Author Topic: A10 Pistons OFFSET ?  (Read 2509 times)

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8269
  • Karma: 106
  • Lake Conjola NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
A10 Pistons OFFSET ?
« on: 13.07. 2009 14:38 »
G'day Blokes,
                  This might seem like a silly ? Are the gudgeon pins in A10 pistons offset ? The reason I ask is I was about to put the pistons on the rods and was giving them the last once over, when I noticed the pin was not central in the piston but back 2.5mm. These are nos MC forged 10.5/1 from the states bought from the junkyard (eBay). I have never noticed this before, was it a performance mod at some stage ?
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline A10Boy

  • Wise & Enlightened
  • *
  • Join Date: Jun 2008
  • Posts: 1075
  • Karma: 11
  • Solihull, Near Birmingham England.
Re: A10 Pistons OFFSET ?
« Reply #1 on: 13.07. 2009 15:53 »
10.5:1 !! Sounds like some aftermarker fire breather to me.

Ordinary A10 pistons have the gudgeon pin central.
Regards

Andy

1958 Super Rocket
Plus
1974 Kawasaki Z1a
Yam XJR 1300

Offline MikeN

  • A's Best Friend
  • ***
  • Join Date: Mar 2009
  • Posts: 220
  • Karma: 0
Re: A10 Pistons OFFSET ?
« Reply #2 on: 13.07. 2009 20:40 »
Not heard of that, but I know that Unit AJS /Matchless 250 and 350 singles from the ,60's had off-set (forward)cylinders .Known as "DeSaxe" configuration.
 Also some early radial aero engines were DeSaxe.
 I cant remember the thinking behind this,might have been an idea to reduce vibration.
Mike

Offline beezalex

  • North Carolina, USA
  • Valued Contributor
  • ****
  • Join Date: Jan 2008
  • Posts: 374
  • Karma: 4
Re: A10 Pistons OFFSET ?
« Reply #3 on: 13.07. 2009 20:50 »
While I believe all stock BSA pistons were central, many engines have the wrist pin offset in the direction of rotation (forward in this case).  These usually will have an arrow or an "F" stamped onto the crown.
Alex

Too many BSA's


Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8269
  • Karma: 106
  • Lake Conjola NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: A10 Pistons OFFSET ?
« Reply #4 on: 13.07. 2009 22:07 »
Thanks fella's, the pistons do have an arrow forward. I have seen this before but not to such an extent. May be 0.5 - 1.0 mm. On some rice burners the barrels are offset. I'll try anything once, suck and see ! The motor will be a slightly detuned 650 version of my old 500 race engine. Down from 13/1 to 10.5, 357 cam, chamfered skirts, allot push rods, A7 twin carb head with big valves, lightened & polished everything internal, and THROUGH BOLTED. The only way to keep the top end on. Doh secrets out.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline beezalex

  • North Carolina, USA
  • Valued Contributor
  • ****
  • Join Date: Jan 2008
  • Posts: 374
  • Karma: 4
Re: A10 Pistons OFFSET ?
« Reply #5 on: 14.07. 2009 17:51 »
Muskrat, the only thing I would check is clearance between the rods and the skirt.  BTW, how did you through-bolt the head to the case?  I'd like to see how you did that since I'm building an A7 race motor.

Cheers.
Alex

Too many BSA's


Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8269
  • Karma: 106
  • Lake Conjola NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: A10 Pistons OFFSET ?
« Reply #6 on: 14.07. 2009 22:00 »
G'day Alex, bit of a secret, I haven't seen anyone else do it. Quite simple really you just move the four outside bolt holes outward 1/2 a hole. Took a couple of donor head/barrel/cases to work it out. I will take some pics and post soon. Are you in Oz ? I also used A10 rods shortened 2.5mm and Trident std high comp pistons to run methanol. The crank is the weakest link, hence now building A10 with lge journal crank.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline beezalex

  • North Carolina, USA
  • Valued Contributor
  • ****
  • Join Date: Jan 2008
  • Posts: 374
  • Karma: 4
Re: A10 Pistons OFFSET ?
« Reply #7 on: 14.07. 2009 22:35 »
G'day to you muskrat, but I'm in the United State of Nawth Cackilacky...sorry.  The weather's good for riding here, anyway.  Interesting you should mention the triple pistons since that was precisely my plan.  The other part of my plan is to use an A65 crank...seeing as to how the pre-unit cranks, particularly the A7's are a bit spindly for my tastes.  So the two front and two rear head bolts go all the way into the cases, then, eh?  I'll have to take a look at that.

Cheers.
Alex

Too many BSA's


Offline coater87

  • Wise & Enlightened
  • *
  • Join Date: Jul 2008
  • Posts: 1181
  • Karma: 6
Re: A10 Pistons OFFSET ?
« Reply #8 on: 14.07. 2009 23:25 »
 Hi,

 I dont know the rules, or the type of racing you guys do- but if you are worried about blowing the head off wouldn't it be possible for you to strap it down? Might take some engineering, but I can envision 1/2 inch iron straps mounting to the barrel base studs and traveling over the head to the opposite studs. Might not even be able to do it in one piece of "strap", but I can see other gains with this method.

 I realise this is an "ugly" method, but I think we have all seen 6-71 blowers mounted this way in drag racing.

 Lee
Central Wisconsin in the U.S.

Online groily

  • Wise & Enlightened
  • *
  • Join Date: Jul 2007
  • Posts: 1082
  • Karma: 18
    • www.brightsparkmagnetos.com
Re: A10 Pistons OFFSET ?
« Reply #9 on: 15.07. 2009 00:13 »
Belt or Braces or Belt and Braces . . .
Interesting that the 2 UK marques who gave us through-stud vertical twins - AMC and Royal Enfield - ended up with worse reputations for reliability than the NoTriBsa triumvirate, even though the formers' designs were in many ways purer - and far truer to the sporting genre of the fast singles. Easier to put together too, with separate cylinders.
Haven't seen a through-stud engine rip its whole top end off (in road use that is), although they can sling their head gaskets into the hedgerows and leak Olympically from the barrel/crankcase joint. Conversely, loads of non-through-stud engines have head gasket failure AND stud/bolt trouble (like our As), and also barrel base stud/nut/flange trouble (like our As) . . .
Ergo, I reckon through-studs are better in principle as there is only one lot of fasteners to fail, and the loads are evenly distributed and shared. But I also reckon the engine and its castings have to be designed that way.
Ergo, 'straps' are good for engines not designed for through studs or bolts, just as, we hope, braces hold when the belt has long gone. But in a motorcycle, head steadies made of sturdy farmyard scrap iron instead of pretty bits of useless shiny stuff, do  much the same thing. The frame is stronger than any strap could ever be . . .?
Bill

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8269
  • Karma: 106
  • Lake Conjola NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: A10 Pistons OFFSET ?
« Reply #10 on: 15.07. 2009 11:05 »
True Groily I also welded a 3/4" nut to the top frame rail, placed a 1/2" steel plate over the rocker box then screwed teh bolt down till the frame started to flex !!! Then the crank blew. It had to go somewhere !!! The A50/65 crank would be the answer, I just didn"t want the extra width.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online groily

  • Wise & Enlightened
  • *
  • Join Date: Jul 2007
  • Posts: 1082
  • Karma: 18
    • www.brightsparkmagnetos.com
Re: A10 Pistons OFFSET ?
« Reply #11 on: 15.07. 2009 12:36 »
Blimey! But yup, the crank has to go somewhere! Never had anything as hairy as you describe, by miles, but in 35 years of AMC-ing, although I've broken a crank or 2 and blown a few head gaskets etc etc along the way, I've never had the top end fly away into the  atmosphere (or my tenderer parts). Can't say quite the same for our twins, love mine lots though I do!
Good luck with your bottom end retention . . . nasty mechanical - or medical! - problem
Bill

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8269
  • Karma: 106
  • Lake Conjola NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: A10 Pistons OFFSET ?
« Reply #12 on: 15.07. 2009 13:57 »
Ah I remember the sound. Whats that rattle, at 120MPH down Eastern Creek straight. Oh, it's the rockerbox hitting the bottom of the tank. Or the second or two of quiet hum then bang as the crank exits the cases in three different directions, same straight only faster. They should have run the classics up the hill instead of down. Flat out, little load, not good.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online RichardL

  • Outside Chicago, IL
  • Wise & Enlightened
  • *
  • Join Date: Nov 2007
  • Posts: 5062
  • Karma: 48
Re: A10 Pistons OFFSET ?
« Reply #13 on: 15.07. 2009 14:46 »
I think Muskrat has truly inspired me. In an effort to keep up with the performance options he has made, I have been scampering to make a few changes of my own in hope of creating a true racing machine. Do you think I can still refer to my bike as "near original"?

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8269
  • Karma: 106
  • Lake Conjola NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: A10 Pistons OFFSET ?
« Reply #14 on: 15.07. 2009 15:10 »
Richard, you might need to upgrade the brakes. wow a Rocket 8
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7