'47 XA7 280 Early engine failures

[Brandis]

 I know there are not many of the '47 models left with numbers below 600. I'm told it was because there was an engine design issue that caused serious failure. I picked up XA7 280, matching frame, from an old warehouse in Connecticut.  It looked complete and seemed like an easy restoration.  It didn't take long to realize there was no crankshaft, exhaust cam followers, (photo attached), valves and one exhaust valve guide missing. 
It was then that I heard about the mysterious issue that resulted in major failure in the first 600 units.  Before I try to rebuild, I'd like to know what the issue was.  I have to assume it was a crankshaft problem.  Anything else and they wouldn't have split the crankcase.  Oiling perhaps. 
I'm sure the first owner put things back together with thoughts of a later rebuild, but back then there was almost no BSA support in this country.
With a model this early, I'd like to do it right, because it's the grand daddy of all the vertical twins that followed.  Also, no clutch.  Transmission, of course, is perfect.

[Greybeard]

I look forward to hearing more about your bike and the history of the engines. Please will you wander over to Introductions and tell us a little about yourself and how you came to get the early A7
 

I read somewhere that the first A7 was designed by Mr 'A' and when the design was reviewed by Mr 'B' he redesigned it to have the layout that we all recognise. I'm sure someone will paint the full picture for us.

Update. A Wikipedia article about the bike.
https://en.m.wikipedia.org/wiki/BSA_A7

[JulianS]

This 1946 magazine article gives some insight into the early A7 including the crankshaft.

[JulianS]

These pages from D W Munros book BSA Twin Motor Cycles gives some further insight.

The parts books show that the first crankshaft design was replaced after engine XA7 600.

[groily]

I've never seen one of these A7 cranks, so know the square root of diddly squat about them specifically.
But I did build a not-dissimilar Triumph 5T engine for a friend some years ago - built-up crank and white-metalled rods. That wasn't at all a bad thing to play with (after I'd got over my surprise, having had no idea what I was letting myself in for!)
After having the crankpins ground back to 'round', the rods redone, and replacing all the fasteners with new and correct grade, the engine was totally reliable for the next few years and as far as I know still is. It's now been sold on .
So as long as the oil system is up to it (to your point - but probably as good as any piston-pump early '50s Trihard?), I'm thinking there may not be an inherent design-induced reason for such a crank to fail? But, where to get one is obviously another question entirely. No idea if the one-piece job can be used, with or without mods. Maybe the parts books have something to say on that, or maybe there are useful dimensions in some of the literature, haven't looked, but you're in the right place because someone on here will know.
I had the machining work and white-metalling done (at some expense as it's a rare-ish procedure these days) by T&L Engineering in the UK, my favourite people, and they didn't let me down  . . . well worth the freight costs.

[Brandis]

Thanks.  The A7s had a gear oil pump which looks to be pretty tight.  It was not changed after #600 so I assume the engineers thought it was ok.  I found a correct (?) crank on Ebay.uk and it is on it's way.  The rough measurement by the seller assumes  it's an 82mm long stroke part.  Time will tell.  One of the reasons BSA went to a 2 piece con rod was that the crankshaft, once assembled in the factory was almost impossible to disassemble, even by the factory techs.  I assume that once I confirm a correct crank, I'll measure the journals and the cylinder bore and try to turn the problem over to Draganfly.
The weird, and quickly discarded exhaust valve cam followers are another item.  I have the guides coming but they must fit into something that is never shown in old workshop manuals.  The parts book is also on the way and it might better describe.

[RDfella]

Don't understand why the original crank should have proven to be difficult to dismantle; after all, the singles were similarly mounted (on a taper). In my view the original crank was a superior design. I'm inclined to believe the replacement crank was cheaper to manufacture.
Disappointing to note the article writer includes the myth regarding that con-rod oil hole. There's no way it can lubricate the cylinders - all it'll do is add friction by spraying onto the LHS of the flywheel (remember it sprays x2 at half stroke, not TDC). Which is why I place the hole facing outwards.

[Brandis]

Aha !!  Now we might be on something.  Poor cylinder oiling could have done in the engine at 12,000, which is what the odometer reads.  And, given the one piece rods, the guy in Connecticut pulled the crank and gave up when he couldn't disassemble.  Because it clearly wasn't a broken con rod. It would explain everything. Let me follow this up.

Heaps of thanks.

[groily]

A search using 'Long Stroke A7' produces a list which might have 'stuff' that helps you. As a know-nothing, I don't know if the followers were the same on other / all long strokes or not, but you'll see there are people who do know these things - and whose judgment you can trust.
I thought there were some premature camshaft & follower wear issues on these early  engines, despite the 'oil flooding' them quotation in the pages cited, but will leave to those who actually know! Might even explain why there are no ex. followers  . . . hope the cam is OK  . . .???

[Brandis]

Camshaft looks great.  Intake followers are fine.   

[Brandis]

I've got the Roy Bacon BSA Twins Restoration book.  Doesn't show what you are talking about.
A scan would be welcome at your convenience.
And, thank you, Roy

[chaterlea25]

Hi Brandis,
It just so happens that I have  2 NOS followers available
Yous for what they cost me plus postage

John

[Brandis]

Thanks.  The exhaust cam followers look like the the 2 I have waiting to be shipped from the UK. That's the best drawing I've seen.

[Swarfcut]

Brandis.. The Forum Literature Section contains parts lists for the early A7 Longstroke, as your engine is  designated. Unfortunately in their early format these lists include every BSA Model, but contain part numbers and good line illustrations of the parts, so you can see what's what. 1949 list should give best selection of bits that will fit. Reference to later model parts lists can also identify parts carried over and are common to the earlier bike. Inlet Valve Guides are the same (67 31) and this guide is also used for the Exhaust Valves in the later engine. As are the valve caps, collets and spring seats. Crankcase design changed with the later engines, but the basic layout and much else stayed the same. Even a modern SRM Oil pump or a late A10 Petal Gear type pump fit straight on.

 Cam followers appear identical for inlet and exhaust. All have one curved outer side. This matches the radius of the central  (missing in your case) tappet carrier shown in mitch's  exploded view, which appears in Roy Bacons's book on page 38.  Looks very similar to a Triumph type but isn't. All the tappet carriers are held in line by a small metal plate which you may have to fabricate, again this is illustrated.

 The crank you have coming shares all the same timing bush and big end journal dimensions as the later so called shortstroke engines fitted with small journal cranks. Timing bush and big end shells appear readily available. Drive side main bearing is a ball race, later engines have a roller race. These do not interchange, they are different sizes
  The advice on removing the plugs to clean the central oilway "Sludge Trap" holds good, blockage here is the prime cause of crank failure. Crank only fits this engine, no other type will do. Has a characteristic flat in the flywheel by the big end journals and counterweights that look like add on's to the flywheel side rim.

 Longstroke conrods are steel, split shell type is what you want. Conrod bolts are also unique, so may present a problem tracking them down.

 Plenty in the Longstroke Section for you to explore.

 Swarfy.

[Greybeard]

It's great to see this forum doing what it does best! 👍