Piston Skirt Clearance

Banzaibob816 · Thursday at 5:09 PM

'71 750. .020" over. Engine ran like crap, anemic, inconsistent. When I disassembled it, there were there were seizure marks on both pistons. Mostly around the wrist pin. I have always called this type of seizure "cold seizure." It is the result of poor break-in, not letting it warm up before pinning the throttle or simply poor piston clearance (or all of the above).

The bore was actually still in good shape with no scratches, no metal transfer. I measured the piston skirt clearance....a .002" feeler gauge slides smoothly, a .004" could BARELY be coaxed in there but was REALLY tight.

I am going to be using garden variety EMGO replacement pistons. At first glance, opinions about piston skirt clearance appear inconsistent....so I'll just say it: I am going to hone the bore to .005" piston skirt clearance. Anybody see any problems with this?

robertv · Thursday at 5:55 PM

In my book, that is excessive. 1.5 to 2 thou. Make sure the piston pin is a good fit, oiled, it should be a nice slide oiled.
Not fall through and hit the table.

DennisMo · Thursday at 8:05 PM

My new Hepolite + 0.020 850 pistons/rings came with instructions for a 4 thou clearance.
The machine shop did this and I checked and clearance was 4 thou.

When I checked the ring gaps, they were all towards the tighter side of spec but within spec except for one ring that was
9 thou gap instead of 10.

I am not sure if Robert is saying your 5 thou is excessive and should be 1.5 to 2 thou clearance, or he is saying excessive by 1.5 to 2 thou (meaning he suggests 3 thou to 3.5 thou clearance.)

My 850 was my first air cooled engine rebuild and it was all learning for me. But my gut feel is that clearance under 4 thou is too tight. But then there
is the material the piston is made from and its expansion characteristics
Dennis

kommando · Friday at 1:48 AM

5 thou will be fine, but if the seizure marks are in the right place to count as 4 corner seizure then also check your jetting too and fuel feed to the carbs.

In the factory manual fresh fit clearance is 4 to 5 thou with a wear tolerance of another 5 thou. Considering the low miles we now all do eating up an extra thou of wear is not an issue.

Four corner seizure
Symptom – The piston has vertical seizure marks at four equally spaced points around the circumference.

Cause – A four corner seizure is caused when the piston expands faster than the cylinder and the clearance between the piston and cylinder is reduced.

Reason – The main reasons for this problem are too quick warm-up, too lean carb jetting (main jet), or a spark plug with too high a heat setting being used

Emgo pistons are made by JCC and are very close copies in sizing and materials to Norton OEM. Their rings are suspect so ask whose rings are fitted.

Fast Eddie · Friday at 2:23 AM

Well worth fitting Goetze rings IMO even if it means throwing the suspect ones away and buying the Goetze rings separately.

My T140 with brand new pistons and bore burnt oil, causing fouled plugs and was coked up like hell inside. I fitted Goetze rings and problem solved.

baz · Friday at 4:00 AM

Fast Eddie said:
Well worth fitting Goetze rings IMO even if it means throwing the suspect ones away and buying the Goetze rings separately.

My T140 with brand new pistons and bore burnt oil, causing fouled plugs and was coked up like hell inside. I fitted Goetze rings and problem solved.

Did the goetze rings have a solid oil control ring or 3 piece?

Tigernut · Friday at 5:13 AM

As far as I know, Emgo (JCC) pistons have been sold with Goetze rings for a couple of years now. The pistons are very good (as good as the old genuine Hepolites) but the rings they used to come with were rubbish. The T140 JCC pistons I've bought in the past few years have had Goetze rings, with an expander spring on the oil rings. For T140s, I use .0045 to .005" skirt clearance - a tad more clearance seems to do no harm, and minimises worries about nipping up while running in.

Fast Eddie · Friday at 5:41 AM

baz said:
Did the goetze rings have a solid oil control ring or 3 piece?

Can’t remember

Fast Eddie · Friday at 9:11 AM

Tigernut said:
As far as I know, Emgo (JCC) pistons have been sold with Goetze rings for a couple of years now. The pistons are very good (as good as the old genuine Hepolites) but the rings they used to come with were rubbish. The T140 JCC pistons I've bought in the past few years have had Goetze rings, with an expander spring on the oil rings. For T140s, I use .0045 to .005" skirt clearance - a tad more clearance seems to do no harm, and minimises worries about nipping up while running in.

If that’s true it’s great to hear.

But folk should confirm when buying though, stock doesn’t always flow, it can sit around for years!

Banzaibob816 · Friday at 9:57 AM

Tigernut said:
As far as I know, Emgo (JCC) pistons have been sold with Goetze rings for a couple of years now. The pistons are very good (as good as the old genuine Hepolites) but the rings they used to come with were rubbish. The T140 JCC pistons I've bought in the past few years have had Goetze rings, with an expander spring on the oil rings. For T140s, I use .0045 to .005" skirt clearance - a tad more clearance seems to do no harm, and minimises worries about nipping up while running in.

They are EMGO pistons with Grant rings.

linker48x · Friday at 4:26 PM

I’d bore the cylinder .0045 bigger than the properly measured piston. I learned a long time ago, adequate clearance or even .001 or .002 above factory spec is fine, and has no negative consequences, and the factory even contemplates a wear tolerance maximum quite a bit greater than that. But not enough clearance, even .001 or .002 LESS than factory spec, certainly can have very serious negative consequences.

This is an aluminum piston in a cast iron, air cooled cylinder, with “issues” regarding perpendicularity and roundness. There are plenty of reasons to give it enough clearance. Same goes for ring end gap, several thou too much is fine, no consequences, but too little can be a problem.

DennisMo · Friday at 5:00 PM

Every 0.001 thou you overbore it past spec will open up the unworn ring gap by 3 thou.
Dennis

ashman · Friday at 5:01 PM

When I did my rebore back in 82 40 oversize Hepolite pistons and what ever rings came with them the min clearance was .003 I started working at a tec college (TAFE) and the motorcycle section got to do the rebore for me I told the Tec teacher I wanted a .002th final clearance after piston bore, he said that might be too tight but I insisted, it was winter at the time and after firing the bike up for the first time I went out on its first run in at 2 am on a very cold morning to clock up the first 100 miles to bed the rings in with cheap motor oil, I froze my arse off but the pistons didn't lock up.

My motor was very tight and lots of compression with all the other work done to my motor, after the first 500 miles without any problems and changing to oil to my normal running oil I always used as well adding STP with my oil, today I am still running with the same pistons, my motor is still the same today as when I did the rebore all them years ago and I still mix STP in with my oil, still feels tight and still lots of compression, just over 18 years now I had to replace my crank cases from a few hair line cracks and a broken piece of crank where the middle stud stripped but when I pulled the motor down the bores showed no signs of wear as well still had the hone marks from when it was done in 82, I just replace the rings and a very light hone before putting the motor back together, I didn't even remove the pistons off the rods but all the bearing were replaced, even the rod slipper bearings showed very little wear.

My Norton has always been an everyday rider till 2013 from new and averaged about 6 to 10k miles a year riding, so my tight clearance has helped in getting long life out of my bores as well mixing STP in with my motor oils has proved to me it works in prolonging the life of my motor, but tight bore clearance has to be run in with extra care that it don't go tight, but too much clearance and your bores will wear out quicker in the long run.

This of course is my own opinion and experience and most engine builders will do more final bore to hone clearance as not knowing the owner care in running in new bores but to me .005 is too much of a clearance, but the original Hepolite pistons you could do tighter clearances back in the days.

Ashley

Tigernut · 2026-09-26T04:42:35-0600

ashman said:
When I did my rebore back in 82 40 oversize Hepolite pistons and what ever rings came with them the min clearance was .003 I started working at a tec college (TAFE) and the motorcycle section got to do the rebore for me I told the Tec teacher I wanted a .002th final clearance after piston bore, he said that might be too tight but I insisted, it was winter at the time and after firing the bike up for the first time I went out on its first run in at 2 am on a very cold morning to clock up the first 100 miles to bed the rings in with cheap motor oil, I froze my arse off but the pistons didn't lock up.

My motor was very tight and lots of compression with all the other work done to my motor, after the first 500 miles without any problems and changing to oil to my normal running oil I always used as well adding STP with my oil, today I am still running with the same pistons, my motor is still the same today as when I did the rebore all them years ago and I still mix STP in with my oil, still feels tight and still lots of compression, just over 18 years now I had to replace my crank cases from a few hair line cracks and a broken piece of crank where the middle stud stripped but when I pulled the motor down the bores showed no signs of wear as well still had the hone marks from when it was done in 82, I just replace the rings and a very light hone before putting the motor back together, I didn't even remove the pistons off the rods but all the bearing were replaced, even the rod slipper bearings showed very little wear.

My Norton has always been an everyday rider till 2013 from new and averaged about 6 to 10k miles a year riding, so my tight clearance has helped in getting long life out of my bores as well mixing STP in with my motor oils has proved to me it works in prolonging the life of my motor, but tight bore clearance has to be run in with extra care that it don't go tight, but too much clearance and your bores will wear out quicker in the long run.

This of course is my own opinion and experience and most engine builders will do more final bore to hone clearance as not knowing the owner care in running in new bores but to me .005 is too much of a clearance, but the original Hepolite pistons you could do tighter clearances back in the days.

Ashley

If you averaged 6-10K miles a year over 31 years (1982-2013), that's a minimum of 180000 miles, which is a truly exceptional wear rate, if your engine is still on the same bores & pistons. I'll have to look into this STP additive!

Triton Thrasher · 2026-09-26T06:55:01-0600

I try to find the maker’s stated clearance range and (if I have a choice) aim for the looser side of that.

ashman · 2026-09-26T15:14:38-0600

Tigernut said:
If you averaged 6-10K miles a year over 31 years (1982-2013), that's a minimum of 180000 miles, which is a truly exceptional wear rate, if your engine is still on the same bores & pistons. I'll have to look into this STP additive!

The Norton had a few little breaks like when the crank case had hair line fractures around the main seal area and broken front middle stud part of the crank case, the broken piece was rolling around on top of the crank at idle but flowed in between the bottom of the cylinder wall when riding, had a clunking noise when it was idling so took a while to finding a good set of replacement cases and rebuilding the motor for the new cases but as I said I put the old pistons back in with new rings and a light hone so yes the bore would have high mileage on it and its still pulls like a train.

When I was building the Commando/Featherbed in 1980 I brought a new 81 Triumph Thunderbird and for 9 years I was riding both bikes, but the Norton was my main bike in them years, the Thunderbird was more my traveling bike in those 9 years and it also clocked up a lot of road miles, 250k km before doing a top end rebuild before selling it, I also ran STP in its motor oil, to this day I still regret selling that Triumph, it had the same size bore as a 750 Bonnie but shorter stroke, it was quicker than a Bonnie and a lot smoother being a 650, I also ran it with a older 2 rare earth Joe Hunt magneto on it for 9 years was a very reliable bike.

I have used STP in both my old air cooled British bikes and it has proven to me that it works in prolonging a long life with my air cooled motors, I also ran lochead oil coolers on both bikes all year round, our summers are hot and our winters are very mild.

Pics of both bikes, pic of the Norton was taken 2 years ago on our annual kick start only bike ride.

Tigernut · 2026-09-27T04:27:50-0600

ashman said:
The Norton had a few little breaks like when the crank case had hair line fractures around the main seal area and broken front middle stud part of the crank case, the broken piece was rolling around on top of the crank at idle but flowed in between the bottom of the cylinder wall when riding, had a clunking noise when it was idling so took a while to finding a good set of replacement cases and rebuilding the motor for the new cases but as I said I put the old pistons back in with new rings and a light hone so yes the bore would have high mileage on it and its still pulls like a train.

When I was building the Commando/Featherbed in 1980 I brought a new 81 Triumph Thunderbird and for 9 years I was riding both bikes, but the Norton was my main bike in them years, the Thunderbird was more my traveling bike in those 9 years and it also clocked up a lot of road miles, 250k km before doing a top end rebuild before selling it, I also ran STP in its motor oil, to this day I still regret selling that Triumph, it had the same size bore as a 750 Bonnie but shorter stroke, it was quicker than a Bonnie and a lot smoother being a 650, I also ran it with a older 2 rare earth Joe Hunt magneto on it for 9 years was a very reliable bike.

I have used STP in both my old air cooled British bikes and it has proven to me that it works in prolonging a long life with my air cooled motors, I also ran lochead oil coolers on both bikes all year round, our summers are hot and our winters are very mild.

Pics of both bikes, pic of the Norton was taken 2 years ago on our annual kick start only bike ride.
View attachment 131177 View attachment 131178

I like the idea of the Thunderbirds and I'd like to have a go on one. Not sure how many were made, but not many, so they're pretty rare. How much STP do you add to your oil?

ashman · 2026-09-27T14:59:52-0600

There was 2 models of the 81 Thunderbirds made one model for the Euro market and a model for the USA market which mine was but they never made it to the USA and 5 Thunderbrids were on the production line when the workers got locked out, the Euro model had a bigger tank and a red and black paint job, the US model had the smaller peanut tank and the blue and silver paint job , at the time I was looking at buying a new Bonnie but the Thunderbird took my liking sitting in the show room they had both models but the blue one got me and was cheaper by $500 than the Bonnies, it was a lovely bike to ride with a single Amal carb and a drum rear brake and a lot smoother to ride than the Bonnies, with the shorter stroke it was a very quick bike for it's size, paid $4100 new.

I did a lot of long distant trips on that Thunderbird, it clocked up a lot of miles in the 9 years of ownership always started on first kick every time and never let me down, I made a big mistake when I sold it at the time I wanted a new 91 1200 Harley Sportster the first of the 5speed and belt drive, but a Harley just wasn't me and I only had it for 3 years before selling it, I got the Harley before I sold the Thunderbird and to this day I regret selling it, it was a lovely bike to ride, when I traveled I had a larger bread box fuel tan I put on for longer miles between stops.

My round alloy oil tank on my Norton is only 3ltr cap, so 2ltr of oil I mix 500ml (1/2 a bottle) of STP in my oil, I mix it in my oil before putting it in the tank I also use a mixture when assembling rebuilt motors the STP puts a protected coating on moving part and it sticks to it, I also get longer life out of my oil.

Piston Skirt Clearance

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