Weight of original Mk2 Sq4 piston?

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Graham.Goral
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Weight of original Mk2 Sq4 piston?

Postby Graham.Goral » Thu Nov 22, 2012 11:00 am

Hi all, This post is on behalf of Eric Spencer who some may know from the Lancs branch...

Can anyone tell me the weight of an original piston for a Mk2 square four please, if anyone has one still lying around or they have the technical data? Eric is using later repro pistons but wants to try as much as possible to match their weight to that of the original, in order to achieve the original crank balance.

I guess it would be useful to know the weight of the gudgeon pin separately, as perhaps these will vary too.

Thanks :-)

Graham

bob cottam
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Postby bob cottam » Thu Nov 22, 2012 10:21 pm

Graham- Hi, I have my four down at the moment and its fitted with Hepolite +30`s, yep, +30 thou-odd size but they are as new and i can weigh them tomorrow if this is any good.Did Hepolite supply the originals? I suppose a piston manufacturer could maintain a constant weight for std/=20/+30/+40 etc by machining a bit more out of the skirt of the larger pistons, but did they (Hepolite) bother? It would be interesting to establish a "standard"(no pun) for piston weight for the MK2-or any motor for that matter.It would give something to work to.I am about to re-build poss with new rods from the club,which i believe are lighter than the original rods, which i hope will improve on the standard MK2 balance-i read somewhere Ariel didn`t bother re-balancing for the later rods thus MK2`s are not so smooth as standard as earlier fours? I`ll post the weights tomorrow, regards, Bob.

Graham.Goral
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Postby Graham.Goral » Thu Nov 22, 2012 11:04 pm

That'd be very much appreciated... Thanks! :-)

bob cottam
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Postby bob cottam » Fri Nov 23, 2012 7:57 am

Hi Graham, been out to the garage armed with electronic kitchen scales-should be accurate to within a Gramme i expect. Here`s what i got:
Gudgeon pins: 2@47.5Grms, 2@48Grms.
Pistons (+030 O/S with all rings and both circlips):228.5G, 229G x2, 230G. Thus total for each assy. 277-278Grms .Hope this helps,no idea how that compares with original factory ones..... Out of interest,what weight are the repro ones?,
Regards, Bob Cottam.

Graham.Goral
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Postby Graham.Goral » Sat Nov 24, 2012 11:17 am

Thanks Bob, I've passed this on to Eric. I'll get those weights for you.

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Postby Graham.Goral » Fri Nov 30, 2012 2:53 pm

Info from Eric... Apologies for the formatting

WEIGHT OF RECIPROCATING PARTS ( in grams)

CYLINDER NUMBER 1 2 3 4
Piston with rings 242 241 241 244
Gudgeons 47 51 48 46
Con Rods 319 320 319 319

Totals 608 612 608 609

Graham.Goral
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Postby Graham.Goral » Fri Nov 30, 2012 2:58 pm

More info from Eric...

Interesting that (Bob) does not have originals but they are still more than 10 grams lighter than mine (These were supplied by Drags in 2004 - unsure of the manufacturer).

The other very interesting fact about mine, is that at first I did not bother weighing the gudgeons, thinking they must all be alike , ground o/d, machined bore, controlled length, but as you will see from mine there is a big difference between them. More surprising is that the heaviest piston has the lightest gudgeon, so obviously the manufacturer is selectively assembling them.

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Postby nevhunter » Sat Dec 01, 2012 1:14 am

The SQ 4 is potentially one of the best balanced engined built. It would pay to get the figures right if possible.Nev

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Re: Weight of original Mk2 Sq4 piston?

Postby david.anderson » Sat Dec 01, 2012 7:49 am

Original 7.2 Mk2 std size pistons weight (with electronic scales)
Bare 188 -189 g
With rings 203 – 204g
with rings and gudgeons 251 -252g
While the square 4 is generally regarded as an ideal balanced engine you should check and make all pistons the same weight and check the balance of the crankshafts before building an engine that you run. I built my 4 and did not check the crankshaft balance. By the time I had the bike run in at 1000 miles I was so disappointed with the vibration above 55mph that I stripped the engine again to check balance. The front shaft required a 3/8" hole 3/8" deep to achieve static balance. The rear shaft was correct. That was all it took to make the bike unpleasant to ride. After the balancing the bike is very smooth.
David

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Postby Graham.Goral » Tue Dec 04, 2012 6:27 am

Thanks David :-)


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