About 40 years ago the owner of my 19S decided to over bore it to 85mm and fit a pre-war Triumph T90+40 piston, giving it a capacity of 641cc, a silly compression ratio of near 9:1 and a few other "go faster" mods equally out of character with its original concept.
Now more than 20K miles later, the last 2.5k with me, time to return it a less challenging original spec by fitting a custom made liner and standard 82mm piston. I'd already replaced its TT Carb with a 32mm Monobloc.
Original Hepolite pistons seem long gone and I have just taken delivery of a Gandini GMP piston as shown below.
The supplier assures me that it is correct for a 1957 19S but I have never seen a Norton OHV piston with a concave top and no valve cut-outs. The M50 and ES2 pistons that I have worked with, and the T90 one already fitted have a slightly domed top and valve cut-outs. Without fitting it to prove otherwise, the concave depression does not seem deep enough to achieve valve clearance. The supplier says the original Hepolite catalogue lists a concave piston, but Parts Lists show otherwise !
Any thoughts appreciated.
The Gandini piston shown looks very like the original fitment Hepolites. They didn't have valve pockets either. 82mm liners are (at least they were a couple of years ago) available from RGM. If you want a little more compression you should still be able to find compatible NOS Matchless flat top pistons with valve pockets (for G80, std bore 82.5mm). I am using one of those in my 19S. I would recommend comparing your Gandini piston weight with an original Hepolite - around 447g. If the new piston is more than just a few grams heavier you might want to consider the balance factor. Standard Norton balance factor 50%. When changing to the Matchless piston I rebalanced to around 63% which on my bike makes things pretty smooth from about 55mph upwards. With the mods you have described the crank may well have already been rebalanced to suit the Triumph piston so check for additional holes drilled either side of the crank pin.
Further to Ian McD's details, all the '50s Model 19 pistons were concave, pre-war were flat top. The reason there are no valve clearance cut-outs is because the piston stops about an 1/8th inch before the top of the cylinder to keep the compression low, so there is no need for them.
Ian is correct about the GMP pistons as they are usually heavier than the originals, and as he indicates, the higher the engine speed the more balance weight is needed.
Thanks for some very useful information which helps to explain some of the problems found with my over capacity / over compressed 19S engine.
I decided to go down the Matchless G80 82.5mm piston route and have now acquired one of these and a bespoke Westwood liner in the pipeline to accommodate the present bore. I know RGM had liners some years ago but these were not big enough to fit the already over bored barrel, the reason I didn't return the engine to standard spec at the time when I stripped it for inspection. Interestingly there was no evidence that the crank had been balanced for the heavier Triumph piston, no wonder I've lost a few teeth fillings over the last couple of years! Time now to sort things properly.
Contrary to popular opinion I found that the standard Gandini piston with pin and clips was actually lighter than the others mentioned, details below:
Gandini Std. 82mm 433g
Original 19S (I.McD) 82mm 447g
Matchless G80 Std. 82.5mm 480g ** (456g with a Norton pin)
Triumph T90+40 85mm 500g
Thanks again for your help.
** Postscript: having subsequently removed the Matchless gudgeon pin from the piston to ream the new small end I see that whilst the same diameter it is a different design with a narrower constant internal diameter and much heavier than a Norton one, weighing 109g against 85g It is also slightly longer, 70.6mm rather than 65.8mm. Machining a 7º internal taper each end, á la Norton, reduces weight to 89g making the piston weight above nearer to that of Ian McD's.
Hi Peter, the tapered bore Norton gudgeon pin does save weight. I had the Matchless pin modified with similar internal tapered profile. The weight figures you've quoted should result in a balance factor close to 50% so rebalancing not essential. If you do want to consider increasing the balance factor I could let you have a copy of the spreadsheet I used to calculate the amount of weight to be removed and the size of holes needed to achieve the weight reduction. Just drop me a PM if interested.
Cheers, Ian McD