SQ 4 piston advice needed

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pauldridge
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SQ 4 piston advice needed

Postby pauldridge » Fri Oct 07, 2016 11:46 pm

When my MK II SQ 4 crankshaft decided to part ways with one connecting rod, the rod managed to reduce the associated piston to bits of rubble.

The P.O. claims to have only ridden this bike 2,000 miles since he "overhauled" the engine back in the mid-90's. He even provided a DraganFly invoice for parts, including pistons. Sure enough, the remaining 3 pistons do look just like new. So, I ordered just a single piston from Dragan (.20 over to match the others).

Today, when I gave a close examination of the piston I find its design is considerably different from the ones they were supposedly selling in the 90's. The new piston doesn't have the diagonal slots cut in the upper area, AND, it doesn't have an arrow on top showing direction of installation:

oil lines (5 of 4).jpg


The one with rod attached is the earlier piston. I can't imagine that these pistons would weight anywhere close to the same, but can't check until I get the rod out of the first piston.

My questions:
1) Is this "new" design considered superior to the old?
2) should I just go ahead and pony up for 3 more pistons so I have a matching set? (these babies aren't cheap!)
3) Does it matter which direction I orientate the piston in the cylinder, since there doesn't seem to be a locating arrow on top?
4) Does it matter which way I install the rods on the crank (i.e., with reference to the bearing locating slots in both rod halves?

Any help would be appreciated.
Phil Auldridge Austin, Texas - USA
Visit the North America Ariel forum: http://arielnorthamerica.org/phpBB3/
53 SQ 4 MK II, '72 Norton Commando, '78 BMW R100/7, '79 Honda CBX, 2014 Indian Chief Vintage

david.anderson
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Re: SQ 4 piston advice needed

Postby david.anderson » Sat Oct 08, 2016 9:19 am

Phil
The thermal slot in the piston is where the failure will start with an engine that is ridden hard. So yes the new piston is superior but will require a greater piston to bore clearance. The thermal slots allow a tighter running clearance and are quieter when the engine is cold.
The most important thing is that the piston weights must be the same on the same crank. But 4 all the same is a lot better. the new pistons will require a minimal bore or possibly a cylinder hone for the greater clearance required.
It does not matter which way you orient the new piston
It does not matter which way around the rod goes, but once assembled, slipper bearings in the rod and rod cap should not be turned 180 degrees as the bearing has bedded in.
David

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chris.shearwood
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Re: SQ 4 piston advice needed

Postby chris.shearwood » Sat Oct 08, 2016 12:48 pm

The pistons that I bought from Draganfly for my Sq4 quite a few years ago have no arrows to indicate orientation but the gudgeon pins are offset. In 2004 Knud Degnbol wrote in the club e-mail chat group of a simple way to check for offset. I quote him:

" If there is an offset, it will be around 1mm. It is most easily discovered by laying a piece of paper around the piston, mark the pinhole on the paper with a pencil and turn the paper to the other side of the piston. Then you can easily see, if there is a difference."

If your piston has an offset, the pin should be offset to the thrust side.
Regards, Chris
1946 4G, 1950 NG and 1951 VH

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Re: SQ 4 piston advice needed

Postby john.bebb » Wed Oct 26, 2016 11:29 am

Phil; - I'm dubious of David Anderson's comment "It does not matter which way you orient the new piston" ...

IF there is an Arrow - it is there for a reason!!! - either the gudgeon (wrist) pin is offset or the construction of the piston crown or skirt has been biased for expansion or loading on the thrust side. There are pistons without any pin offset but with construction bias.

Incorrect orientation of Arrow marks will certainly increase the likelyhood of failure...

Orientation requires a little forethought on a Square - the REAR crank may be considered as a "normal" twin or single - the thrust side is the direction of rotation as viewed from above... i.e the Arrow to point forward. However on a Square the FRONT crank is reversed in rotation; - as the thrust side remains the direction of rotation as viewed from above the Arrows on the FRONT pistons must point backwards! - Think about it!

So; - if you fit a set of Arrowed pistons to a Square when viewed from above ALL Arrows must point TOWARD the camshaft. HTH, John

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Re: SQ 4 piston advice needed

Postby david.anderson » Thu Oct 27, 2016 7:09 am

John
Paul stated “Does it matter which direction I orientate the piston in the cylinder, since there doesn't seem to be a locating arrow on top?” refering to his singular new piston. In response to that I replied "It does not matter which way you orient the new piston" with "new" clearly being stated in my reply . So my point is quite clear that advice only relates to the new piston.
If there is no arrow then it should not matter which way to orient the piston, unless like Chris stated he had come across pistons that the gudgeon was fitted out of centre. In such case the piston manufacturer has failed to mark the piston so that should be a rare occurrence (but worth checking). The pistons that I obtained for my 4 were old stock Hepolite and they have a central gudgeon, they are thermal slot and they have direction arrows which I followed.
I have not seen offset gudgeon pistons for many years and have never seen a newly manufactured one. The practice was discontinued as it is bad engineering practice. An offset gudgeon increases vibration and bore wear. In fact in the preparation of some race engines, the piston is balanced before fitting and the weights of all pistons are matched. The piston in my VH is balanced as below.
This is a photo from the net showing how to balance a piston. the piston is first mounted in a lathe and a centre drill indent is put in the underside of the crown. The piston is then mounted on a pin and when it hangs plumb it is in balance, provided of course that the gudgeon is central. Note the small balance weight on the piston. That weight should be removed from the other side of the piston.
David

piston balancing.jpg
piston balancing.jpg (32.74 KiB) Viewed 1148 times


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