Doctor, could you give me a diagnosis please?

Status
Not open for further replies.
Bonzo

Bonzo

VIP MEMBER
Joined
May 8, 2022
Messages
890
Country flag
I've removed the head in order to get the exhaust threads sorted. Here's what I found.

I wonder if the experts could take a glance at these pictures and say what they see?

Doctor, could you give me a diagnosis please?


R/H

Doctor, could you give me a diagnosis please?


Doctor, could you give me a diagnosis please?


L/H

Doctor, could you give me a diagnosis please?


Doctor, could you give me a diagnosis please?
 
"STD" ('B' grade) piston in the LH bore but maybe you'd seen that already?
 
Bonzo said:
Haven't stripped the head yet, but will investigate.

Cheers
Click to expand...
It's probably split....(Seal). Whats the mileage?
Check the bores for ovality as per edit: Shop manual spec. If these are not out of spec then re-hone cylinders and use new rings. Best use a machine shop if you don't have the measuring tools.
 
Last edited:
Yes definately I.D.13.

So, STD means it isn't bored out to a +20,+40 etc, and grade B is the finished piston size? To the food retailer (for example) grade B would perhaps indicate a sub quality item, but in this case it wouldn't mean that the pistons were 'seconds' or something. Good to know.

Would Grade B's have been grabbed off the parts shelf and fitted at the factory, or are they something a PO would have installed?
 
CanukNortonNut said:
It's probably split....(Seal). Whats the mileage?
Check the bores for ovality as per owners manual spec. If these are not out of spec then re-hone cylinders and use new rings. Best use a machine shop if you don't have the measuring tools.
Click to expand...
Numbers on the clock show 6,633. Looking at the bike (from the outside), I'd say there's a strong chance that could be right.

No measuring tools as such in my man cave, but my youngest is an engineer at a well known aerospace company, so I'm sure we'll come up with something ;)
 
Bonzo said:
So, STD means it isn't bored out to a +20,+40 etc,
Click to expand...

Yes, STD = Standard bore size.

Bonzo said:
and grade B is the finished piston size?
Click to expand...

Finished bore size so you may find the cylinder(s) is/are also stamped with a corresponding 'B' etc.

Bonzo said:
To the food retailer (for example) grade B would perhaps indicate a sub quality item, but in this case it wouldn't mean that the pistons were 'seconds' or something.

Would Grade B's have been grabbed off the parts shelf and fitted at the factory, or are they something a PO would have installed?
Click to expand...

'STD' either 'A' or 'B' are likely to be factory.

When reboring, cylinders are usually bored and honed etc. to a finished size that gives the required clearance for the supplied pistons, however, factories don't spend time matching bores to pistons but do the opposite, so the bores are measured after machining and graded according to the finished size in this case either A or B (as above), Triumph for instance used L, M & H, (Low, Medium & High, later just L & H).

Pistons are then selected to give the necessary clearance.
 
Probable intake valve oil leak. Look at leak of head gasket/rings? If my memory is correct, 750 don't have flat top pistons. They have cutouts. 850 pistons are flat tops. Looks like a 750 barrel to me.
George
 
Bonzo said:
Yes definately I.D.13.

So, STD means it isn't bored out to a +20,+40 etc, and grade B is the finished piston size? To the food retailer (for example) grade B would perhaps indicate a sub quality item, but in this case it wouldn't mean that the pistons were 'seconds' or something. Good to know.

Would Grade B's have been grabbed off the parts shelf and fitted at the factory, or are they something a PO would have installed?
Click to expand...
Lab beat me to it
It's factory fitted parts by the look of it ,the mileage is probably correct
 
baldy said:
Probable intake valve oil leak. Look at leak of head gasket/rings? If my memory is correct, 750 don't have flat top pistons. They have cutouts. 850 pistons are flat tops. Looks like a 750 barrel to me.
George
Click to expand...
It's an 850 barrel and pistons if ever I saw one!!
 
I understand
L.A.B. said:
Yes, STD = Standard bore size.



Finished bore size so you may find the cylinder(s) is/are also stamped with a corresponding 'B' etc.



'STD' either 'A' or 'B' are likely to be factory.

When reboring, cylinders are usually bored and honed etc. to a finished size that gives the required clearance for the supplied pistons, however, factories don't spend time matching bores to pistons but do the opposite, so the bores are measured after machining and graded according to the finished size in this case either A or B (as above), Triumph for instance used L, M & H, (Low, Medium & High, later just L & H).

Pistons are then selected to give the necessary clearance.
Click to expand...
Yep! That makes sense to me.

Thanks for explaining :)
 
Bonzo said:
Numbers on the clock show 6,633. Looking at the bike (from the outside), I'd say there's a strong chance that could be right.

No measuring tools as such in my man cave, but my youngest is an engineer at a well known aerospace company, so I'm sure we'll come up with something ;)
Click to expand...
If the clock is right, that engine has just been broken in. I'm guessing the oil in R/H cylinder is from a split or damaged or popped off oil seal on the intake valve on that cylinder. Look for scratched in the cylinders. If there is nothing you can feel with your fingernail, I say re hone and put new rings in.
should be good for 50 thousand miles. that is after you fix the head.
Cheers,

Tom
 
Bonzo said:
I did see that.

What does it mean?
Click to expand...

The grades are in the range of 4 10ths to 5 10ths of one thousand of one inch (around half a thou)
A human hair might be 0.003", 6 times that.
So wouldn't lose any sleep over it.

Could there be a potential problem (I put NOS stock size graded AE pistons in my 850) only with a standard bore if an unknown standard bore piston was used like a JCC or reboxed JCC as a Hepolite as a replacement which might not come in grades like OEM parts.
You might then have a bore clearance a half thou either way plus any bore wear.

It is a walk in the park with oversize pistons dictating the bore size and done to suit.
Standard bores and standard crankshafts are a little more complicated but might be as simple as a hone (not fine) and new rings but that depends on what the pistons actually look like skirt and ring land location wise along with the actual condition of the bore. (not corroded or excessively worn)

Did 850's come with copper head gaskets or maybe the head has been off before but looks like a typical hardly used, been sitting might have worn guides (or the seals died of old age) Brit bike top end to me.

I wonder if the big end shells are showing copper.
 
Last edited:
Was the bike running or did you just buy it? Not an expert BUT I have never seen a piston that clean with 6,000 miles. I do agree with valve guide or seals on dirty cylinder. Good luck
 
Status
Not open for further replies.
Back
Top