850 Bore Marking & Smoking Exhaust

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My original thoughts on the culprit for the smoke upon start up was the inlet valve guide or seal, hence I removed the head for further inspection. I found a lot of oil in and around the inlet port throat and the top of the piston too, but of course after seeing the barrel bore i had to go deeper. I would say there is a fair bit of play on the inlet valve guide, however the standard issue seal looked intact, hence definitely one source for the smoke although cant say if bore/piston is contributing. Piston ring gap couldn’t help either, see below

Next step will be to clean up the bore probably with just a quick/light hone to see if that marks disappear. Once clean ill check the ID’s with a 3 point bore gauge to establish if serviceable.

The fact that measuring the piston ring gap gave me .025” in a used part of the bore and also same in an unused part of the bore suggests little wear, well apart from the ring gap which I believe should be probably half of this.
 
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Lovel said:
Next step will be to clean up the bore probably with just a quick/light hone to see if that marks disappear.
Click to expand...
Try the hydrochloric acid on the marks first, just test a small area first, if it fizzes then your bore will end up with less being taken out by a hone. Honing on an aluminium contaminated bore will mess up the stones and cause more issues.
 
Lovel said:
My original thoughts on the culprit for the smoke upon start up was the inlet valve guide or seal, hence I removed the head for further inspection. I found a lot of oil in and around the inlet port throat and the top of the piston too, but of course after seeing the barrel bore i had to go deeper. I would say there is a fair bit of play on the inlet valve guide, however the standard issue seal looked intact, hence definitely one source for the smoke although cant say if bore/piston is contributing. Piston ring gap couldn’t help either, see below

Next step will be to clean up the bore probably with just a quick/light hone to see if that marks disappear. Once clean ill check the ID’s with a 3 point bore gauge to establish if serviceable.

The fact that measuring the piston ring gap gave me .025” in a used part of the bore and also same in an unused part of the bore suggests little wear, well apart from the ring gap which I believe should be probably half of this.
Click to expand...
Have you checked the tappets drain is clear from the top, right rear of the head.
 
Tonight’s report. Two types of piston fitted A & B both standard.
850 Bore Marking & Smoking Exhaust


I’m assuming that the bottom of the barrels are marked A & B to suit each respective piston? Each piston was fitted this way anyway.
850 Bore Marking & Smoking Exhaust


I tried removing any aluminium smeared on the bore wall but none came of so I then proceeded to hone at a slow speed with some light oil, and the scuffs started to vanish, the remains scuffs cannot be felt tbh, and I’m tempted to leave as is for now. I’ll measure the bores accurately in next day or so.
850 Bore Marking & Smoking Exhaust


Manufacturers Piston skirt spec should be OD of 3.028 -3.0271”

A - piston measurement OD at bottom of skirt was 3.0235” quite a bit out of spec. To be expected.
B - piston OD 3.027” I’d say close enough for an 8,000mile bike?


I then placed piston B in bore A just to see what clearance there was with a feeler gauge on the skirt and the bore = .003” tight sliding fit, correlating the two suggesting there isn’t much wear?
850 Bore Marking & Smoking Exhaust


Then fitted piston B into its own bore and got .003” feeler tight sliding fit between skirt and bore.
850 Bore Marking & Smoking Exhaust


Checking ring gap with ring from piston B in bore B I got .020” which again is out of spec which should be .010-.012”
850 Bore Marking & Smoking Exhaust


So pending a proper bore measurement in the next day or two and perhaps a little more cleaning up of bore A perhaps I just need 1 new std piston and a set of new std rings?
 
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It'd be a bit like fitting One Shockabsorber . Tho Std. Size you would need to be carefull you dont end up over clearance .
Looks like the real stuffs still ' on the shelf ' there . Many Sizes .

850 Bore Marking & Smoking Exhaust


259 bucks . Inc. rings & gudgeons ! . Dosnt pay to be a skinflint .theyve 20 over too .
https://www.classicbikespares.com.au/product/piston-set-standard-norton-commando-850-hepolite/

Of course the sizing comes into it again , clearances . WOULD PAY TO GET A MEASURE .
Dunno if there was ever a , say , 2 thou. oversize , where a vigorous honing gets the bore ' to size ' to match the piston .

jsmotorsport.com

Lightweight Pistons & Long Rods

The best piston/rod combination on the market. Our lightweight pistons (1/3 lighter than stock) are forged aluminum, making them stronger than original cast aluminum pistons.
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drivle on piston / cylinder fit .

850 Bore Marking & Smoking Exhaust

archive.org

Cycle Magazine USA 1979-02 Volume XXX No 02 : Free Download, Borrow, and Streaming : Internet Archive

Cycle Magazine USA 1979-02 Volume XXX No 02
archive.org

Now if a Nortons set up properly , it should eat onne of them for breakfast .With a bit of other work . :(
 
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Matt Spencer said:
Looks like the real stuffs still ' on the shelf ' there . Many Sizes .

850 Bore Marking & Smoking Exhaust


259 bucks . Inc. rings & gudgeons ! . Dosnt pay to be a skinflint .theyve 20 over too .
Click to expand...
That looks very like the artificially distressed box that Wassells’ “Hepolite” branded Taiwanese pistons come in.
 
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You really need inside and outside micrometers or at least an inside caliper to measure piston clearance. Feeler gauge is not accurate because clearance should be measured at the center of the skirts and the same spot in the bore.
 
Danno said:
You really need inside and outside micrometers or at least an inside caliper to measure piston clearance. Feeler gauge is not accurate because clearance should be measured at the center of the skirts and the same spot in the bore.
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That is my plan as mentioned earlier. I was using rudimentary measurement devices just for reference.
 
Rudimentral Fred Flintstone ' Comparative ' measurements can be fine . For Farmers with Arc Welders .

A ' U ' steel ( say three bits of 1 x 1/8 in. ) and a ' Stick ' ( Steel Rod )

A ridgid U & a feeler gauge , along with a felt pen or notebook & pencil , Can get good comparitive outside comparisons , for equality ! disparity !

A Steel Rod or Two , cut , ends deburred / radiused , will check bores for ' how round ' along across & up & down , with tightness / loosness .

If you measured a accurate parrallel U , and steel rod or two , down the local machineary shop , you could start getting pretty accurate .

Just to put a point across ! :p

---------------------------------

Cave at emptor ! Gee Willikers ! The muddle aeroplane crowd went to Taiwan . Ended up top line .
But name changes , and financial deals etc . No Longer There ( or elswhere ) .
Seemed to be who was at the helm ( and hip pocket ) IF THEY INVESTED IN THE EQUIPMENT
they could make top line stuff .

Think its worth doing a 100 % ' matching ' job . But investigating the ' bottom of the barrel ( figurative - Sump Plug ) to see if ' standards match ' .

GRADED !? did Norton ' do their own pistons ' ? .
 
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I shall be using one of these three point Bower bore gauges measuring at different points in the bore and recording in a table form. It just takes a little while to get this in place due to it being a favour from someone.

850 Bore Marking & Smoking Exhaust
 
Lovel said:
I shall be using one of these three point Bower bore gauges measuring at different points in the bore and recording in a table form. It just takes a little while to get this in place due to it being a favour from someone.

850 Bore Marking & Smoking Exhaust
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"Intramic" here, ok, but not what a dial bore gage accuracy level is.
 
Have the place that can do the boring (if needed) do the measuring. They will know what they are doing and have accurate calibrated measuring tools. Buy pistons only after measurements are made per their recommendation. Do not assume it is just one oversize up. The bores can wear very unevenly from the center axis. It can all be on one side so sometimes you need to go at least twice the measured wear to have it end up on axis and round. I recently went thru this with a Kohler big single.
Forget about using any spring loaded T bore measuring tools. They and the large micrometers can introduce errors. I learned this the expensive way.
 
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And there are two tolerances for bores.

1. The original tolerance band for a new bore with new pistons and rings which is the Workshop figures.

2. A wear tolerance for the bore, which is hidden in the text of the Workshop manual and I think refers to a 5 thou limit over top limit for a fresh bore. You better read the WM to be sure, heaviest wear is typically just below the top travel of the rings.
 
Batrider wrote: "Forget about using any spring loaded T bore measuring tools. They and the large micrometers can introduce errors. I learned this the expensive way."

Micrometers and telescoping or snap gauges will work fine if you know what you are doing. They require touch and experience, which you can't get watching a video on the internet or pulling them out of their boxes once a year.
Do the shops who do the boring and honing to size use different measuring tools? Sure.
But that is not what we are talking about here, as far as I can tell.
 
Lots of differing opinions here on measurement by the look of it.;)

My initial readings from the Bowers gauge gave me some weird readings and also good readings, so need to revisit and also try it with a T bore type too.

I have no access to any fancy boring shops, well none that are geared up for any tight tolerance machinery, where most cater for the agricultural side of things.
 
kommando said:
And there are two tolerances for bores.

1. The original tolerance band for a new bore with new pistons and rings which is the Workshop figures.

2. A wear tolerance for the bore, which is hidden in the text of the Workshop manual, and I think refers to a 5 thou limit over top limit for a fresh bore. You better read the WM to be sure, heaviest wear is typically just below the top travel of the rings.
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The two specs kommando refers to are commonly called:
New Limits - what the factory and a machinist doing a re-bore work to.
Service Limits - what a mechanic [aka engineer] uses to decide if the current parts can be re-used, or if a re-bore is needed.

New limits for Commando pistons suggest to me that they use Hypereutectic [12.5 % or more Silicon] alloy to reduce thermal expansion. Anyone know what alloy is used?
 
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