Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board BSA A70

Very interesting. I have a few more Qs.

Primary chain, single or double row?

Clutch? Stock or fancy?

Con Rod length ?

Outrigger plate for mainshaft behind clutch.?

Thanks for the extra pics, it all looks teek aleek.

Instead of adding a second plug on my triple I welded and reshaped the crown to push the charge over to the plug. It also replaces a squish band around the edge.

Interested in what the finished crank weighted after balancing? Please and thank you

I have dreamed about that adjustable cam gear for many years! I would be first in line for a couple if you made them. Cheers, PRT

+1

It looks too well made for me to want to try and copy it. Be interested in investing in one or two though.

The 68-473 (lightning/spitfire) cam works really well when retarded a few degrees, however with using the broached method as mentioned earlier you need to spend a lot of time removing the timing case to make adjustments and check timing when you do, I made one adjustment and the bike went like a rocket but had a huge flat spot at certain revs, I should have advanced it a few degrees but it took too much messing about and I wanted to use the bike....

With the verniner gear you can have a window cut out of the inner timing cover with a screw on disc over it for easy access. - have the timing changed in 30 minutes or less (if you have a cut away outer cover also)

Hi All,
Back in the late 90's built a couple of GSXR Suzuki's that had slotted cam sprockets, they just have two 6mm (?) bolts holding the sprockets
They did not give any trouble
A lot of modern auto engines (European) do not have keyed cam pulleys, they rely on the clamping force of the pulley bolt to keep them in place

John

In my H***** twin race bike engine I built in the 90s the professional engine builder who was advising told me to slot the cam sprocket for adjustment. That never gave any issue with 2 x 6mm bolts to hold it.

Ray

Hello, Will try to answer.

Primary chain is double row.
Clutch is Barnett with additional plates, Springs are Barnett.
Rods are stock A65 length
Did not install outrigger bearing.

It seems the JE Pistons as supplied by E&V have crowns similar to those shown on the triple. Didn't give it much thought when I put them in but the charge concentration concept makes sense.

Misplaced the document with finished crank weight. Will poke around for it.

Had some discussion with E&V regarding a production run for some indexable timing gears. Have a design in the works. The access port is a terrific idea. Gear would need to be fabricated that has all of the socket head screws and the slot/hole for eccentric tool on the same center to minimize the size of the access hole.

I made one years ago, just turned up 2 cam gears and used the centre of one in the outer of the other.
Couple of 6mm allen screws locked it ok.

last time we talked, greg blagus at ro dy has a number of billet A10 crank blanks that he can machine for you to make a stroker A65. he did the crank for my 650 triumph, and reground it for me after i threw a rod at the races. easy to work with, and does excellent work.

im ignorant with respect to BSA cams. what megacycle cam are you using in this?

and on the adjustable cam wheel, the holes appear to be slotted under the fixing allen bolts. so the thing at the bottom is a removeable eccentric tool to put into an adjuster hole to be able to rotate the outer cam wheel very finely on the inner hub?

Snag with the standard cam is the centre angle could do with being closed up a bit.
If you retard the splatfire cam more than a few degrees the exhaust side is too late,
101-102 degs on the inlet is good but anymore pushes the exhaust out too far and
you loose more than you gain. That's why slightly larger inlet valves work well with
the standard setup. But then, that's only my opinion.

Nice crank, a few bob's worth i would think.
You may get away without the outrigger on dirt but my own view is they are a must,
i screwed too many boxes and cases without one.

Outrigger clutch plate is probably a worthwhile investment, Someone was asking about cam. Attached is screenshot from a megacycle catalog. I used Cam #542-X1.

The 'X12' would have been my choice, looking at those numbers.

My data shows stock cam at IN 98.5 centre Ex 110.5
But there are so many ways of measuring cam setups, i'm sure they are correct eh?

I had the SRM race cam, which from memory timed up with the same LCA as the spitfire cam but with .400" lift and more duration. It worked really well and gave the bike a lot of power. Same setup but with the X12 made for a more sedate bike compared to the SRM race cam. Still got more poke than a stock bike but nothing like what it was.

I think when you start working with cams with the power band aimed at higher revs you need to start thinking about what gearbox your using, so a CR or 5 speed will be your friend.

Thats the cam I am using in my A50 LSR bike. Hoping it works out.

Be interested to see how you find it. Are you using the recommended 3/4" radius followers?

I've known the spitfire cam works well in a twin carb A50, so it might go well.

The X12 in my Lighting responds best to certain exhausts, it works really well with my short 2-1 with dunstall mega, looses some power with the standard exhaust setup.

The SRM cam I only ever ran with the standard exhaust setup, it gave some really high revs.

The OP is building a flat track motor. Maximum power is not necessarily the best idea…..especially with a 360 degree parallel twin.

Finding usable torque/HP for that critical drive off the corner is vastly more important. That can be elusive with the BSA twin. The firing order isn’t your friend. Even with today’s tires. Get it wrong, and you simply cook the edge off your nice, new dirt track tire.

There is a lot more to it than the engine power, but get the engine wrong and there is nothing you can do with tires, Shocks, swing arm angle, etc. to get it to hook up.

Oddly, according to a lot of the veterans racers of that era, a near stock BSA twin made a pretty respectable flat track bike. The tires of the era would work with the HP/torque available and get traction for that critical drive off the corner.

Not only tractability, but an absolute requirement for doing well in a race is finishing the race. Especially if you will have a racing series with points totaled up at the end.

Lannis

Interesting. I have an X12 cam in a box waiting to go into an A50. Would like to see more of the A50 LSR build. E&V Now has forged JE A50 pistons. Below is a photo.



Here are a couple more photos of an A70 crank from RO-DY. Greg has been making cranks haute couture for a long time.

It would be interesting to see how they cut the relief in the central web. It appears the depth of the relief is almost the same as the space between the end web and the flywheel. That does not leave much room for tool holder width.

Thats useful to know, I have some original BSA and some Omega pistons for my A50 Daytona build but not been too keen on using either... I like Eds JE pistons and fit JE pistons in all my bikes.. It might push me to getting the A50 started (or at least as my next motor build)

SRM 'Race-Cam' @0.020in

Overlap 90°
Intake Duration 303°
Intake Lobe Centre 106.5° ATDC
Exhaust Duration 306°
Exhaust Lobe Centre 108° BTDC
Lobe Separation 107.3° (Cam°)

Lift at cam 0.372 over base
Rocker ratio 1-1.125

They changed their cam over the years. I bought mine second hand from a side car racer as he didn’t get on with it and preferred a different one. I think I have the card somewhere which Gary sent me.

This was 2014 so my memory may be a bit vague. But I did time the cam up at the time and was certain I got about 98.5 inlake LCA and about 110 exhaust. I’ll see what I can find.

Yes, the original SRM cam i used was setup at 102 degs on the inlet.
That was 25+ years ago. They have changed them.
The Tighe cam i like is different again, but aren't they all? Newman's
race cam looks quite hairy.