Norton commando 750 1972 piston compression and brand help

Hi there,

I am about to rebore my barrel and I am looking for new pistons at the moment I have on a pair of Hepolite +20". I could not understand which is the compression ratio of my motorbike since it changed over time across the same model. In addition, I would like to fit the genuine Hepolite +30" but it seems like the oversizes are +/- 202 (+20", +40", +60" etc etc). I could not find the +30" oversize.

Is it 8.9:1 or 9:1 or 10:1 ?
In case, there would be a choice among these ratios which one would you suggest? and why?

Thank

Francesco

Re: Norton commando 750 1972 piston compression and brand he

Hi, welcome. Do you NEED .030" to clean bore damage?

Re: Norton commando 750 1972 piston compression and brand he

There have been no .030 oversize pistons made for quite some time.

Hepolite pistons all have the same compression height, the compression ratio was determined by the surface cut on the cylinder head. Jim

Re: Norton commando 750 1972 piston compression and brand he

+.030 pistons are available from the usual suspects in the UK but they are GPM not Hepolite copies (Wassell, Emgo, JCC, Teikoku, JP, whatever). The GPM pstons are OK but the one piece oil control rings are somewhat heavy. Atlas type three piece oil control rings can be used but these are quite narrow. The best solution is to cut the bottom groove a little deeper and fit the late type 3 piece oil control rings.

Re: Norton commando 750 1972 piston compression and brand he

With modern fuels having low octane rating, it would be best to go with 8.9 or 9.0 (no significant difference between these).

If you do not have bore damage, or undue cylinder wear , stay with +20. You can always bore again in the future....you cannot go back except at great expense.

Re: Norton commando 750 1972 piston compression and brand he

1972 was Nortons death stroke by Combat Bombs and a lot if not most of them sold were Combats. if 2S Combat cam then it needs 10CR, if std cam and 10CR its detonation prone. If 9CR + 2S cam then is soggy low down to give the great 2S cam a bad reputation as gutless down low. Anything could of been done to it the last 4 decades. 10 CR has CHO milled head so fin gap is less at seam than other fin gaps, there should not be a base plate and the gasket should be FlameRing thin. Copper kind works ok just lowers CR a couiple % points. Going up in bore size alone gives more pump volume so ups CR a percentage point or so. I love Combat spec sensations now and then but will wear more if enjoying its 2S kick all the time, ie: deep into 6000 and on into red zone.

Glad to know there is a source of .30 over jugs/rings but what difference does it make if somewhat heavy or weaker than Hepolites ->> if ya don't know about Norton cam + CR reasons vs octane, then likely not wring the snot out of it to benefit or enjoy a Combat 2S 10: CR set up or any extra reserves in piston heating or rpm tolerance, nor do I for that matter on otherwise bone stock Cdo, Combat or not. I'd say get the .30 so next owner that wears it out can get 2 more over bores out before barrel is memorial decoration weight.

Re: Norton commando 750 1972 piston compression and brand he

IF +.030 over pistons are available, does anyone make +.010 over pistons? I recently discovered that my 750 was bored .010 over however they fitted std pistons. :roll: Maybe that's why the top ring was broken on both pistons.

Re: Norton commando 750 1972 piston compression and brand he

sidreilley said:

IF +.030 over pistons are available, does anyone make +.010 over pistons? I recently discovered that my 750 was bored .010 over however they fitted std pistons. :roll: Maybe that's why the top ring was broken on both pistons.

Click to expand...

I don't know of anyone making .010 OS pistons. You might find some NOS somewhere.
The only currant production I know of is standard, .020, .040, .060. Jim [I have seen some .080 but I wouldn't use them]

Re: Norton commando 750 1972 piston compression and brand he

I had a 30 over set not too long ago. It is too fussy to maching out only .010" and honing that much is asking for trouble with an inconsistant bore. In theory, you will have to do .010 again to go to 40 over. That doesn't make much sense either so if you did go to 30 over from 20 over you will need to make the big adjustment sooner or later.
If your bore is so bad that that you need to go from stock to 30 over to clean up, then ya better go to 40.

Re: Norton commando 750 1972 piston compression and brand he

comnoz said:

There have been no .030 oversize pistons made for quite some time.

Hepolite pistons all have the same compression height, the compression ratio was determined by the surface cut on the cylinder head. Jim

Click to expand...

ah ok so the compressions ratio 10:1 that my norton is supposed to have is not determined by the pistons, but instead only by the cut of the cylinder head?

Having said that, why do I always see pistons' descriptions reporting the compression? sometimes 8.9:1 or 9:1 or 10:1?

:?: :?: :?: :?:

Re: Norton commando 750 1972 piston compression and brand he

chiccogarz said:

comnoz said:

There have been no .030 oversize pistons made for quite some time.

Hepolite pistons all have the same compression height, the compression ratio was determined by the surface cut on the cylinder head. Jim

Click to expand...

ah ok so the compressions ratio 10:1 that my norton is supposed to have is not determined by the pistons, but instead only by the cut of the cylinder head?

Having said that, why do I always see pistons' descriptions reporting the compression? sometimes 8.9:1 or 9:1 or 10:1?

:?: :?: :?: :?:

Click to expand...

Aftermarket pistons can have different deck heights. Older manufacture pistons such as Power Max pistons have a dome and are considerably higher compression. Custom forged pistons can be had with any compression height you would desire.

All Commandos came from the factory with a standard deck height piston - including the Combat. The different compression ratios listed were due to the head cut or the presence of a cylinder base gasket. The common piston made by JCC and sold under many brands all have the same compression height as the factory piston. They may quote different ratios depending on the specific application. Jim

Re: Norton commando 750 1972 piston compression and brand he

Good question but as Jim says it the stack height of barrel and head that determine how far up in chamber the pistons go for how high the static CR. I know Combats all have pistons stick up .050" proud of the cylinder top so pass through the gasket which better not rub on the pistons. The hi end after market speicalty pistons do come in several CR's so these do have different crown thickness or pin placements and valve cuts to alter CR w/o changing the stack heights, unless a collision of course. W/o much trouble about any 750 can be built from ~8.5 to ~10.5 static CR by base plate and top gasket and or by milling. Off the shelf 750 over bore pistons will leave your 750 CR pretty close to what it was issued with. Just a few years ago I was pretty pensive on my plain Jane Combat sans base gasket but with copper gasket, detonating in summer heat Mt climbs on rural 87 octane but not no more with just normal avoidance of lugging on high throttle hi gear. In practice likely can't hardly tell a 9 or 10 CR difference w/o fine tuning carb and exhaust for each configuration but either one dialed in will certainly please you and surprise the moderns with its performance up to the ton or so. JSM has both pistons and gasket thickness to make life easier for those wanting to spend for it.

Its not that hard for a good machinist to creep up on what ever bore dia the pistons need down to .o00th's and even finer by the hone finishing. My Trixie came with .040 barrel which I had touched up a few years ago on complete rebuild of old comma oil hole pistons and Aluminum gasket [unknown till destructed at easy cruise] then again after rod bolt broke at 50-ish coasting looking at fields of flowers, then again when carb boots cracked to let THE Grit in yet her bore are still a few .000"th's under size for std .040 over specs. Last ring gaps [3rd set] out the box were .122" which is just right for 750's.

On Ms Peel also .040 over her bores ended up too big for the pistons I had so had hot rod shop knurl them and worked great for ~ 10k miles till over rev event messed her up. Pistons had rubbed off much of the knurls by then but boy howdy was she ever pleasing w/o rattle or smoking or blow by leaks with just a cheap PCV in breather hose near tank.

I say make a long suffering vendor happy to unload the slow moving .030 kit, maybe at some discount bargaining on that issue, and seek out a decent equipped skilled machinist and get some bragging rights on extending the life of limited barrel. Leave base gasket out and fit good FlameRing head gasket for a bit over 10 CR and a purring to SNorton Combat by a bit of spark time diddle and carb and float level fiddle for the octane available. Each change of stacked items changes the push rod length effect or where rockers land on valve stems so might consider that extra effort diddling but I never bothered and stem wear lasted as expected and rev'd into Peel red zone routinely where I dared. I do not run my stock Combat like that as Peel had other stuff done to stand it.