Gap on an 850 with +20 ?

NORBECER · Apr 7, 2014

Good day Gents, My 73 850 has +20 Hepolite pistons. I just measured the gap between my piston and cylinder wall, came in at .007. Then I slipped the top ring in and placed it about 2 inches into the barrel from the top and it came in at .016. What should I be looking for if I have the cylinder honed and replace with Andover GPMs ? You can see in the reflection of the picture the lines on the current pistons however the barrels are clean. Would you re-use these pistons with new rings?

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concours · Apr 7, 2014

They look like mine did.. rust stained, but not rusty. Get a bright light and magnifying glass. :shock:

NORBECER · Apr 7, 2014

concours said:
They look like mine did.. rust stained, but not rusty. Get a bright light and magnifying glass. :shock:

So tell me what was your remedy :?: Inquiring minds would like to know.

kerinorton · Apr 7, 2014

whats the bore wear like. whats the taper etc. If you hone the bores you will wear more off them. Modern technology is to fit new pistons and rings, throw it together and ride the bike hardish. I blew both piston on my 850 some years ago and simply just fitted new pistons and rings. [ note the bores were hardly worn and the marks were not deep enough to worry about. I have done another 10,000 miles trouble free. I did not hone nor deglaze the bore.
DONT HONE THE BORES. [ bore honing is only done after a rebore ] If anything, rub high spots off only, but looking at your bores, they seem ok. Ive been a Mechanic for over 40 yrs. have a good time. Believe it or not, I reused the head gasket as well. Did it right and still don't have leaks. Bike can do 73 MPG. [ imperial gallons that is ]

NORBECER · Apr 7, 2014

kerinorton said:
whats the bore wear like. whats the taper etc. If you hone the bores you will wear more off them. Modern technology is to fit new pistons and rings, throw it together and ride the bike hardish. I blew both piston on my 850 some years ago and simply just fitted new pistons and rings. [ note the bores were hardly worn and the marks were not deep enough to worry about. I have done another 10,000 miles trouble free. I did not hone nor deglaze the bore.
DONT HONE THE BORES. [ bore honing is only done after a rebore ] If anything, rub high spots off only, but looking at your bores, they seem ok. Ive been a Mechanic for over 40 yrs. have a good time. Believe it or not, I reused the head gasket as well. Did it right and still don't have leaks. Bike can do 73 MPG. [ imperial gallons that is ]

Good to know

Thanks for your input.

pete.v · Apr 7, 2014

What are the actual dimensions of the bore and piston.
If your bore is within spec and the pistons are out, then maybe just new pistons.
If the bore is out new piston won't matter and it's time to go to 40 over.

NORBECER · Apr 7, 2014

pete.v said:
What are the actual dimensions of the bore and piston.
If your bore is within spec and the pistons are out, then maybe just new pistons.
If the bore is out new piston won't matter and it's time to go to 40 over.

Looks like 77.50 as per a +20...the cylinder is not glazed from the looks. I understand that the gap should not be less than .005 and no greater than .010 they are at .007

pete.v · Apr 7, 2014

The book says standard bore is 3.030. 20 over would be 3.050.
Pistons 20 over would be 3.028/3.027, by the book that is.
Seems a little tight compared to the .004 clearance of the 750.

NORBECER · Apr 7, 2014

What should the ring gap be with a fresh set of +20 :?: The worn set is at .016

pete.v · Apr 7, 2014

NORBECER said:
What should the ring gap be with a fresh set of +20 :?: The worn set is at .016

Book says top ring .010 to .012, middle ring .008 to .012.

Rohan · Apr 7, 2014

Those bores don't look too flash either.

Might be serviceable, just...

dennisgb · Apr 7, 2014

Not sure what measuring the ring gap on a worn set of rings will tell you. Ring gap spec usually is for new rings in fresh bores or bores that measure within spec. I would be more concerned about the measurement of the bore at multiple locations.

NORBECER · Apr 7, 2014

pete.v said:
NORBECER said:

What should the ring gap be with a fresh set of +20 :?: The worn set is at .016

Click to expand...

Book says top ring .010 to .012, middle ring .008 to .012.

Thanks Pete, The engine only smoked out the left side and the obvious reason was due to a valve guide leaking into the exhaust manifold as visible in the picture below. From the pic of the pistons included would you consider this a light carbon build up ?

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NORBECER · Apr 7, 2014

dennisgb said:
Not sure what measuring the ring gap on a worn set of rings will tell you. Ring gap spec usually is for new rings in fresh bores or bores that measure within spec. I would be more concerned about the measurement of the bore at multiple locations.

I checked the gap just as I was curious to know at what measure they currently were. I will definitely have the bores properly measured and see what is recommended. If I require a re-bore so be it, if they are serviceable with a fresh set of rings even better. As you can see from my questions I lack knowledge in this area.

dennisgb · Apr 7, 2014

NORBECER said:
I checked the gap just as I was curious to know at what measure they currently were. I will definitely have the bores properly measured and see what is recommended. If I require a re-bore so be it, if they are serviceable with a fresh set of rings even better. As you can see from my questions I lack knowledge in this area.

Wasn't trying to sound like a jerk. Just couldn't figure out what the ring gap would tell you, because if the bores are worn, the gap will be wider along with ring wear. Usually a good check of measurement of the bore will tell you a lot. If the bores are in spec then the ring gap measurement you got will tell you the rings are worn.

NORBECER · Apr 7, 2014

dennisgb said:
NORBECER said:

I checked the gap just as I was curious to know at what measure they currently were. I will definitely have the bores properly measured and see what is recommended. If I require a re-bore so be it, if they are serviceable with a fresh set of rings even better. As you can see from my questions I lack knowledge in this area.

Click to expand...

Wasn't trying to sound like a jerk. Just couldn't figure out what the ring gap would tell you, because if the bores are worn, the gap will be wider along with ring wear. Usually a good check of measurement of the bore will tell you a lot. If the bores are in spec then the ring gap measurement you got will tell you the rings are worn.

Not at all, your comments and suggestions are very welcomed and appreciated

dennisgb · Apr 7, 2014

If your new to rebuilding an engine then it is difficult to figure out. You'll get a lot of different opinions, all of them may work but I always err on the conservative side once I have an engine torn down. The amount of time is more valuable than the parts, and peace of mind comes into play. It looks like you need head work for sure. I would look at new pistons and possibly a rebore as there is some pitting in the cylinders. That might be okay, and even the existing pistons might be fine with new rings if the bores check out. I wouldn't chance it though.

NORBECER · Apr 7, 2014

dennisgb said:
If your new to rebuilding an engine then it is difficult to figure out. You'll get a lot of different opinions, all of them may work but I always err on the conservative side once I have an engine torn down. The amount of time is more valuable than the parts, and peace of mind comes into play. It looks like you need head work for sure. I would look at new pistons and possibly a rebore as there is some pitting in the cylinders. That might be okay, and even the existing pistons might be fine with new rings if the bores check out. I wouldn't chance it though.

The head needs a complete over haul that's for sure, and trust me I would rather have the new parts as well. I've read far too often here on the Forum about bad re-bores and engines blowing up on their maiden voyages. Makes me nervous since +40 puts me one away from scraped barrels. I would prefer to go new pistons and rings with the present barrels if possible. Cheers

NORBECER · Apr 7, 2014

Found this article on the Norton Owner's Club site regarding determining wear without the use of instruments. It will tell you if you require a rebore, however it doesn't help determine if the cylinder is oval or not.

J.B. Nicholson - Modern Motorcycle Mechanics

Cylinder Block Measuring and Reboring:- Cylinder wear can be fairly accurately determined without micrometer equipment by placing a piston ring in the lower unworn end of the cylinder, measuring the gap with a feeler gauge and then shoving the ring with a piston to the top end of the ring travel in the cylinder and measuring the gap in this postion. The difference in joint gap measured at these two positions will represent approximately three times the cylinder diameter wear. For example, if the ring gap at the lower end is 0.010" and the top end measures 0.040", the difference of 0.030" circumference increase indicates approximately 0.010" diameter wear, and reboring and oversize piston fitting is indicated. Generally cylinder wear of over 0.008" calls for reboring for fully satisfactory results.

concours · Apr 8, 2014

NORBECER said:
concours said:

They look like mine did.. rust stained, but not rusty. Get a bright light and magnifying glass. :shock:

Click to expand...

So tell me what was your remedy :?: Inquiring minds would like to know.

Bore .020" over, hone to proper fit.

Gap on an 850 with +20 ?

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