened 1/2" to suit the rods thus removing the top fin, flat top pistons are fitted.
The bore/stroke remains at 70X84 so swept volume is still 650cc. The trouble with finishing a project is its not a project anymore so:-
A couple of days ago I posted about through bolting the head/ barrel and muscrat kindly replied with some info. This is for my next project, basically a big bore A7, using A7 crank and rods, A10 pistons with a thick flange "Daytona" barrel previously bored to 71mm
This configuration gives an almost square bore/stroke 71X72.6=560cc. Why am I doing this stuff? Either I need therapy or in its self it is a kind of therapy, I go nuts if I dont have something interesting to do.
Best Regs
Chris P
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Thanks Chris, I get the picture, so assume the piston skirts ( if using standard parts) required shortening to clear the crank, so how about balance? Lucky chance, rough as junkyard dog or meticulous attention?
Your other Big Bore A7 presents a problem to me with the slightly weaker small end rod eye and a bigger A10 piston pin. It will be interesting to see how you swerve round this conundrum. I am all for this off the wall suck it and see, nice to push the boundary. Well done.
I'd take the simple way, A7 crank, small journal A10 rods and pistons, plus an A10 barrel (good use for one with busted top fins) trimmed down to suit.
Cheers,
Swarfy.
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attached some photos of my thick flange barrel with the mod. Has been extremely reliable over the last five years and happily pulls into the mid 7's when running on a 4 speed box.
currently running steel capped rods, but probably dues for some steel rods.
With a six speed I keep it to 6500 and its never been through bolted. I'v seen evidence of thin flange a10 motors being run with +100 10:1 and surviving for quite a bit of abuse before the flanges went. If you're sticking with thin flange and going to 14:1 you'd need to through bolt. A lot of the thin flange failures were purely poor maintenance -not keeping all the base studs tight.
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Hi Rocket Racer
Thanks for the interesting pics, looking particularly at the center head bolt tapped hole do I understand correctly that the thick flange barrel casting is beefed up at the top as well as the bottom? When the top fin is removed that there is enough material left for the head bolt, which would make this conversion much easier to not having to weld in a plug.
Mine has loads of midrange though I feel it does not rev out to what I maybe expect (though I am used to the Trident) but I do not have a rev counter at the moment so don't really know. Things that may be acting as a rev limiter 376 cam? more of a midrange cam? 27mm carbs on the Shooting star head may also be a bit restrictive for a 650?
Hi Swarfy
Yes spot on I had to M/C the side skirts to the dia of the flywheels, the front/rear thrust face remains in full. It is I think a really smooth engine but it has been dynamically balanced. I have been told the A7 is the smoother twin but this short rod is far better than either of my (unbalanced) Shooting Stars.
There is a problem that it is a complete pig to start despite fitting a Trispark E.I. yet once going ticks over like a watch. A Triple expert I know assures me this will be cured by fitting a bigger pilot jet so the concentrics are off to be converted for the new screw in type.
Best wishes for the New year and thinking of the people in Aus
Chris P
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Hi Rocket Racer
Thanks for the interesting pics, looking particularly at the center head bolt tapped hole do I understand correctly that the thick flange barrel casting is beefed up at the top as well as the bottom? When the top fin is removed that there is enough material left for the head bolt, which would make this conversion much easier to not having to weld in a plug.
Mine has loads of midrange though I feel it does not rev out to what I maybe expect (though I am used to the Trident) but I do not have a rev counter at the moment so don't really know. Things that may be acting as a rev limiter 376 cam? more of a midrange cam? 27mm carbs on the Shooting star head may also be a bit restrictive for a 650?
Chris P
Chris,
I recall there was a little welding involved which I recall ties up with the how to article from 1959. My engineer (who also did my timing side bearing conversion and built my replica BB32R frame!) knocked it out fairly promptly and also fly cut my pistons to clear the flywheels.
I doubt the pair of 27's are restrictive and the 356 is an excellent cam and stronger up to 6k and still pull through comfortably to more. I suspect your airfilters are too restrictive, I had a similar issue on an atlas race sidecar (filters being useful for trying to keep the kitty litter out, aka the gravel traps) and a dyno session confirmed the airfilters were putting the brakes on for eveything over 5k.
No its not a triple, but it has plenty of midrange and the life of these engines above 6500 starts to be measured in minutes and above 7k in seconds. On a road engine you shouldnt be expecting to be pulling the revs you pull on the track on a wing and a prayer.
Something to be conscious of with short rods is to time the bike by degrees not by distance btdc . the rod length alters the dwell at tdc.
When I was running a 4 speed I could keep it between 4500 and 7000 in top on the straights, anymore had me really sweating she would stay together while I kept the throttle pinned and watching the final climb from 7 to 7500 was fairly intimidating on a long straight. Now with a six speed I can gear more conservatively and keep it to 6500 although she is rarely below 6.
My head came off a shooting star but had been previously radiused. These heads wont take the big valves necessarily either but they will still boogie