Expanders for lower ring on pistons...

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James P
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Expanders for lower ring on pistons...

Post by James P »

One of the aspects of trouble Doug (bozbridge) was having with pistons (viewtopic.php?f=1&t=12634) has me intrigued; The breakage of the expander for the lower piston ring. I'd never given much thought to the expander up to this point - if it was supplied with a piston kit, I fitted it, assuming that the supplier knew best and would not supply it if it wasn't necessary. While piston rings are often of generic types, any piston should be specifically designed to suit the intended rings.

Unfortunately, it seems that Mitaka is now advising against fitting the expander supplied with its new-style KR-1 piston kit. I am wondering why an expander is supplied at all if it is detrimental to the operation of the piston and/or rings.
I haven't got any of these new-style Mitaka piston kits for the KR-1, so I can only make measurements of pistons which I have got (those which are supplied with expanders).

Anyway; I took some measurements from the following pistons:

KR-1 (Mitaka old-style, 1KT-based piston, marked 01-2010, with Riken rings)
KR-1 (Mitaka intermediate-style piston, actually branded "Aoki" on the box, perhaps the first of the Taiwanese production, with Riken rings)
TZR/TDR/R1-Z (OEM ART piston with Teikoku rings)
TZR/TDR/R1-Z (Pro-X branded aftermarket piston with Riken rings)
TZR/TDR/R1-Z ("SA" branded aftermarket piston with Riken rings)
SDR200 (OEM ART piston with Teikoku rings)
IT175 (Pro-X branded aftermarket piston with Riken rings)

Instead of taking up loads of space with measurements, I'll make a few statements of my findings. All measurements have been rounded-off to the nearest 0.1mm:

The thickness of the expander in every case (whether Riken or Teikoku) was 0.3mm.
Generally, the "horizontal depths" of the upper and lower ring grooves were the same.
The widths of the upper rings were about 0.3mm less than the horizontal depths of their associated grooves (i.e. after the piston ring becomes flush with the skirt of the piston, it can be pushed a further 0.3mm into the groove).
In most cases, the width of the lower ring was 0.3mm less than the width of the upper ring (i.e. adding together the thickness of the expander and the width of the lower ring gave the same result as the width of the upper ring).

There were two notable exceptions to the above; On the old-style Mitaka KR-1 piston and the "SA" branded TZR piston, the lower ring groove was an additional 0.2mm "deeper", meaning that the expander would be compressed less. I wonder if that lessens the chance of the expander breaking... :?

Could someone who does own one of the new-style Mitaka KR-1 pistons make some measurements to see whether this piston and its rings conforms to any of the general patterns mentioned above? With such measurements, it MAY be possible to decide whether the new-style Mitaka KR-1 piston has had its grooves incorrectly machined, or whether incorrect lower rings have been supplied, or whether there is some other reason that the expanders have broken (material too brittle possibly).

Thanks & regards,
James
DougB
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Re: Expanders for lower ring on pistons...

Post by DougB »

I'll get onto it this morning James, still got 2 pistons of the new type in the garage.. Have to dig them out of a storage box, might have to dig out the storage box, might have to move a bench to get to the box, might have to move a couple of bikes to get to the bench! Then I've got to get to the measuring gear :( :(

NEED A BIGGER WORKSHOP!!!!
DougB
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Re: Expanders for lower ring on pistons...

Post by DougB »

Ok I've measured the pistons as best I can. Top ring & groove first:

Ring width (id to od) - 2.24mm
ring groove depth in the piston - 2.5mm


Lower ring & groove:

Ring width - 1.8mm
Expander ring - .3mm
ring groove depth - 2.5mm

Approx though, close as I could.
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James P
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Re: Expanders for lower ring on pistons...

Post by James P »

bozbridge wrote:Ok I've measured the pistons as best I can.
Thanks for that Doug. Your measurements seem to indicate that the machining of the new-style Mitaka KR-1 piston is "correct" (based on my own measurements of a range of pistons, which I assume are correctly machined :? ).

Compared to the upper ring groove, there seems to be an extra 0.15mm space available in the lower ring groove (with all rings fitted and compressed flush with the skirt).

Is the expander perhaps too "long" for the piston? Does it bind up on itself or the ring peg when installed? If so, could improvement be gained by cutting a few millimetres off one end of the expander?
If the expander fits OK as it is, could the material be too brittle and prone to cracking/breaking in use? Anyone else got any suggestions as to the reason for the expander breaking on this type of piston, or on any other type of piston?

Regards,
James
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