Letter F on pistons

[Ghaur]

Hello to all,
Does the letter F engraved on the face of the pistons of my TR3A of 59 means that they are in standard size ?
These pistons are also engraved and numbered in order from 1 to 4.
Thank you.

[Ransomes256]

I would think they are numbered from a previous rebuild. 

The letter F may be a guide as to the orientation (front) of the piston by the builder.

Neil

 

[john.r.davies]

I believe that, Triumph machining tolerances being no higher than any other part of industry at that time and bores therefore being different sizes, a selection of slightly different sized pistons were provided for assembly, labelled with stamped letters .     So if this is a stamp, then it may be a larger (or smaller - I don't know which way the letter order went) original piston.     But if the letter is engraved then more probably added by a previous rebuilder.    Stamping is much quicker than engraving - you would never have the latter on a production line!

John

[RogerH]

Tolerance size - F, G, H  -   I think the range was.

 

Roger

[stuart]

17 minutes ago, RogerH said:

Tolerance size - F, G, H  -   I think the range was.

 

Roger

Correct.

Stuart.

[Motorsport Mickey]

+1

As Roger says, this is typical in “ batch” engineering, the liners were made and came out to have a small difference in finish diameters ( maybe fractions of a thou). So they would be graded F G H from bottom of the allowed range to the top, ( or the other way round) then the pistons were made and came out to have a small difference in finish diameters (maybe fractions of a thou) and these are graded also to correspond with liner diameters. This allows the best operating fit at a reasonable cost without the costs per unit becoming unreasonable.

The tools making any units are “indexed” meaning their cutting faces ( which wear when machining 10s of thousands of components… even tungsten when cutting aluminium) are closely controlled. But unless a finish surface needs the best precision finish with another component ( and piston/ liners don’t) then a small amount of tolerance can be allowed, which helps reduce the costs of the unit.

Mick Richards

[Charlie D]

I've seen this on Herald pistons as well. Is it just a Triumph thing to use F, G and H ?

I wonder why not  A, B, and C?

There MUST be a reason...

Charlie.

 

 

[Motorsport Mickey]

1 minute ago, Charlie D said:

I've seen this on Herald pistons as well. Is it just a Triumph thing to use F, G and H ?

I wonder why not  A, B, and C?

There MUST be a reason...

Charlie.

 

 

Charlie...get a hobby 

Mick Richards

[RogerH]

Hi Charlie

for the same reason that electron shells start at K and stop at O  - but there are no elements with an O shell.

 

Roger

[Charlie D]

Mick,

I’ll have you know that I already partake in a hobby.

I spend my leisure time trying to unravel the intriguing mysteries of life.

(Why Triumph pistons are graded F, G or H, rather than A, B or C is one such mystery)

One day I’ll return with the answer.

Roger,

Electron shells? Do you mean the fiberglass body shells made by Fairthorpe in the 1950’s. I believe that they had Standard Triumph engines, so no doubt they had pistons marked F.G or H.

The chassis were made of a magnesium alloy, called “Electron”.

But I have never really understood why it was called “Electron”.(Another great mystery of life.)

[RobH]

28 minutes ago, Charlie D said:

The chassis were made of a magnesium alloy, called “Electron”.

Actually the metal is called Elektron  - viz Wiki:

"Elektron is the registered trademark of a wide range of magnesium alloys manufactured by a British company Magnesium Elektron Limited."

https://en.wikipedia.org/wiki/Elektron_(alloy)

 

However, Roger was referring to the orbit bands of electrons around the atomic nucleus, known as shells:

https://en.wikipedia.org/wiki/Electron_shell. 

 

 

Edited March 18, 2022 by RobH
typo

[Ghaur]

I have to change the segmentation.

Do I have to order standard size segments?

[Motorsport Mickey]

2 hours ago, Ghaur said:

I have to change the segmentation.

Do I have to order standard size segments?

I think we are all wondering “is this a really obtuse comment which relates to differential piton to liner clearances” … or what is it ?

Mick Richards

[james christie]

If it helps:

'segments' in french = piston rings

 

james

[BlueTR3A-5EKT]

3 hours ago, Ghaur said:

I have to change the segmentation.

Do I have to order standard size segments?

First step is accurately measure the cylinder liner bore.
If 83 or 86 mm they are standard.   This would tie in with the F, G, Or H  grading you have found on your pistons.

Oversizes go +0.020” ( about 0.5mm)  + 0.030” (about 0.75 mm) +0.040” ( about 1.0 mm) 

Is there a step at the top of the cylinder bore where the old piston ring/segment has run?   If so you will need stepped or ridge dodging or Apex ringset with a stepped top ring to avoid that cylinder ridge.   If you fit regular rings you risk breaking the top ring the first time the engine runs.  
 

https://www.thn.fr/en/product-line/piston-rings/piston-ring-types

 

 

Peter W

Edited March 19, 2022 by BlueTR3A-5EKT

[Ghaur]

3 hours ago, james christie said:

If it helps:

'segments' in french = piston rings

 

james

Yes, correct and sorry...