NH - KH conversion

[harrybarry]

Hi chaps,
I have a '56 Nh. I've had it for years and commute on it regularly.
It's a darlin. reliable, comfortable etc etc.
So then my dad gives up riding and gives me his '57 BSA B31.
yup, peas in a pod in so many ways that i cannot justify keeping both ( at least I cant justify it to the wife), unless.....
.. I whack a KH 500 engine in it and make a whole new beast.
So, my point... would anyone happen to know if
A) it is possible?
B) if it is, is it easy?
C) If it is, how much would/ should i expect to pay for a ally head engine.
D) would everything else (primary drive, gearbox etc) be the same?

Any advice apprieciated hoping I don't have to sell it......
thanks
Harry

[nevhunter]

Are you talking about the single 500 or the twin? KH. The single is easy. You just need the head, barrell amd crank and rod. I have converted a 54 to a 500 single but I used all iron parts. Everything fits. The twin is a wider engine so may present more difficulty. Nev

[brenton.roy]

Hi Harry,
if you go with a KH conversion, you'll need at least both sets of engine plates. I have a feeling the front down tube might be a different length - although I'm not sure of this.
Webmaster John has the reverse - a KH frame, single motor, so he will probably know.
Primaries are the same. You'll need the KH cush drive.
KH parts aren't expensive. An ally motor went quite reasonably on US ebay a few months ago.
Another way to go would be to sell your bike and buy a KH?
regards, Brenton

[nevhunter]

The engine is much wider than the single. I think the primary chaincase inner is different and the gearbox offset too. Alloy KH motors tend to pull the middle stud out of the barrell. (The one between the cylinder sleeves). Next year was iron barrell, alloy head, which may have been to rectify the problem. Nev

[harrybarry]

Thanks for the ibfo guys.
I think you may have a point Brenton, sell mine and buy a kh.
its just finding the right one when i've not long finished undoing the bodges and ironing out the nasties on mine.
that said nor do i have oodles of time to muck about swapping them over.
Was over optermistic in the whole simple swap idea perhaps!
Harry

[pete.collings]

An alternative is to put a Huntmaster 650 motor instead of a KH 500 twin, as in general Huntmaster parts are easier to find, as the engine internals are the same as a BSA A10. You would need (as well as the complete engine) a set of engine plates, 24t engine sprocket and Huntmaster inner primary chaincase. I have done this in the past, admitedly when Huntmaster parts were easier to find, and found a Huntmaster a good reliable bike once set up correctly. BSA had had a few years to sort out the A group twins before the Huntmaster was launched. One downside is the increased weight compared with a single, although I am told the BSA singles are heavier than their Ariel equivalents.

[brenton.roy]

Harry, we're all optimists - or we wouldn't be looking for new, exciting ways to spend our hard earned cash!

[john.mitchell]

Sorry Brenton, taking my life in my hands, I have to disagree with a holder of all three anoraks! The primary cases for singles and twins are certainly not the same. The twin inner case is slightly stepped for later S/A KHs and FHs. (And probably similarly for earlier KHs) I have never been sure whether the swinging arm FH and KH use the same inners. Unfortunately the relevant parts lists cannot be compared in the parts book repository. As an aside I remember using a twin engine shock absorber setup on a rigid Trials bike (an easy way to get a small sprocket choice). However when I smashed the outer case, the spare outer case would not fit. I guess this means that the early twin outer case differs in the size of the shock absorber bulge and parts books seem to suggest the outers were different too.

[harrybarry]

Cant go wrong with a bit of carefree optomism! in my case, that is, usually until just after it's all in bits...
The B31 certainly feels heavier, indeed given the choice the ariel wins hands down in almost every way... however the beesa was my old dads and i dont think he'd be too chuffed if I sold it on!
I did consider a 650. but i have a dommi 600 already and thought the fieldmaster would fill the hole in the shed rather better.
All in , a shame really. I love the Ariel just how it is....
still, nice to have the machines to give the quandry in the first place of course!
happy days....

[brenton.roy]

Well John, it looks like I'll have to check my facts a little more before feigning wisdom!
You will certainly be correct and as you know, the label is tied to posts - not a lifetime of experience..
Nev was trying, very politely also - to set me straight.
I have both sets of cases in another shed and will go and have a look.
By the way - are there only three? - Should I stop now?