running 10-1 pistons

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1972 Commando...10:1 pistons. How streetable would these be on city roads? the bike has a single 36MM mikuni. Problems?
 

Fast Eddie

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Depends what the fuel is like in your area. I run 11:1 in my 920 on pump fuel without issue, but high octane ‘super unleaded’ is easy to find here.
 
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I bought a Triumph Tiger that had 11:1 pistons in it, and for some stupid reason got the motor bored over and put stock pistons in it. Never ran as strong or sounded as good as it did with the 11:1 pistons. That said, fuel was a little different in 1970's.

I've run high compression pistons in street cars without a hint of detonation on pump gas. I would think 10:1 in the only hot two weeks out of the year Seattle would not be a problem on premium pump gas. I'm around 9:1 and no issues on a shaved Fred Barlow ported head, thin barrel base gasket 2S cammed 750. I know, not the same. Point is I sure would do it, but I'm not exactly the bell weather poster child for intelligent things to do.
 
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1972 Commando...10:1 pistons. How streetable would these be on city roads? the bike has a single 36MM mikuni. Problems?
Depends on your riding style. high cr is real noticeable at high rpm. with more cr you`ll get more forces to the cylinder head and the bolts. so....more likely to get the head blowing. Had my Dunstall pistons (10.5:1) (my moped is also a 1972..A combat at that time) trimmed for a less cr after some seasons. I like it going fast and doing many km on trips. But I am no racer pushing the bike constantly to the limit. Reliabelity is important for me. If you consider to change only the pistons ...only and just for higher cr...price-performance-ratio isn`t right. much work for little output. I wouldn`t do it...Although you are on the faster side of the riding style spectrum. if you want to go faster then I would rethink the whole operation....2 carburetors (maybe pumpers, FCrs, RS, Delorto)...maybe longer intake path...porting the head....decrease reciprocal weight...camshaft..... my 2 cents. But return to your question: of course it is streetable...
 
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Since you already have the domed pistons, use a thick barrel base gasket and copper head gasket if you want to knock some compression off. If you could find a good set of Atlas dished pistons you could probably get the compression down around 7:1 or lower. Stock P11 was 7:5:1, and the pistons were dished. Wish I would have kept those old dished pistons. I gave them to Phil Radford though, ... but I digress.
 

Onder

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Wasn't the idea of the 850 to give 750 performance without so much compression and stress on the engine?
 

texasSlick

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I ran 10.5:1 from the 60's until pump fuel octane became too low and I got tired of "nursing" the throttle.

I would think 10:1 would be streetable with modern fuels, particularly if you run 28 degree ignition advance. I was running 32 degree advance when I decided to change pistons to 9:1.

Slick
 
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Instead of trying to answer the question, and use what you have, here's another thought...

If I were actually in your shoes seattle##gs with a motor apart, I'd put lighter weight flat top pistons in it. Those domed pistons could be on the heavy side and not real vibration friendly in a Norton motor going up and down at the same time. Less reciprocating mass in an engine is always a good thing for me. That is just me though, and not helping at all.
 
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With modern fuel you should have no issues at all. Many modern 4 stroke 125's run far higher compression than what could sensibly be achieved on a Commando.
 

illf8ed

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1972 Commando...10:1 pistons. How streetable would these be on city roads? the bike has a single 36MM mikuni. Problems?
Other than the 36mm single Mikuni, is the rest of the engine to combat spec? 10:1 compression is as supplied for combats. I would not run a single 36mm it’s too large, but if it carburets OK for you then no problem.
If you’re concern is fuel, yes it could be a problem. I also have a ‘72 combat and in the last 25 years running it the engine pinging problem has gotten worse using premium pump gas. I’m using two copper head gaskets right now that lowers the compression to 130psi....180psi with one copper head gasket. The reason for two gaskets however is one of my shortened exhaust pushrods broke and I’m having to use four standard length rods until I can cut a new chrome/moly tube for a repair.
 
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I do not know anything about the motor other than what I can see with the cylinders and head off. The owner does not have a lot of money to work with and NO previous experience with Nortons. I would like to build for her a Norton that does not give problems but the unknowns for me are a 36MM Mikuni on a 750 and 10:1 pistons. Ideally I would go to the 34 Mikuni and a set of 9:1 pistons. I should also check the bottom end and see if the suspect main bearing has been replaced. Anybody want to buy a pair of hardly used Powermax pistons?
 

Onder

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Low compression means a longer lasting engine and probably one that is a bit more flexible. I cannot really see needing that extra amount of power for everyday riding and especially if the owner is skint because if you go high performance he soon will be.
 
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I do not know anything about the motor other than what I can see with the cylinders and head off. The owner does not have a lot of money to work with and NO previous experience with Nortons. I would like to build for her a Norton that does not give problems but the unknowns for me are a 36MM Mikuni on a 750 and 10:1 pistons. Ideally I would go to the 34 Mikuni and a set of 9:1 pistons. I should also check the bottom end and see if the suspect main bearing has been replaced. Anybody want to buy a pair of hardly used Powermax pistons?
Build for HER?? How to start it? Not much money ? Future rebuild cost? Keep it to the sensible / safe side of specs. is my suggestion.
 

gortnipper

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When I was a buyer/exporter of 70s and 80s bikes, a mate of.mine had an 86 Yamaha SRX600 to sell and he was out of state so I agreed to sell it for him. It has no estart and I found it easy to kick over.

The first people to come and look at it were a couple of guys on bikes with a pretty, slight but stacked young thing. It was for her. One of the guys started it rode around for 5 min, came back and she then went for a ride.

We made the deal, and while signing over the title and making the BOS, the dudes left. Slackers.

She couldn't kick it over, and i had to give her lessons for 20 mins till she could. She was all of 5'3"", and ended up so puffed she was in her singlet... what a view. :-D

Return it to stock compression unless she is a shot-putter.
 
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