850 Bore Marking & Smoking Exhaust

I recently acquired this machine and have been working through a number of fine details on the cycle parts, but I have now started looking at the engine side of things. Upon cold start up the left cylinder smokes heavily then clears which I thought was perhaps an inlet valve stem seal so I removed the cylinder head to try and establish the cause. The inlet valve had a bit of play in the guide but the seal was intact. Looking at the left hand bore these scuff marks appear around the circumference of the bore. The bore still shows cross hatching marks and there is very little lip to be felt at the top of the bore. Does anyone have any thoughts on the markings in the photos below? The marking cannot be felt with a fingernail.

I feel a rebore is the next step along with new valves and guides, are there any specific machine shop that are recommended in the UK?

The English magazine had a article on the Steve Wynn NORTON franchise .

On their ' warranties ' section it mentioned a ridged bore like that . Cause unknown .
Ive seen one other similar thingo here . Id think it related to ' ring chatter ' , perhaps
inadaquately salubrious lube on assembly . Or some morons used the choke on a
fresh engine . Or left the taps on and flooded / washed the bore . However wotever .

YOUR problem looks like that side swipe / scour, in the first photo . Id assume the rings mayve suffered similarly .
Therefore a INSPECTION is obviously in order . Any scuffing gets it unsuitable for racing .

A hone might clean up the bore , and the rest of it could be servicable . If ya wanna save loot . Minor irregularites
on a road machine thats not gonna be going interstate arnt gunna stop it running well . If you aint a redline merchant .

Id take a microscopic look at the rings , lands etc . And ya gotta be real carefull to seat the wrist pin circlips correctly on assembly .
No forign Matter . If all the bits are new . And MEAURE UP satisfactorally , seems unnessesary , perhaps , to go the whole hog .

N Z I think it was Cortina 1300 flat top crossflow pistons they used , maybe theyed go 920 unlinered . And the 1500 pistons , using sleeved barrels
got you 960 or sumsuch . The shop that ran the speedway big bore sidecar winner , said he had NO Blow Ups . ! So a less expensive solution .
If youve got a good machine shop nearby .

Plus , you get TWO sets of pistons , for the price ! . ( Actually I think you can get Qualty aftermarket ones , & in pairs )
Formular Ford and 10.000 rpm Cross Flow fords , mean there are quality pistons avaliable .
Linered the boring for the liners breaks through in a place or two . But why worry .

First Step after dismantling is to wash then inspect all the LABLED componentry under strong light with your strongest glasses ! Who'd of thought ! .

finally ,
AS THE PISTONS SAY " STD " , theres a chance ancient top quality ( Hepolite ) pistons may still be around . So if the BORE ' Cleans Up ' straight ,
Std , or 5 thou over , if ya can gettem , might do the trick . Sidey H D's ran , then honed to 1 1/2 over , to take the pistons supplied . After the cylinders had stress relieved ,

The issue will be: Are there scuffs on the piston skirts to match the marks on the bores? If not, check your piston skirt diameters. Those are grade A [smaller by a few .ooo1" than grade B] pistons. Hone the cylinders, install new rings if the piston skirts are not scuffed or collapsed. If there are issues with the skirts, measure the bores after honing. Odds are new standard pistons and rings will cure it. FYI, I've found that many machinists, when given the piston to wall clearance, will always strive to hit the minimum clearance during a re-bore. That's because most of them only deal with liquid cooled engines. I feel that on an air cooled engine, it's better to strive for the middle of the piston to wall clearance range.

I suspect that your engine was not broken in properly. You won't really know what you have until you pull the cylinders and measure the bores and pistons to determine what the clearance is.

Is this a Mk I , Mk II or Mk III 850?

The spacing appears to be from the ring gap, if you can't feel it with your finger nail it probably won't hurt a thing. Piston skirts won't run that far up the cylinder. insufficient ring gap could cause some minor scuffing.

Mike

Hard to tell but the marks seem to go to the top of the top ring and all the way down the bore that is visible. If you pull the cylinders, you can see where the marks stop in relation to the rings. I suspect that it has nothing to do with the piston itself. Maybe the top ring is poorly made or maybe there was some crud in it when the engine was put together. If you can't feel the marks and if nothing else seems wrong, I would hone in a new cross-hatch, get new rings, and try again.

BTW, since you can see where the rings all sat with the piston at the top, I suspect that the engine was put together dirty or the rings rusted, or the rings were poorly made, or the rings were upside down. What does the other bore look like?

Lovel said:

I recently acquired this machine and have been working through a number of fine details on the cycle parts, but I have now started looking at the engine side of things. Upon cold start up the left cylinder smokes heavily then clears which I thought was perhaps an inlet valve stem seal so I removed the cylinder head to try and establish the cause. The inlet valve had a bit of play in the guide but the seal was intact. Looking at the left hand bore these scuff marks appear around the circumference of the bore. The bore still shows cross hatching marks and there is very little lip to be felt at the top of the bore. Does anyone have any thoughts on the markings in the photos below? The marking cannot be felt with a fingernail.

I feel a rebore is the next step along with new valves and guides, are there any specific machine shop that are recommended in the UK?

Click to expand...

Where are you?
Your namesake, Pete Lovell is based in the midlands and is a great aground Norton engine man.

Norman White is based darn sarf.

There are others. Just be warmed that the good guys will all be busy, so don’t expect a quick turnaround…

Lovel said:

The marking cannot be felt with a fingernail.

I feel a rebore is the next step along with new valves and guides,

Click to expand...

It might only need a hone and a new set of (Std.) rings but of course, it depends on what the piston looks like and as marsh246 said, the marks appear to go right to the upper edge of the top ring.

That should clean up with a hone but whoever does the honing will know if its needing too much honing to clean up and can then bore to next oversize. If the hone cleans it up then new rings plus a check of the pistons for marks and ring groove dimensions to decide is they need replacing.

Lovel, did you clean the piston tops, or were they like that already? If it was the P.O. I'm just wondering if some carbon or whatever abrasive was used has dropped down the side, got trapped and is making these marks as they seem very randomly spaced. Maybe it's just a trick of the light, but looking at your first picture there does appear to be some debris coinciding with one of the marks at least....

Thanks for all your input.

I’ll try and get that barrel off tonight, anticipating broken ring/s perhaps, rags to cover crankcase mouth will be in place so nothing can fall into it.

The other bore does not have these marks and is more normal. I cleaned up the top piston to establish if standard, the carbon was removed quite easily as it was saturated with oil, so was inlet valve guide tract.

I found a previous receipt in the vehicle history file for head gasket and valve stem seals from only a couple of years ago

My gut feeling is worn piston ring grooves, or piston rings having a faulty thickness, allowing one or both to cant and twist against the cylinder wall. The sharp edge(s) may then produce the wear marks at the cylinder wall.

- Knut

Looks like piston seizure at some point. Top ring gap .025” in the worn part of bore, and the untouched part of the bore. The smear marks on the wall of the barrel do not have and grooves or high spots, feels very smooth. Top piston ring removed for measurement.

One other question should the crankcase mouth have Helicoil inserts where the long socket head screws thread into the casting?

Lovel said:

One other question should the crankcase mouth have Helicoil inserts where the long socket head screws thread into the casting?

Click to expand...

Yes.

I had the same situation. Your piston was too tight so when hot caused wear pattern. Looks like new pistons and rings for you. Make sure you resize the bore correctly to suit your pistons.
George

A quick polish & a coat of paint , and it should be fine .

Bin thinkin , hows the reciepts for the bottom end ?
Turns it into a major , But IF you could get O.E.M. Crank & Rod BOLTS , youd know where you were .
How Unusual .
That head face needs resurfacing . If only lapping on glass sheet with coarse valve grinding paste .
Seems neglect / lack of use is the main culprit . If it were a horse the R S P C A would get involved ! .

At Least , pull & inspect the OIL PUMP & Timing Cover Seal , And Strip & overhaul ( Clean & Oil )
the Auto Advance Mechanism .

Ign. & Carburation must be spot on , after an overall .

-------------------------------------------------------------
First Step'd be INSPECT THE FILTERS & Oil Tank , if theres not a iota off debri , your pretty safe .

Wondering if the bottom ends ever been apart ? Then theres the ISO's to check & shim .

Lovel said:

Looks like piston seizure at some point. Top ring gap .025” in the worn part of bore, and the untouched part of the bore. The smear marks on the wall of the barrel do not have and grooves or high spots, feels very smooth. Top piston ring removed for measurement.

Click to expand...

Is the other piston also stamped STD A?

'STD' with an 'A' stamp would normally indicate a factory-fitted (Grade A bore size) piston.

RatedSpeed said:

The spacing appears to be from the ring gap, if you can't feel it with your finger nail it probably won't hurt a thing. Piston skirts won't run that far up the cylinder. insufficient ring gap could cause some minor scuffing.

Mike

Click to expand...

That's what I thought when I saw the pic of the bore. I would hone the marks off and then check the bore to see what piston clearance I ended up with. Too much and you're in for an overbore and oversized pistons.

Danno said:

That's what I thought when I saw the pic of the bore. I would hone the marks off and then check the bore to see what piston clearance I ended up with. Too much and you're in for an overbore and oversized pistons.

Click to expand...

Yes, they appear to be witness marks of a condition that should be looked into but I don't believe it's causing the oil control problem.
Most of us have been in this situation where we tear into the motor before doing the compression / leakdown testing and that leaves us wondering...Do I reassemble and test or start more disassembly and hope I find something obvious.

The marks may be aluminium from the piston, hydrochloric acid (Muriatic acid) will remove that and not affect the barrel.

kommando said:

The marks may be aluminium from the piston, hydrochloric acid (Muriatic acid) will remove that and not affect the barrel.

Click to expand...

Fizzes it off in seconds!