General opinions on A10 rebuild choices I am trying to make

[BSA_Owen]

Hi All!

I'm in the process of piecing together my A10 plunger engine for my rigid.

I have done a fair bit of reading but have found mixed opinions on various things... just trying to pick a nice combo that has a bit of poke but also will be happy enough to plonk about for decent rides.

I have early cases (probably obvious due to plunger)
I have picked up a neat set of thick flanged barrels, 356 camshaft and a rebuilt alloy head.

now I come to selecting the rest of my parts mainly pistons, rods and crankshaft.

1. I am thinking of locating a large journal crank, is it worth the effort?
2. I am looking at new rod options (as per above) I like the map cycle version being H beam steel forgings with replaceable small ends and lower small end weight.
3. Pistons - since I have the ally head I am wondering if I should be looking at 8.5:1? or 7.25:1?, then if I throw aftermarket rods into the mix most are 6.5" long then that will shift the compression ratio up slightly again although maintaining the same swept volume of course?

new to preunit twins so just hopeful of some guidance and maybe input from people running similar combinations?

cheers for your time
-Owen-

[Greybeard]

My Plungie is standard tune apart from a 356 camshaft. I'm 70 next month and my legs are a bit crook. The bike is great for my ability to kick start it easily and going fast enough for my riding style.
I know that many 'mature' bike owners have trouble starting high compression engines so if you are in the middle-aged demographic you may like to ponder living with a HC beast.

[Swarfcut]

Hi Owen.   Looks like a custom build here so choice depends to a large extent on your budget.

  Higher compression ratios put a greater strain on the motor, this was part of  the thinking behind the introduction of the big journal crank and thick flange barrels, as the original design showed its failings as power outputs were pushed higher.

 The big journal crank offers greater physical rigidity, improved big end lubrication and a proper sludge trap, and lower surface loading to the big end journals, compared to the earlier crank.  Source and choice of bearing shells is better for the big journal crank, so if the only crank you can find requires remetalling, no problem getting shells.

 With new rods, the large journal size is well catered for and choice is only restricted by your budget. All A10 standard rods are the same length, no matter what compression ratio pistons are used.....the compression ratio is altered by the shape of the piston crown.

 The 356 cam is a good cam with flat top and higher compression pistons, but bear in mind a compromise on compression ratio may have to be made depending on the octane rating of your usually available fuel.

 Before you continue,  assemble the cases, gearbox, barrel, head and rockerbox, just to make sure it will all fit in the hole. From experience it is easier the assemble the engine into the frame, rather than struggle loading a fully dressed motor.

Swarfy.

[KiwiGF]

Hi All!

I'm in the process of piecing together my A10 plunger engine for my rigid.

I have done a fair bit of reading but have found mixed opinions on various things... just trying to pick a nice combo that has a bit of poke but also will be happy enough to plonk about for decent rides.

I have early cases (probably obvious due to plunger)
I have picked up a neat set of thick flanged barrels, 356 camshaft and a rebuilt alloy head.

now I come to selecting the rest of my parts mainly pistons, rods and crankshaft.

1. I am thinking of locating a large journal crank, is it worth the effort?
2. I am looking at new rod options (as per above) I like the map cycle version being H beam steel forgings with replaceable small ends and lower small end weight.
3. Pistons - since I have the ally head I am wondering if I should be looking at 8.5:1? or 7.25:1?, then if I throw aftermarket rods into the mix most are 6.5" long then that will shift the compression ratio up slightly again although maintaining the same swept volume of course?

new to preunit twins so just hopeful of some guidance and maybe input from people running similar combinations?

cheers for your time
-Owen-

Not all aftermarket rods are over length. Some are the correct length eg most if not all billet rods. Small journal rods have wider shells than large journal so I think it’s just rigidity that is improved with lj cranks. It seems generally accepted that the 356 cam with flat top (7.25:1 ish) pistons is a good compromise tune.

Personally after investing a lot of time and $$$ into the engine (line boring, other “blue printing” jobs,  Thunder Eng. billet rods) I don’t like revving it beyond 5000 rpm ish so I opted for the 356/flat top combo, iron head, thick flange barrel, new monobloc at 1 1/16” and it’s fine for NZ roads (100kmph speed limit).

[Rocket Racer]

The thin flange barrels and small journal cranks were fine for everyday use and in a stock state of tune.
Some of the early american speed record machines were the semi unit plungers with SJ/thin flange! They got 143mph out of a stripped down A10 semiunit before it blew.

But, there is no doubt a LJ crank is much stronger - be aware there are two types, the common post 58 type and whats often called the transitional crank, that appears to have been used on earlier machines in road rockets once the americans started breaking SJ cranks with higher states of tune . The transitional cranks have a different (inferior) sludge trap but are super solid so create a very smooth machine and IMHO the dogs bollocks for everything but the racetrack (where a lighter crank is preferable)

356 - good choice, the strongest cam up to around 5500/6000, above 6000 fit a 357 for those extra couple of horses at the expense of low down. The higher lift of the 357 can require the cam trough opening up

Pistons -I prefer mild compression 7.5-8.5 depending on riding style. a flat top 7.5 will give you a sweeter motor that wont knock on bad fuel.
I'm a recent convert to the MAP steel rods, no weight disadvantage to the alloy billet ones and very strong.

Dont over tune or over port unless its a race bike! you'll just spoil it. keep it mild and it'll be sweeter.
Just put it together carefully.
Please note my race a10 rarely gets to run below 5000 and I'm only happy when its humming along at 6-6500. Its also routinely pulled down and I recognise is very fragile. It's hung together for 10 years of abuse and I've seen 7800 on the tach.
Enjoy the journey

have just added a pic of a transitional (heavy) crank in this case running 6" T140/A65 steel capped rods

[BSA_Owen]

cheers for the feedback all!

Thanks RR on the crank heads up, I didn't know there was an inbetweeny crank.

I splurged on a set of map rods so now I'm just down to sourcing a crank and choosing a piston brand and then of course sourcing all of the regular rebuild parts required bushes/bearings/etc.

seems mixed opinions out there on pinion brands also so still trying to figure out that next move.

[trevinoz]

The "inbetweeny crank" was only used in the Road Rockets.

[muskrat]

G'day Owen.
Lets get it straight. Are the cases long or short stroke? With the short stroke from 50-51 all parts to make an A10 out of an A are the same (bar crank, rods barrels, head, pushros etc). I made my 57 SS into a RR+. The 51 A7 plunger will end up an A10 later but for now has a SS top end.
Not sure if a later top end and cam will fit a longstroke bottom end.
Thoughts?
Cheers

[Swarfcut]

   G'Day Musky. A good point to make about the cases. We all assume the motor will end up as a a high spec. A10, so no problem mixing and matching the bits.

   Owen, The cases you need can be marked A7 or A10.  They must have provision to bolt the plunger type gearbox to the back. Later S/A cases are no good to use in this build.

    The cases you need for the more common shortstroke type engine have a trough under the camshaft. The cam followers are retained in the cylinder barrel, so easy to identify.

  The earlier Longstoke engine is somewhat rare, being only produced for a short time, and has no cam trough. The tappet guides are located in the crankcases.

  Longstroke A7  is a completely different engine, but shares a few parts with the later design, really just the oil pump and associated bits. Rocker Gear, Head, Barrel, Pistons, Crank, Rods,  Camshaft, Cam Followers and Pushrods are all unique. Timing gears and breather are the same basic components, but timing marks and breather peg drive location are different to suit the longer stroke. All no good at all for your build. Primary drive parts were carried over to the later engine, and are well proven.

 Good chance you have the later type cases you want already, but we all know about Sod's Law. BA Series Crankcases represent the last development in plunger crankcase  design.

Cheers

 Swarfy.

[BSA_Owen]

Hi all, my cases are short stroke a10 plunger so no dramas there. I have a good-er set and the set that are part of my complete motor (have damage to the primary casting to the top, assuming it took a chain hit at some point)
I have a newby setup for primary duties so either cases would work as won’t need lubrication.

[morris]

I have a newby setup for primary duties so either cases would work as won’t need lubrication.

Just curious, but would a Newby clutch fit inside a plunger primary case?

[BSA_Owen]

I have a newby setup for primary duties so either cases would work as won’t need lubrication.

Just curious, but would a Newby clutch fit inside a plunger primary case?

Hi Morris, yep. Bob makes a setup to suit the plunger. Different to the swing arm kit.

[morris]

I have a newby setup for primary duties so either cases would work as won’t need lubrication.

Just curious, but would a Newby clutch fit inside a plunger primary case?

Hi Morris, yep. Bob makes a setup to suit the plunger. Different to the swing arm kit.

Ok. Only saw the Goldstar clutch on his website which should fit on a swingarm but indeed wouldn't fit on a plunger

[Rocket Racer]

I have a newby setup for primary duties so either cases would work as won’t need lubrication.

Just curious, but would a Newby clutch fit inside a plunger primary case?

Hi Morris, yep. Bob makes a setup to suit the plunger. Different to the swing arm kit.

Ok. Only saw the Goldstar clutch on his website which should fit on a swingarm but indeed wouldn't fit on a plunger

i can confirm I did speak to bob some time back and he does a plunger belt drive conversion. while the semi unit cannot move the box for tension it does run a duplex chain so well suited to bobs kits

[Rocket Racer]

Also worth considering whether a nova five speed conversion in the budget. I recall the outer cases need swapping to the pre unit types...  but those are common enough
http://www.novaracing.co.uk/ProductBSAGoldStarTouring.html