I have recently bought a Bantam in need of total restration. It's D reg (September 1966 according to a DVLA vehicle check). The frame number (on the engine mounting lug) is D10 followed by three digits and the engine number is GD7 followed by 5 digits. Is it possible that late D7 frames had a D10 frame prefix or does this suggest that the frame has been replaced at some point? The last change of owner was 35 years ago. I've tried to look through the datbases but I can't work out if the D7 engine is correct for the 1966 reg or the frame number could possibly be correct for 1966 or both. Any help or guidance would be greatly appreciated. I am happy to provide the complete frame and engine numbers if needed.
The D10 frame would have been despatched from the BSA factory with a D10 engine, the number reading the same as the frame. The GD7 engine would have been despatched from the BSA factory with a frame bearing the same number as the engine. The GD7 series was the first Bantam, other than Post Office models, where the frame and engine numbers read the same.
The GD7 series ran from late 1965 until July 1966 when the D10 was in introduced. So you find both D7 and D10 models with "D" suffix plates.
Some of the bigger London dealers were still selling the D7 in 1967.
Apply for a BSAOC dating certificate to see where you bike was snet by BSA.
https://www.bsaownersclub.co.uk/machine-cert
Okay thanks for that. So the D suffix plate on my bike is most likely the correct one for the frame (D10) and the GD7 engine isn't the original but is from a slightly earlier GD7 machine (late '65 to July '66). Just out of interest, could there have been a cross over period in July 1966 when they changed from the GD7 to the D10 models and they may have fitted the last GD7 engines into D10 frames or is that something you've never heard of?
Your info is very interesting and helpful. I will definitely apply for a dating certificate.
No, nothing like that in the despatch records - the D10 had improved performance and better electrics than the D7, who, back in 1966, would want to buy a new D10 with a D7 engine given that?
Yes, I never thought of it that way. It was just that the GD7 engine in my bike is probably only a few months older than the frame. I wasn't aware that the D10 had better performance than the D7 although I did know about the electrics. Do you know how they improved the engine? Higher compression?
Hi
The D10 bikes have a different generator with a much smaller flywheel than the D7, this allows the engines to rev more giving an increase in top speed (but at some cost to flexibility).
I believe the flat topped piston of the D7 was replaced with a slightly domed arrangement which, together with a noticeable squish band on the head extracted more power.
Regards
Thanks for that. I didn't realise the two engines were so different. That obviously means the cranks and crankcases are not interchangeable between them because of the alternator mountings and ign points location etc.
As you say the cranks are not interchangeable ... BUT ... my bike has D14 crankcases with a D7 crank! This necessitates using the older style outer cases to fit the larger rotor but gives a flexible four speed motor ... I like it anyway!