Sq 4 mkii piston compression ratio

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pauldridge
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Sq 4 mkii piston compression ratio

Postby pauldridge » Thu Oct 27, 2016 1:10 pm

Dragan's pistons are back ordered so I am ordering from JP in Australia

They offer two different compression ratio.. roughly 6.0 or 7.2 (just going by memory from their catalog)

Need opinion on which choice to make
Phil Auldridge Austin, Texas - USA
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53 SQ 4 MK II, '72 Norton Commando, '78 BMW R100/7, '79 Honda CBX, 2014 Indian Chief Vintage

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Re: Sq 4 mkii piston compression ratio

Postby Knud.Degnbol » Thu Oct 27, 2016 4:42 pm

Take the low one.
You don't know how many times your cylinderblock and cylinderhead have been ground flat.

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Re: Sq 4 mkii piston compression ratio

Postby paul.jameson » Thu Oct 27, 2016 7:38 pm

Check out IMD Square Four pistons from the UK. These are recently manufactured but are very close to the original spec for the MK II, including their weight. Unfortunately, they are only available in 7.2 : 1 compression ratio but the manufacturer, when I spoke to him, told me that he had made the pistons with plenty of meat on the top so that they could be turned down to reduce the compression ratio.

Like Knud, I prefer lower compression ratios as they make for a much nicer engine.
Paul Jameson
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Re: Sq 4 mkii piston compression ratio

Postby david.anderson » Fri Oct 28, 2016 7:43 am

I can say that my mk2 4 runs 7.2 pistons and the head and cylinder were shaved a combined total of .070” to clean up pitting and rectify warpage. At that my compression ratio is up to 8.2 to 1 (with an otto headgasket and possibly a little higher with a standard headgasket). The engine is tractable down low and easily sits on 30mph in top gear (26 tooth engine sprocket). The engine also clearly puts out more power and will run away from a standard 4. But crankshaft flexure would be increased and the engine may run a little hotter. I have never ridden a low com 4 so can someone please fill me in on the advantages/ what is nicer about the low com engine.
David

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Re: Sq 4 mkii piston compression ratio

Postby paul.jameson » Fri Oct 28, 2016 10:49 pm

The classic answer is that a high compression engine feels harsher than a low compression one. The high compression gets you more speed (and more to the point, better fuel economy) but the low compression is more forgiving and smoother. This applies to all engines, not just Square Fours. I have had a couple of periods in my life when I have run 2 bikes with essentially identical engines, apart from the compression ratios. Both times I have found the low compression version the pleasanter to ride. I should add that I am no speed merchant so the extra 5 or maybe 10mph at the top end means nothing to me but I do like a smooth bike. My RH500 is low compression and lovely to ride, so much so that I find myself resting my left hand on my leg, rather than on the end of the handlebars. I have only ever done this with one other bike - also a low compression 500. But I ran the RH500 as a high compression 350 for many years (probably over 20) and never once in that time found myself with the left hand resting on my leg.

What is also evident though is that some people are much more tolerant of harsh, vibratory machines than others. I will not name names, but I can think of two very well known names in the Ariel Club and for each of those, I have ridden two of their bikes and wondered how their owners put up with them. Yet the owners in question did many thousands of miles on these machines. To be fair, my own KH vibrates badly - I know why and how to cure it but have yet to find the time to do so. Meanwhile, I avoid using it but its sheer convenience factor means I still do almost as many miles on it as my other bikes.
Paul Jameson
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Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
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Re: Sq 4 mkii piston compression ratio

Postby Brian.Fosh » Sun Oct 30, 2016 12:52 am

Hi,

I set out to use original NOS low compression concave topped pistons from an iron barreled SQ4 in my MK1

..but drags liners were annoyingly to std size for honing and left no room to accomodate the couple of thou reduced diameter of the earlier pistons.

... So I went for IMD pistons because T&L had mentioned they were impressed by their pistons and also because this forum suggested the IMD weights were closer to original. They also arrived really quick.

The machining, tolerancing, Piston ring types, offset gudgeoon pins and even the packaging left me very pleased with IMD.

Although I left them at 7.2:1 on my Mk1, I'm pretty satisfied so far that Coupled with T&L's diamond honing, club rods, Otto gaskets etc.. that I've an engine that incorporates many best practices from current times, not so much in terms of "design" but certainly in terms of "workmanship".

I've yet to ride the bike (tank came back from chromiing today.. and i'm very pleased with S&T's superb work)..

So far then, I have only had the opportunity to notice how quickly the piston friction in the diamond-honed bores reduced after just 5-10 minutes of gentle tick-over 'blipping" and how, despite the silencers being 6" off a white painted wall, there is no sight or witness of any smoke "whatsoever".

..Time will tell.

Regards, Brian

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Re: Sq 4 mkii piston compression ratio

Postby paul.jameson » Sun Oct 30, 2016 9:09 pm

Brian,

Please advise what time tells in due course.
Paul Jameson
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Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.

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Re: Sq 4 mkii piston compression ratio

Postby john.bebb » Wed Nov 02, 2016 3:24 pm

Phil - Would suggest the higher compression ratio - the motor will run a little cooler with these...

Knud warns to ckeck for clearance if the head & block have been skimmed (... possibly numerous times in their lifetime!)

... measure cam lift above base circle to ascertain whether the valves still remain within the head when fully lifted and work out the clearance you've got.

Cheers, John


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