Thursday, 12 December 2013

"The Lord of the Piston Rings".......

Well, I've raided the works of C.S.Lewis for post titles in the past: now it's his old chum J.R.R.Tolkein's turn.......

The replacement engine for my Alvis TA14 has been completely stripped down.

In the process, we were delighted to discover that it had had very little use. All the engine internals were sufficiently within tolerance to go again after a good clean.

This, sadly, could not be said of the piston rings. A couple of them broke up on removal.

(It doesn't matter how careful you are: they are brittle, and get more so with age.

Besides which, there was sufficient detectable wear in the cylinder bores to convince us that a new set was needed anyway.

Further, there was about a 5-7 thou wear ridge present at the top of the cylinders. This isn't enough to warrant a rebore, but the top rings had been hitting this ridge causing some damage to them.)

So a new set of rings plus all available gaskets and seals were ordered from the marque specialist.

While we waited for DHL to do it's stuff, I took a file to the wear ridges and filed them off.

Now, any precision engineers reading this will have probably just spat their dentures out in horror at the very idea. But the rationale behind it is this:

Back in the day, you used to be able to buy 'ridge-dodger' piston rings which were machined with a lip that allowed the top ring to avoid making contact with the wear ridge.

Now we live in the present, when most of this stuff is being re-manufactured.

One might suppose there just isn't the demand to make the re-manufacture of ridge-dodgers viable.

Or, less charitably, assume that the people doing the re-manufacturing are also making first oversize pistons, which they'd much rather you bought from them than supply you with a means of making the old ones go again.........

Taking down the wear ridge with a file is very Old School indeed.

It's the sort of thing Grandad Eric would have done.

Which is why I did it!

Anyway, Roger inspected my work on the bores with his digital gauge and pronounced them okay.

All we needed to do was to give the cylinder walls a very light hone, drop the original Specialloid pistons in with a new set of rings, and Bob's yer uncle.

Er, no.

Not quite.......

We got the digital gauge on the new rings as soon as they were out of the packaging.

They were far too loose.

Something was clearly very wrong.

Switching the gauge read-out from Imperial (thousandths of an inch) to metric gave a clue.

The rings were correct for a standard cylinder bore at 74mm, but were way too small at 4mm height x 2mm radial depth. I don't have the original data in front of me for what it should have been, but rest assured, the piston ring grooves were a lot wider.

We needed Imperial spec. rings and had been supplied with metric.

The rings need to be an absolutely snap-tight fit in the piston grooves or you will build an engine that will self-destruct on start-up.

(If you've been zoning out, think Star Trek and The U.S.S. Enterprise's warp engines having hair-line cracks in their Dilithium crystals built-in.......

What do you mean, "That hasn't helped"?........)

If the rings had been oversize, rather than under, it would have been possible to machine the grooves in the pistons to accept them, as Roger has a lathe and the skill to use it. But this was not to be.

A call to the marque specialist was made. Dave the parts man said he'd go and have a look at the original drawings and call us back.

Meanwhile, Roger and I dived over to Cottenham to pick up the freshly cleaned and painted engine block. (It was at this point he checked my work on the wear ridges was up to snuff).

We were just about to leave when my phone rang. It was Dave the parts man. I handed the phone straight to Roger as he really knows what he's talking about, whereas I merely bluff.....

The upshot of the conversation was this: yes, the rings were all wrong, apologies all round. However, Dave had found an original, new-old-stock set of 74mm plus 10 thou pistons complete with rings. If we would like them we good have them at a very reasonable price, less the sum paid for the 'wrong' piston rings, with carriage waived as Dave felt he'd messed us about.

Would I like them?

Cue sound effect of my wallet snapping open and shut so fast it nearly had Dave's hand off over the phone.

So, what does all this mean?

Well, it means brand new pistons!

We will hone out the bores the 10 thou required ourselves, pausing every now and again to check the fit with one of the new pistons.

And we remembered to ask for a set of locking tabs to replace the ones which will be U/S once we've bent them back to release the gudgeon pins.

But I must not be smug.

T'was none of my doing, this all seeming to turn out so well.

I detect the ever helpful shade of Grandad Eric in all this.

"The Lord of the Piston Rings", indeed.......



  1. FILE - files are for wimps

    REAL engineers would have used a hammer 'n' chisel and finished of with a lump chipped of an old mill stone


    1. Believe it or not, the mill stone option was considered, (albeit briefly!)

      The file may indeed be for wimps, but it did at least give a semblance of control over the removal of the wear ridges.

      I'm going to devote another blog post to the relation of "The Tale of Grandad Eric and The Knackered Morris Bullnose Engine", in which lump hammers make more than a guest appearance......

      Stay tuned!


  2. Reading your blog above we thought if you are looking for piston seals, then a good place to buy stuff is from our website darcoid

    1. Thanks! I had a look, but your product lines are flexible seals made from rubber.

      Ideal for hydraulic applications, I'm sure, but no good for for the pistons of a petrol engine!

      Thanks again, John.

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