Here we go again....

[Simon.Gardiner]

From the back, left side piston on the left

From the front, left side piston now on the right

Left side cylinder bore

Back of bore

Front of bore

Managed a bit over 2000 miles (over a year) before breaking the 650 again. (Previous best 800 miles.) Then managed just another 70 miles....

So, 2000 miles in (actually 3000 miles for the bores and pistons as they were reused in this build) inexplicably seized the left-side piston 'big time'. This has all been relatively light use, not much above 60 for any length of time although there have been a couple of motorway trips at 55-60.

However, this was the first attempt at anything sustained at over 60-65 for any length of time. It happened after about 25 motorway miles of 65-70. Very ironically, just as I was marvelling at the way the bike was romping along with the throttle barely open.

Up to this point the engine's had a set of JP pistons, fitted as part of a major rebuild with much of the work done by SRM. SRM didn't want anything to do with the JPs and warned me they'd seize as they'd had a series of failures with them even when they'd increased the bore clearance. And after 416 miles (all very careful and controlled) they were proved right when I suddenly noticed some metal flakes in the oil tank gauze and a top-end strip showed the right-side piston had 'nipped up' exactly where SRM said it would (very top of the skirt just under the oil ring).

As that was a 'nip' I cleaned it up and put everything back together again, and that was OK for another 400-odd running-in miles before 'other issues' stopped that iteration.
(See viewtopic.php?t=14293)

My 'cleaning up' shows fairly clearly in the pics, that piston was OK this time but the other one (previously undamaged) decided to quit.

Concluded the fault was with the JPs, honed ('glaze busted') the bores, fitted a pair of new Wassell/Hepolites, oil and filter change, everything back together, a first fairly gentle 45-mile trundle was trouble free so a day or two later off for something longer. 15 miles in, after a few minutes cruising at 45-50 on a very light throttle, suddenly no engine and a brief squeak from the back tyre.

I assume it's seized the pistons again - which suggests that it's the bores that are the problem (despite having been done by experts with lots of experience with this type of engine) and I need to exonerate the JPs. (In the pics, the front and back of the 'good' JP piston have a polished look - it's not a trick of the light - which might suggest running 'tight'. I'd expect the same matt appearance all round the skirt, but I'm not familiar with how JPs look in use).

I haven't got the time (or inclination) to do anything with the bike at the moment, and anyway I can't see an easy fix as the bores surely can't be very far out and having spent several hours combing through original Hepolite catalogues and looking at the specs from current manufacturers (JP/Wasell/IMD) I can find enough different bore specs to amount to effectively a thou difference in size so I can see possibilities for debate over exactly how much correction mine might need. (I did check the piston/bore clearance with a feeler gauge and it seemed to be as per the spec on the instructions that came with the pistons but maybe there's an art to it that I haven't got.)

(And to think I've avoided Square Four ownership because they're just too costly and troublesome! )

SG

[nevhunter]

Not quite enough Taper and not enough clearance. . It's an iron motor so needs more. IF the cylinders are sleeved a bit more for that. It's likely it may have to rattle a bit when cold to NOT Nip Up especially IF you have a chair on it or move it at freeway speeds. The timing side RHS always gets more oil unless you do something to compensate to let more oil through the LH Big end Bearings. I like the type of pistons shown there but they need more clearance than slotted ones (which I Don't use no matter WHO made them). The Hone pattern looks fine also. Try a BIT of 2 stroke oil in the fuel Nev

[Mike Nash]

Simon, what precise oil are you using? Please note, I ask this in view of Nev's remark about differences in lubrication between the two cylinders ("the timing side always gets more oil . . ") and not to start another "oil war".
MikeN.

[Simon.Gardiner]

Mike, there's an 'unless' qualifying the comment about the timing side always getting more oil and this engine has all the qualifying items in place as originally provided by the manufacturers (ie bigger oil holes in drive side big-end journal, additional oil bleed hole in drive-side conrod and a big-end shell with a hole to connect to the conrod bleed).
It's also got a new (2000 miles ago) Wassell/Hepolite oil pump (these have a good reputation) and a new (although 3000 miles ago now) SRM pressure-release valve.
This time around (last 2000 miles) the engine's been on NAPA Classic 20-50. The time before that it was using the Comma Classic 20-50 (as came in the traditional metal gallon cans).
I've agonised over the oil since forever. I won't run a single on anything but straight grades (although apparently these days many people use multigrades in them without any problems). Much searching of the internet (for BSA A10 use) shows about half using straight grades and half using multigrades. My final decision was made on what is readily available locally (although that's started changing too frequently) but mainly on what Steve Carter was using in his very well travelled bike.
I'm still not totally happy with the oil choice but after the first 'lunching' (2017) I sent an oil sample away for analysis and except for a lot of hydrocarbons ('recommend an oil change and checking for rich mixture') it was all good, so I'll probably stick with 20-50 although I might change the brand.

SG

[nevhunter]

Clearance is the Problem and extra oil MAY cool the Part better, but it's a marginal thing in any case. It takes a lot of miles to wear a bit of metal off a piston skirt. Once a piston starts to nip it rapidly gets a lot hotter on the skirt by it's own friction so you have a Vicious cycle. I seriously doubt using a different oil is going to change much though some oils will be better than others. The cams and followers are one of the Most critical Lube film areas. Could the ignition be a bit retarded? Did both Pistons have the same clearance when assembled? Is the left muffler more baffled than the right one? (back pressure). Is the Paint on the cylinders too thick and over rust?. Iron head motors run a lot hotter than alloy ones. Nev

[dave.owen]

Simon , did you use running in oil?

[Simon.Gardiner]

Simon , did you use running in oil?

Absolutely not! I'm not a convert to running-in oil, I just gave these bores 3000 miles running in with old-fashioned API SE/CC spec oil (which isn't very far off running in oil spec anyway?) The glaze-busting hone actually showed up the beginning of wear ridge at the tops of the bores!
Possibly more to the point (and avoiding 'oil wars') the guys who did the bores (SRM) must have done hundreds of BSA/Triumph/Norton twin barrels and they didn't say anything to me about using running-in oil or else!
(Similarly they also blasted and blacked the barrels and heads, which would have been just like they do all their stuff so I don't think that's a factor.)
I am noticing that the pistons seem to have picked up at the bottom of the bores below where the rings have been rubbing, so maybe where the bores are tightest.
I'm strongly suspecting that the bores are too tight but I've run out of time for this engine for now and it's going to have to wait before it comes apart to see if that's exactly what's happened with the Wassell/Hepolites.

SG

[nevhunter]

The way that piston grabbed it was too tight unless the circumstances contributed . Hot day, running hard or downwind etc Solid skirt pistons need more clearance but I prefer them. Are the bores sleeved? If so, extra clearance for that is needed. (extra .001") Make sure the skirts are tapered. More clearance at the top... Nev

[tony.barnett]

For me, a company like SRM should know how to bore a barrel with the correct clearance.

Perhaps an obvious point, but you have checked you aren't running lean? Carb settings etc. I know its a twin but often the fuel flow has some bias. I've ridden a bike for several hundred miles only for it to nip up ten miles from home because I've been inadvertently holding the throttle on a weak spot in the carburation. Even if you don't think that is a contributing factor, its easy enough to check against the standard spec for the bike.

[Simon.Gardiner]

For me, a company like SRM should know how to bore a barrel with the correct clearance.

For me, also - that's one of the reasons the work went to them!

And I'm not completely comfortable with the carb but I'm running the settings advised after a discussion with Burlen and if anything it should be rich rather than weak. (I'm using a 926 concentric, Burlen advised same settings as monobloc but with smaller main jet - 190 instead of 240; I went for a 200).

However, after a LOT of checking with the remaining good piston and a feeler gauge I'm convinced the barrels are 2 thou too tight, so it seems that SRM don't know how to bore to the correct clearance despite their many years of experience. (These Wassell Hepolites seem to have quite pronounced ovality insofar as with a feeler gauge just slightly off-centre from front/back there's an extra thou or so of clearance; at least there is with mine and that might be how I didn't pick up on the lack of clearance straight away).

Pics of the seized Wassells:

From the back, right hand piston seized

From the front (right side now on the left)

It was the other side that picked up this time, and as it didn't have chunks torn out of it (unlike with the JPs) I've cleaned it up and we'll see if it'll run on OK.
The barrels have been off to Hamlins to be honed out to 4 thou clearance (as per spec for the Wassells, and the same as for the JPs!). Just got them back so now to try yet again...

SG