Saturdays fun (dismantling engine)

[BSAmoto]

Happy Easter to all of you!
So I spent saturday in the workshop and took the A10 that I bought in France to pieces. The headgasket leaked anyway and the noises from the engine were a bit "abnormal". All headbolts were lose, too much valve play as well. As I intended to check the cylinder and bottom end, I removed the primary drive (4-spring clutch) and the timing stuff (SRM oilpump) as well. Cylinder came off and showed a scuffed piston LH, with some remedy marks of a file over the worst places.... Pistons are the cheap asian models at 0,040". As the barrel is not ruined much, I might get away with a hone if someone lets me know the exact diameter of a 0,040" oversize piston and the desired maximum clearance. I don´t want to go to 0,060" straight away as this is a thick flange cylinder. Speaking of thick - the big ends are thick as well and of course the RH mainshaft bush (replaced on the bill that I got with the bike) has about 0,4mm play. The result was a slightly ruined LH conrod, having play on the big end and turned the bronce bush in the upper end -thereby reducing the lube for piston pin to zero.  Have not split the cases yet but will do in the next few days, looking forward to repair all this mess and have a nice engine to put back into the frame. Frame and engine plates are powdercoated so I have a lot less parts to spray-paint. Might replace both chromed mudguards (if you know a good source where to get proper parts?)
The non-working dynamo will be looked at or replaced by the Alton system - not decided yet.
Might take some pics tomorrow and post them here.
cheers, Harty

[Swarfcut]

Harty.  I think you mean you have a big journal crank when you say a thick big end.  I hope the big end shells are not on the max undersize, as that makes for bigger expense.  Shame if the rod is ruined, it may clean up for re-machining. But then I reckon you are the sort of guy to go for a nice set of modern rods and this is a minor problem.

 Good luck with this one.

 Swarfy.

[BSAmoto]

Swarfy, yes, I meant a big journal crank. Does anyone have a precise measurement of a new +0,040" piston?

[KiwiGF]

Swarfy, yes, I meant a big journal crank. Does anyone have a precise measurement of a new +0,040" piston?

That may be a tricky question to get answered, pistons are not round or particularly uniform in diameter. The brand will make a difference as well. Generally the barrels are bored to suit the particular piston, but I have heard of barrels being bored to the required size, without the piston being “known”.

From memory I had about 007” clearance (using my feeler gauges, and the machinest’s internal mic to measure) this after I had a “marked/seized”, lined, barrel honed, it turned out not oversized after the marks were nearly completely gone, so I used new JP pistons on std with no subsequent oil burning problems, despite using the JP rings (which do not a good reputation). I ran it in over 1000 miles, again not recommended by all, especially with JP rings.

You can read up here on opinions on a recommended minimum clearance on this forum, from memory 004 is the minimum, but’s it’s more for some brands.

I suspect the best you can do is buy some branded pistons, but make sure you can return them if they are too small in diameter (but I don’t think that’s likely).

[RDfella]

Engine rebuilding is becoming a nightmare these days. Because engines (cars/boats/motorcycles) are way more reliable and long-lasting mechanically (but not electronically) than their predecessors, machine shops have gone out of business and finding one (with a machinist old enough to know what he’s at) is getting difficult (apart from the specialist restorers, that is).
For example, the only crank grinder around here is a farmer who bought machinery when machine shops closed. He has a lovely Churchill – impressive machine but takes a while to set up because, unlike the Prince which follows a big-end, on this the stroke has to be set manually.
As for pistons / bores it used to be the case that bores were specified and pistons made to suit. Boring to suit a piston was usually reserved for blueprinting / racing requirements. At the factory pistons with variations of around .001” or so were used to match to bores enabling blocks that had been bored slightly out of spec during production to be used. Rather like the Ford 2700 diesels of the 60’s, where some blocks had main bearings .015” oversize on OD to save rejecting a cyl block. Similar with their core plugs.
These days we have no idea of the material specification of the pistons we buy. I get the impression some started life as saucepans – rather like in WW2, when pistons would ‘grow’ just by being left on a shelf. For others the price suggests they’re some sintered rare-earth material. Short of actually measuring a piston’s coeff of expansion, we can only rely on forums like this. For example, if Fred says his XYZ pistons seized at .003” clearance, then others can try .005”.

[BSAmoto]

Well, I got the crankcases split and took the crank apart. Of course the idiot who ground the bigends never ever heard of the needed radius and to make matters worse, the crank is on its last leg, the shells are 40s... Luckily the bigends are round and allow 5/100 mm play to the new shells so I can live with that. In the lousy workshop that did all this "work" nobody even knew that there is a sludge trap with access from both sides, at least they could not figure what the big slotted screws are for, so they decided to "best leave them alone". Sludge trap was more than half full, the stuff inside the tube was soft so probably not too old  but on the outside it was rockhard and showing signs of rust - so the bike stood quite some time before the stupids "overhauled" the engine.
Anyhow, due to the problems that idiots without proper tools have but sadly never realize, the crank nut on the primary side was not tight enough so the bearing wore about 4/100mm off the shaft. Another case for the laserweld affair. The crank needs a grind session anyway as the timing side is 5/100s out of round and around 6/100s conical as well - did I mention that a new timing side bush plus work is on the bill that came with the bike... having almost 10 times the correct play on the shaft.
So I will order a set of pistons and go from there.
cheers, Harty

[KiwiGF]

Just a “FYI” on shells (if you’ve not ordered them yet)
https://www.a7a10.net/forum/index.php?topic=5028.15

[chaterlea25]

Hi Harty
If the crank was ground to a sharp corner on the big ends rather than radius, it is scrap
It will break as sure as the Sun rises
I have seen this happen 

John

[berger]

none or little radius snapped mine in 1980, it fell on the floor when I was taking the engine apart

[groily]

Hi Harty
If the crank was ground to a sharp corner on the big ends rather than radius, it is scrap
It will break as sure as the Sun rises
I have seen this happen 

John

The sun did rise and I had one break too, years ago, after a dodgy regrind.
Radii can't be reclaimed once eliminated.
Ouch.
Good machine-shop, good radii, plus crack testing is what you want imho.

[KiwiGF]

It’s pretty upsetting for BSAmoto, is the crank usable at all? Can these cranks be fitted with non std rods and/or shells and a radius (of a sort) made?

I’ve heard the small journal crank when it gets to max undersize can be fitted with Trumpet rods for some more life.

[BSAmoto]

I will machine the sludgebore to fine tolerance, shrink in a ground tube of a strong tempered steel and into that tube will the new plugs be screwn in with a step to pull on the crank cheeks as well. Thus the crank could break inside the engine without destroying everything. A smaller sludgetube could still be fitted so no real loss and at least with low compression this crank might survive the summer.
cheers, Harty

[Swarfcut]

   Don't recall ever seeing a crank ground without a radius on the corner. It is simply a matter of dressing the grinding wheel edge. Seen plenty of cranks with tapered journals, where the wheel grinding face has not been dressed accurately  parallel to the axis of the machine, or the poor old machine was simply worn out. Also seen finished cranks where the journals looked to be multi faced rather than cylindrical! All done by guys who should know better, maybe didn't care a fig or with a vested interest in making the customer return for another wallet busting empty out. The same guys that never mentioned sludge traps when you went back a second time.

  Harty... Reckon that's a good scheme to prolong the life, well worth a try.

 Kiwi... Interesting thought about Triumph Rods. I suppose it all depends on the difference in local costs and parts availability. I have a SJ crank worn to below -40, which is effectively unusable as no further underside bearings are available as far as I know.

 Very interesting to read about big end materials.  We always reckoned if they were " down to the copper" they were worn out...that may have still been good bearing metal, and the nice smooth silvery layer straight from the box was just a flash coat, as explained.

 As for my crank Metal Spraying is possible, so anyone know if this could be taken up to large journal size as bigger shells seem more plentiful ?   Then again, the costs of this exercise could prove more prohibitive than  just obtaining another crank.

 Swarfy.

[chaterlea25]

Hi Harty,
I saw this post on Britbike with the details of a man who has "Billet" A10 cranks for sale

http://www.britbike.com/forums/ubbthreads.php/ubb/showflat/Number/805474/gonew/1/crankshaft-regrind#UNREAD

John