Author Topic: Conrod length - solving the compresion plate mystery  (Read 3091 times)

Online KiwiGF

  • Last had an A10 in 1976, in 2011 it was time for my 2nd one. It was the project from HELL (but I learned a lot....)
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My '56 A10's conrods have the numbers on them (possibly casting numbers not part numbers) of 67-270 R272R (has oil hole) and 67-270R3R and are 6" long - are they the correct rods?

My 56/57 parts book lists the pr numbers as 67-1203 and 67-1204.

I've not seen the engine I'm rebuilding run, but the seller said it ran fine. I have stripped it down anyway as it had hardly been used in over 30 years. It's got quite a few problems but the strangest is that it had a 3mm compression plate fitted under the barrel, but high comp style pistons. Pic attached.

The pistons have over 006 clearance at the bottom but have nevertheless partially seized in the past, which is a (expensive) pain as the cylinder has already been lined. I think the sleeves might b be too badly marked to re-use. I'm puzzled at to why it has seized and am looking for reasons...apart from the conrod with the oil hole being fitted to the wrong (timing) side!

The barrel is thick flange whereas thin flange would have been standard for 56 model year

The stroke of the crank is right at 84mm, so at least I have the right crank - I think.....it is small journal type and on -030 big ends, -010 main

It seems that 6.5 inch long rods are used on at least some A10's and I can't see why shorter rods would be fitted on a 56 model given I've read the thin/thick A10 barrels are inter-changeable - unless the cases are shorter on some models? Or different pistons fitted on some models?

The pistons are flat topped domed type (typically 8.5:1 look like these I'm told) and measure 1.334 from the gudgeon pin center to the start of the chamfer on the piston top, and the dome part is 0.175 on top of that. I guess the compression plate was fitted to make the chamfer not protrude too much above the barrel.
New Zealand

1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

Offline muskrat

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Re: Conrod length - solving the compresion plate mystery
« Reply #1 on: 12.04. 2011 11:36 »
G'day Kiwi,
               they are A7 rods. All A10's had 6 1/2", sounds like dodgy pistons. Will measure spare pistons tomorrow (in PJ's now).
If the sleeves were shrunk in as they should you could bore them till their down to 1mm wall thickness or re-sleeve back to std
http://www.cyclewareables.com/pages/street_cylinder_sleeves/cylinder_sleeves.htm
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online trevinoz

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Re: Conrod length - solving the compresion plate mystery
« Reply #2 on: 12.04. 2011 23:38 »
Kiwi,
         Those numbers are correct for Flash rods. Are they cast-in?

  Trev.

Online KiwiGF

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Conrod length - solving the compresion plate mystery
« Reply #3 on: 13.04. 2011 06:55 »
I need to find some new 6.5" rods. Does anyone know the carrillo part numbers for the small journal A10? I asked the aussie distributor in Brisbane for a price and they wanted a sample OEM rod to make up a "custom" order.

And can anyone recommend a brands other than Carrillo? (who I suspect will be pricey)
New Zealand

1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

Offline muskrat

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Re: Conrod length - solving the compresion plate mystery
« Reply #4 on: 13.04. 2011 14:26 »
Have heard these are good http://www.mapcycle.com/ProductDesc.aspx?code=MAP7078/B&type=0&eq=&desc=CONROD%2f-A10-%28EARLY%29-6.5--EXTRUDED%2fBILLET&key=it says not available but I'd send them an email anyway.
Sorry forgot to check pistons, will put a note with keys now.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline mark

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Re: Conrod length - solving the compresion plate mystery
« Reply #5 on: 14.04. 2011 05:49 »
I have fitted a set of billet rods from Thunder Engineering in the UK. They cost 269 pound delivered to Australia. The quality seems excellent. I have only just got the motor running so cannot give any long term report yet.
Mark
55 road rocket




71 Norton Commando Roadster 750


Australia