push rod lengths

[Brandis]

So, head torqued down.   Push rods in place but the exhaust rods are definitely too long.    I can cut, round, and heat treat them, but does anyone know how long they should be?

And, does 'the comb'  work in these early models ?

[Brandis]

PS:  I think the Intake rods are wrong as well.

[chaterlea25]

Hi Brandis,

I can cut, round, and heat treat them, but does anyone know how long they should be?

Have you bought these or did they come with the bike?
The originals and SRM replica's are hollow (for the later models) so I wonder how they could be shortened easily
As to the comb ? I do not know, but I have given up using them preferring a bit of wire with a hook and a good LED torch, plus 2 rubber bands.

John

[muskrat]

G'day Brandis.
This topic from 10 years ago might help. https://www.a7a10.net/forum/index.php?topic=5250.0
Cheers

[Brandis]

Thank you, yet again, Muskie.    That old posting had the dimensions.  180 and 205 mm.
My engine came with a camshaft, no pushrods and no valves or springs.  So the rods came off Ebay.  They are solid and all too long so cutting down to length and finishing the end seems to be in my future.  I assume heat treating would be in order.  Tempering in oil to prevent brittleness.

Thanks to all.

[Brandis]

So . . . .
. . . I took the measurements from the 10 year old post and went to the shop.  180mm for inlet.  That's the length I have.   But when I install correctly, even with the adjusters totally removed, and the rocker arm touching the end caps,  spinning the engine lifts the whole alum casting.   So 180mm is clearly too long.   
So lets get theoretical.   If I set the engine with the rod down and measure the distance between the machined face of the head and the end of the rod
. . . . and set the rocker arm in a mid motion position and measure the distance between the cup to the machined alum casting face . . . .
. . . and add those 2 #s together, I should have the correct length of the rod.
Then repeat for the exhaust.

What's the jury say?

[BSA_54A10]

Time to measure the length ( height if you like ) of the barrels.
Good chance they have ben shortened to raise the CR
And how many fins does your have ?

[Swarfcut]

 Can't understand why the rocker box should lift. It should be bolted down, as it is the fulcrum for the rocker arms. Valve should open  but my expectation for a too long pushrod would be a lack of adjustment and valve spring becoming coil bound.

  There is only one cylinder barrel casting, 9 fins and a pair of raised spigots to locate into the combustion chamber would indicate an unmolested example. Shortening the base is not a practical solution to raise compression ratio. Piston height at the top of stroke also an indicator of correct parts.

 Swarfy

[RichardL]

Without taking anything away from good advice intended to help you resolve the problem yourself, I would give Gary Hearl at SRM a phone call pleading for mercy and help and ask if he can tell you the correct push-rod lengths. I've spoken with him enough to know that he won't jealously withhold information if he knows it. If he doesn't, you haven't hurt yourself by asking. If he does, then solved. At the same time, you've established a relationship.

Richard L

   

[Brandis]

I had to bend one of the push rods to make the left intake work.   Looking down at the top of the head, the access holes for the rods were not in the same relationship fore and aft. I have to assume that was something corrected fairly soon.  The 180mm measurement provided in the Forum proved to be correct. 

It makes me suspect that 205mm for the exhausts is correct.  The ones I have in hand are 250mm,probably from a 500 single. I've taken Forum advice and am asking SRM to confirm.

[muskrat]

G'day Brandis.
I spent an hour or so this morning hunting the world trying to find exact measurements to no avail. Rare little buggers.
Cheers

[Swarfcut]

 Brandis   Something not quite right here with the basic set up. Inlet pushrods run more or less vertically, the exhaust pushrods are inclined forwards. Try it without the head gasket, could be that the pushrod holes are in the wrong place. Pushrods never need a bend or bow to fit, they will bend even more.

 The Longstroke  pushrod arrangement is completely different from the later engines (where all four pushrods are bunched together). Longstroke pushrods  have a cup at the lower end to accommodates the differing angles as the pushrods leave the ball ends of the cam followers. Centre tappet block is fitted with oil hole on the flat to the rear, the  locking plate that also holds the inlet tappet block in place fits the slope at the front.

    Looking thro' earlier posts, it appears this engine came without valves. While the springs, seats, caps and collets are the same as later engines, the part numbers of the valves are not. so the question is whether the head has the right length of valves fitted.
 
    I just cheated and threw together a head (without valves), rocker boxes, barrel and crankcase. Not a real test, my bits fitted together fine, but I did need a flashlight to peer down into the jug and make sure the pushrod cups fitted onto the tappets. I fitted the rear inlet box first, leaving the exhaust pushrods aside. They poked down easily afterwards, exhaust rocker box dropped on. OK, simple with bits missing, but with the right parts it all fits.

 Pictures show one exhaust and one inlet pushrod in their respective locations, plus tappet guides with locking plate. Barnfind  A7 Longstoke Engine.

 Swarfy   

 Additional.  Other Longstroke enthusiasts my find that tappet plate picture useful, I've never seen another one, ever!!

[Brandis]

No question that Swarfy knows 10x more than I do on all of this, but . . . . .

I think it comes down to the old magazine page I saw that talked about the BSA financial troubles and the rush to get the first batch of 200 out the door. They were basically prototypes. In a calmer world, I don't think those machines would have shipped.  I can imagine the assembly line guys looking in dismay at the hole placement on the heads they just received and having to come up with a quick fix to meet the deadline.
I already trimmed the head gasket and the rocker cover basket.  At some point, probably with the next owner, the head will be removed and the hole size increased but it's time for me to do the last bits and have some fun riding before these 76 year old bones can't do it any more.

[fffcycles]

I had the very same problem on my first assembly then after trying twice more and using the flashlight down the hole it came together and was not lifting the valve cover/rockers.
I did relieve the holes in the gasket to make sure the pushrods did not contact and I had to reassemble the valve cover as I had the rocker shaft assembled wrong with a Thackeray washer in the wrong place. Went together like a dream after that and no I did not use the comb, elastics and dental pic (one of my favourites).
Frank

[Brandis]

Problem solved.   

First I checked that the washers in the rocker arm shaft were in the right order.  Then I double checked that the rods were sitting on the correct cam followers. Then I took Swarty's advice on how a bent push rod would not end well.

So I removed the head. The 2 holes for the intake rods were clearly off kilter.  They were 1/2".  So I put them on the drill press and opened them up to 5/8".  There space for it and the cast metal is actually quite thin there. I straightened the push rod,  put it back together.   Bob's your uncle. 

It seems a bit radical but it works.     Thanks to you all.   My professionally rebuilt magneto arrived.  $840 US.  Ouch !