Non Standard Piston

[Adrian Payne]

Many thanks everyone for your useful observations.
When I first posted the question I had just pulled a piston and just wondered what type it was and whether spares were even available. At that point I had not had the chance to measure for wear.
Well things have moved on now and the bore measures 4 thou oversize. At the same time I crack tested the 1 1/8" crank and as the saying goes "it is when it will break, not if it will break",  judging from the photo below, very soon.
So all in all I thing I will do the full job, new crank, re-metalled rods. re-bore and new pistons. should see me out!
Thanks again for your input.
Adrian.
   

[Bob Culver]

If that is the total extent a small crack. As is so common with reground cranks the radii not too perfect.Ttempting to remove with a chainsaw file or Dremell and statically rebalance. Some older books claim fatigue life starts afresh if crack fully removed. As a precaution all the radii on my car have been filed out...
Dunno what expereince is with the new small cranks but lack the lovely  proportions of the original so stresses must be more concentrated in crankpins.
If the pistons are solid skirt the recommended claerance was .004.
Without instruments an idea of wear can be assessed from ring gap change, or nails filed to length etc. I have encountered barrel shaped wear easily missed if just top and bottom measured.

[Greig Smith]

Tony Betts here on this Forum has put an extraordinary amount of effort into having new pistons made in a bewildering array of sizes starting at standard and going all the way up to 58mm in 0.25mm increments. They are of the modern lightweight slipper variety which saves weight. Plus he stocks extra ring sets for them which is handy if one inadvertently breaks a ring.

No affiliation, not even the same half of the planet, just a satisfied customer, having purchased two new sets from him at 7 County Austins --> http://www.7ca.co.uk, very happy with the results

Aye
Greig
Sunny South Africa

[Alan]

Bob’s advice comes from the 60s, when parts and labour were cheap. If you attack that crack with a file you will only remove the crack to the resolution of your crack test kit. The crack will still be there and be active. The crank will break. Maybe tomorrow maybe in several years, but you will be building an engine destined to fail. I know new cranks seem expensive but they are less than one sixth of the cost of a decent engine rebuild. Simply, these days, I wouldn’t even think of an engine rebuild without a new crank.

[Andy Bennett]

i must say I have always wondered about crack test kits, particularly on steering arms.
Whilst in my engineering role I wasn't involved in safety related issues, I was involved in designing joints for rare materials used in vacuum tube manufacture with stress joins from dissimilar expansions at high temperatures (2000 degrees for thoriated tungsten) etc, and micro stress cracks were always a 'thing' that could bite after xx cycles. The only way to find them was sectioned under a scanning electron microscope and a degree of calculation combined with trial, error and experience. It was frightening sometimes to see what cracks were lurking under an SEMs gaze waiting to emerge and cause micro fissures and vacuum leaks.

Appreciating that there is a difference between a micro crack causing a vacuum leak, but equally that wasn't under mechanical duress and it frightens me whenever I see a steering arm for sale on ebay with the reassuring statement 'crack tested'

Andy

[Bob Culver]

From what I have seen have limited faith in the non magnet penetrant kits, esp if outside use by date. But with a bar magnet and black fluid  can find totally invisible cracks in polished surfaces. With steering arms easy to overlook the seating radius (which if fully seated and tensioned should not develop a crack, but do). I have infected members of the Jowett club with crack hunting fever. Uncracked Bradford cranks very rare but of cast iron and seem to operate cracked for most of life. Club recently found one out of 12! On the least cracked of world's 2 bearing 4 cyl 10 hp Jowetts the  local crank showed small cracks. Ground out. Ideally should be peened.
(It is somewaht curious that primitve plodding Bradfords fetch more than very driveable Javelins!)

[Adrian Payne]

Just to tie up the loose ends and in case this is of use to others.

Jon E, the bores were quite worn. Ring gap middle and bottom of bore 25 thou. Gap at top 31 thou.
Bob, yes this is non detergent classic 30 grade oil.

Thanks for the opinion regarding the crank crack. I have some history with this issue which luckily did not result in any other damage but not going to push my luck again.

   

Many thanks.
Adrian.

[JonE]

so will you rebuild as 1 1/8" crank still? And what brand?

Whilst I see the absolute raw sense of Alan's point, I worry what it will gradually do for the A7 movement. Is it going to change the ownership gradually just to engineers (or those WITH engineers or budgets for them...) and racers, or will there continue to be a group who are maintaining on a shoestring? I am torn as I like the problem solving of the latter but need the safety and reliability of the former, especially as I get older and our attitude to risk gradually changes.

[Adrian Payne]

Hello JonE
I think I have decided to go down the Phoenix 1 1/8" route if I can find one.
While I agree that it would probably be sensible to change to 1 5/16" crank, I have a couple of minor issues.
Firstly I would need to purchase rods and a flywheel etc.
Secondly and more importantly to me is the question of custodianship after my time.
While I am not too fussy about originality, in the future when a new owner needs to buy parts they will know what to order (and me as the grey cells diminish).
Just my personal opinion.
Adrian.

[Bob Culver]

The oilways a  complication. Otherwise if always break across crankpin, tension ties could be fitted; fatigue resistance is hugely increased under compression, as big end bolts which would not last long finger tight. 1/2 " ht in a 9/16 hole? Anyone got a stack of thin cranks to play with?
If the cranks would go quietly and not wreck, more inclined to take risks. A noted Seven engine rebuilder here, now deceased, reckoned he never tested cranks.....

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Most service manuals regard .030 gap OK for reuse. .006 variation means possibly .003 or .004 wear, by no means worn out.