Piston fitment

[Forumadmin]

Malcolm Clegg phoned me up and we had a long discussion on stepped rings and arrows on pistons.
My initial recommendation of putting the step upwards on the top ring may not be always correct as MC thinks the chrome ring should be on top. The step IMHO was so the step on a worn bore would not foul a new non-stepped ring. I looked at my many pistons Triumph, Ford and Jowett and have no stepped rings.

This is from Mike Allfrey's Tech Notes part 12 section 50

In the instance where split skirt pistons are being installed, careful note should be made of the word ‘FRONT’ stamped into the top face of the piston. In a horizontally opposed engine, the thrust side is opposite on each bank of cylinders and, great care must be taken during the task of attaching the connecting rods to the pistons.

MC's question was his pistons have an arrow on them so how should they be installed?
The above explanation by Mike does not fully describe what should be done! Are the pistons always installed to the front of the engine in a horizontally opposed engine?
If there is an arrow on the piston is this the same as front?
If the arrow is at 90deg to the gudgeon pin is this indicating the thrust side of the piston?
Is the thrust side the side the skirt split on or not?
see the answer here The split is on the opposite side to thrust. Fig 2.48 explains what is the thrust side and Fig 2.62 the split skirt.
Some pistons have an offset gudgeon pin whose narrow side should be on the thrust side.
The thrust side is that portion of the piston skirt which carries the greatest thrust load. This is on the right side when viewing the engine from the flywheel end with the crankshaft rotating counterclockwise. That definition is for the Jav engine but translates into the top of the nearside bank of cylinders (1&3) and bottom on off-side bank (2&4).

So does that answer where does 'front' go and where does the 'arrow' go on each bank of cylinders?

[Mike Allfrey]

Hi Keith,

Have just read this post and wish to make the following points:

1. All (to my knowledge) genuine Jowett Hepolite pistons were marked 'FRONT' and, therefore, that "side" of the piston should face the front of the engine - crankshaft pulley end.

2. I have never seen pistons marked with an arrow, with respect to Javelin/Jupiter installations.

3. I have never been involved with Ford, Mazda, VW, BMC or Triumph piston conversions, so am not qualified to comment there.

4. In Oz, this is all a bit academic now, because we are now using solid-skirt pistons from JP Engineering Products in Adelaide. The reason we opted for solid skirts was to eliminate the concern where crowns separated at the oil control ring groove. When JP cut the split into the skirt, they also cut a slot into the oil control ring groove. This slot ended with very sharp corners and hence with fatigue, the crown would separate from the rest of the piston - usually miles from home base! Our current pistons now have no split skirt and drilled drain holes in the oil control ring groove. So far these pistons are successful.

I suppose it is best to get hold of a genuine Hepolite piston, clean it up and keep it for reference. It can then be taken for walks around the engine being assembled so that correct split skirt orientation can be established with supreme confidence!

Best wishes,

Mike A.

[Leo Bolter]

When our Jowett engines were reconditioned, I had used pistons from an inline engine (Mitsubishi) which MUST have the existing word "front" stamped on them obliterated. This is especially important as these pistons are the type that has a offset gudgeon! When used in a flat four design two of the pistons must be fitted "upside down" in comparison to the other two and so not only must the front of them be re-identified, but the piston should be numbered to it's cylinder position (ie. cylinder/piston 1, 2, 3, 4.).

The information in this diagram might be helpful:




The theory behind designing pistons using a offset gudgeon might be of interest too:




The written word may be helpful as well . . . .

http://60.234.149.204/Leo/Forum_Files/J ... n_Pins.doc

If using ordinary* unmarked, (or marked with front, inline pistons) that are split, I would suggest that they are re-marked as suggested above when they have been oriented so that the split skirt is on the "least thrust" side of the cylinder/piston . . .

Mind you, I'll happily stand corrected . .

* that is, the gudgeon is NOT offset.

Cheers.
Leo

[Mike Allfrey]

G'dday From a Wet South,

Thanks Leo, for a good explanation. I take it that the terms 'LHS' and 'RHS' relate to the view when sitting in the driver's seat and looking forward.

I think my 'quote' should be qualified a little. When I was referring to the stamped 'FRONT', I meant either genuine Hepolite or our local JP Engineering pistons with split skirts.

There was one point that I neglected to refer to in my last response. It is my understanding that, piston rings of good quality and must be installed a certain way, have an ink printed marking 'TOP' on one face. The face marked 'TOP' must then face the crown of the piston. However, with rings coming from all sorts of places these days, maybe that industry norm has disappeared like several others have -- to my dismay.

Kind regards,

Mike A.