Longstroke re-bore - how big?

[Nourish]

Just how far can a Longstroke barrel be bored out?

[Tomcat]

Mine was bored to 60 thou over with no issues.

[BagONails]

May depend on what pistons and rings you can get, unless you have something non-standard in mind?

[Beeza]

I’m about to get this one bored to suit some speed twin 40th over, so will be 80th over

[Nourish]

That would mean you could get a std set of short stroke A7 pistons in there with an A10 crank and rods?

[Swarfcut]

 In a word.... NO.  Although they look similar, swapping cranks is not possible without radical surgery.

 The Triumph pistons are one of life's fortunate coincidences, even the wrist/gudgeon pin diameters are a match. Piston rings and groove depth are the same and Triumph rings can be selected to match the bore on 1mm and upwards Long Stroke oversizes. Dunno about the finer detail of Crown Heights to fit without reference to Drags' piston chart, but it's all there along with original Hepolite cross references. Useful to know, Long stroke pistons being somewhat rarer than Meriden's finest.

 Swarfy.

[Beeza]

As Swarfy said, no.
Speed twin for anywhere over 0.040 over is the best option, I probably wouldn’t go beyond 0.080 over, but there is the availability from triumph pistons to go bigger.
A7 long stroke bore centers is 3 3/8” whereas A10 and short stroke A7 is 3 1/2” gudgeon pin size and distance to crown vary between all 3 and on it goes.
Try for original pistons or Speed twin jobs, crown height varies slightly as does ring height on the trumpy ones.
Photo for the crew😃
Best of luck

[Swarfcut]

 The Triumph pistons have the extra oil drain holes below the oil ring, absent on original Long stroke pistons and a useful character to note among a load of 62 mm pistons which look the right part, but are not exactly correct. Bit of a minefield mixing and matching without a selection of pistons to hand, but it can be done.

 My opinion is that Long Stroke motors are best built as standard, later Short Stoke is a far better bet for mods.

Swarfy.

[Nourish]

I'm a bit confused about bore size, The Munro Twin book states that the std bore diameter is 2.4795" but that equates to 62.97mm which is much closer to 63mm than the much quoted bore size
of 62mm?

[JulianS]

My copy of Munro, page 36, shows the standard bore as 62mm?

[Nourish]

And on page 123. In fact the later A7 bore size equates to 67mm and not 66mm as it should although the A10 looks correct.