Hi All
Im checking my big 4 bore dimensions against the pistons, and would like your opinions please !
Firstly if im doing it correctly and secondly if i need a rebore or not!
So im taking the reading at the base of the bore to the top of the piston skirt 90' to the gudgeon pin .006" gap


Bottom of skirt .005"


Piston at top of skirt 3.236


Bottom of skirt 3.262


As always thank you in advance for your help
Cheers Stu

email (option): stuartcameron66@hotmail.com

The dimensions you show seem to be about ok, however check it again with your piston nearer TDC (with the barrel turned upside down) as most of the wear on a barrel occurs near TDC as the piston changes direction, you can check for tapering from top to bottom in the bore by using a piston ring either a new or old one, it dosen't matter, simply put the ring in at the bottom of the barrel and use the piston to push it square, measure the end gap, then move it to just below the tiny wear ridge at the top of the barrel and use the piston to square it again and measure the end gap again and this will show you how much wear is present between the top and bottom.

email (option): davmax@ntlworld.com

Personally, I consider the method of using feeler gauges to be a relatively inaccurate and also inadequate method of determining bore dimensions...

Further, a vernier caliper is also fairly inaccurate and prone to variations in the results in an application such as measuring a piston...

Bore wear should be measured at various points (vertically) along the length of the cylinder on both the plane of the thrust faces and along the axis of the gudgeon pin to obtain a clear and definitive answer as to the condition of the bore.

The bore can wear into quite irregular shapes if, for example, the conrod is slightly bent, but primarily tends to end up barrel shaped after extended use...

I would recommend taking the cylinder to an engine shop to have it measured using bore micrometers...Likewise, measurements of the piston are best taken at various points with a micrometer for an accurate result...Ian

email (option): ian@wright52.plus.com

Thank you Dave and Ian for your replies !

Dave, using the ring gap as you suggested the difference from the base of the bore to the top (just under the ridge) is .014 overall ! I recorded the gaps at inch intervals, the largest being .004.5 Radco's book suggests at least .003 p/in of bore and if exceeding .010 p/in then replace rings!

Ian, thanks for your email and bore check template !
I found this instrument in my late step fathers tool collection (he was a toolmaker) and used it with the verniers to record the figures below!


I will heed your recommendation of getting expert advice, being a carpenter and working to mm's i find this very interesting (although at times confusing)
Cheers Stu

email (option): stuartcameron66@hotmail.com

It is just a rough and ready guide, not to measure accurately, but more to show up how much wear there is in the bore without special tools.

email (option): davmax@ntlworld.com

Hi Stuart...I'm also a Toolmaker (now retired)...Your step fathers bore gauges are a useful piece of kit to own, though they rely very much on 'feel' to get a consistent result...

The vernier caliper also has its uses and every engineer will have one. However, although a digital vernier can have .0005" increments it is generally regarded as unwise to rely on them if the accuracy required must be .001"-.002" or less...

At these tolerances the micrometer takes over as the more accurate and desired measuring method...and it is why both verniers and micrometers are available...

However, you now have a general picture of the state of your bore (and piston) and could probably make a decision based on this as to whether to have a rebore or perhaps just a hone and a set of rings...

If you are well inside the laid down wear limits based on your measurements then a discrepancy of a couple of thou won't have much relevance...If you are much closer to the laid down limits for your engine I would consider getting the extra set of measurements from the engine shop to confirm, or otherwise, your results...Ian

email (option): ian@wright52.plus.com

Thanks guy's, much appreciated
Ill check the manual for tolerance's !
Can someone tell me if it's been rebored before going by my piston size ?
Cheers Stu

email (option): stuartcameron66@hotmail.com

Stu, is there nothing on the piston crown ?

Your bore seems to be roughly 83mm - Std. is 82 which suggests that it is already on + .040"

If it won't clean up within tolerances with a hone and new rings then you'll probably be looking at sleeving.

16H piston availability is pretty good but I have no idea how you stand with Big 4 pistons.

A quick search and this is what comes up, a +40 big 4 piston and a 82mm unknown piston..! It might be worth ringing around the usual dealers

http://www.vintage-motorcycle.com/index.php?language=en&site=4&pid=317&id=18356&suchtext=Norton%20piston%2082mm&limit=0

http://en.vintage-motorcycle.com/index.php?language=en&site=4&pid=322&id=10670&limit=60

email (option): horror@blueyonder.co.uk

Thanks Rik and Horror !
Rik,The piston is a Polson brand and It has C 311 both stamped on the top, there is nothing on the underside !
It measures 3.260 consistently across top and bottom skirt in both directions so I'm sure it's +40 !
Thanks for the piston links Horror, I'll seek further advice locally on the wear !
Thanks again everyone from the "novice downunder "
Cheers Stu

Don't all oversize pistons have the O/S or + stamped on the top? I thought they did, but not sure now!

Ron

email (option): ronpier@talk21.com

Hi Ron
I googled Polson, they are/were an Aussie firm ( Maidstone Victoria about 10 mins from me )!
I've only cleaned off the carbon , so it's a bit dark in color as I wasn't sure if I should polish it to find more stampings !
Cheers Stu

G'day Stu,
Where abouts in Melb are you?

Modak sell +.060 piston and rings for the M20/21, not sure of the quality though as mine needed a couple of thou extra clearance to stop it nipping up.

As an apprentice fitter we were always told that your measuring equipment was only ever accurate to 10% of the scale it measures in.

ie a steel rule measuring in millimeters would be accurate to 1cm.
a vernier caliper measuring in 1/10mm accurate to 1mm.
and a micrometer measuring in 1/100mm would only really be accurate to 1/10mm.

Verniers calipers are often called Guessing Sticks because people often credit them with more accuracy than they have.

email (option): spacemonkeym@gmail.com

Thanks Matty !
Im not far from Modak (altona nth), ill get the bore checked professionally and get further advice !
Will contact Dave to see what he has in stock
Cheers Stu

email (option): stuartcameron66@hotmail.com

Not far at all Stu, I'm in Kensington.

email (option): spacemonkeym@gmail.com

Stu,
The polson pistons I have don't have O/S stamps on them.
Are you going to the next moderating meeting.
Are you going to be herding cats again next year?
Cheers Darren

email (option): Darrenacartwright@yahoo.com.au

Hey Matty, flemington ? your almost next door
Hi Darren, Tafe meeting ? no, im over em ! the next job cuts my end will be 10 managers going for 5 positions, my contract ends mid december so ill wait and see.
Id like to keep teaching, maybe after they trim the top end fat there might be more cash for teachers
Whats the state of play your way ?
Cheers Stu

email (option): stuartcameron66@hotmail.com

Stu
We just lost 4 teachers in my department, 10 across the trades. But not 1 manager.
My contract ends 31st December, I won't know if I'm herding cats till a few days before as is usual.
Might just go back on the tools, or work in Grey st St Kilda....

email (option): Darrenacartwright@yahoo.com.au

Darren
Weve lost all the teachers we can spare, if one of us are sick the students go home for the day ( no money for sessional's) great way to run a business
If you do Grey st. keep to your own corner will ya !
Stu

email (option): stuartcameron66@hotmail.com

Just stay out of Chauser St you guys, not that there's anything wrong with that mind you.

email (option): spacemonkeym@gmail.com

Hi all
Thought i'd post an update on my project !
After contacting a local semi retired engine builder the piston/bore clearance turned out to be 7 thou, it was decided to crosshatch the piston, hone the barrel, replace the rings and sleeve the valve guides !
The work was completed in three weeks, once back i set about painting the head and barrel, and rebuilt the motor replacing the bearings, small end bush, oil pump and valve springs.





Thanks to the collective minds on this forum and freshly painted top half and polished case's its starting to take shape!
Cheers Stu

email (option): stuartcameron66@hotmail.com

Well i've never seen that before , cross hatching a piston , is that a recognised practice ?.

No it is a bodge to avoid doing a rebore when you are about 1/2 way between needing one.
We used to get our racing pistons expanded and when the shop said "no more" then it as out with the centerpunch
Kurling is a slightly better methos of dong the same thing.
There is a lot less piston touching the bore and thus the hilled bits wear faster than a normal piston.

Having said that with the number of miles these bikes do it will probably be good for another 20 years.

email (option): wariron@tpg,com,au

That seems a bit drastic to me 7 thou is within tolerance and would have been fine on an 83mm bore, and especially on a side valve that runs hot. I don't know how much of the gap you've closed and I hope what you've done works and lasts more than 1000 miles. But once those high spots wear you'll be left with even more gap than 7 thou and a ruined piston. I would say that's a very short term fix and you'll have a knackered barrel AND a ruined piston. Considering the prices of Big 4 pistons I've seen for sale lately I would have saved the piston.

http://www.ebay.co.uk/itm/NORTON-NOS-PISTON-S-V-SIDE-VALVE-BIG-4-HEPOLITE-8616-STD-633cc-82-mm-31-39-/201206021886?pt=UK_Motorcycle_Parts_13&hash=item2ed8d03efe

http://www.ebay.co.uk/itm/NORTON-NOS-PISTON-SIDE-VALVE-BIG-4-10890-030-596cc-81-9-mm-82-mm-47-54-/201225211691?pt=UK_Motorcycle_Parts_13&hash=item2ed9f50f2b

email (option): horror@blueyonder.co.uk

Hi Stuart

The photo of your piston looks interesting. What you did not mention is that the piston was re-ovalised with a file, removing metal from the sides in the gudgeon pin area. Did your semi retired engine builder drill the radial holes below the oil ring or is that how the original piston was supplied? I sometimes drill these holes on my own pistons but have never seen it on an OEM piston.

I do not think light hex grooving or knurling is necessarily a bad practice, providing the ring grooves are within tolerance and the bore is not excessively scored. If the result is visible engine smoke or more than a pint of oil per tank of petrol, perhaps its time to rebore.

email (option): pvlietstra@gmail.com

Knurling the piston used to be recommended as a cheap 'fix' in many older motorcycle publications back when everyone worked a 9 day week and 25 hour days for ten bob and had to feed a family of 13...

I haven't seen the recommendation in any magazines later than about 1955 though...

Obviously it works to a degree..For how long it works remains to be seen...I think I would rather have had a new piston myself and if I were you I'd start looking for the appropriate piston sooner rather than later...Ian

email (option): ian@wright52.plus.com

Is it me or are the cam followers installed upside-down in the photo ? Don't suppose it makes a great deal of difference at the end of the day but the workshop manual and books seem to show them the other way up with the "curve" profile against the cam rather than on the valve stem.......?

The holes and oval shaping were already in the piston, the ring grooves were recut !
The clearance was 7 thou at the skirt bottom, I thought 6 thou was the maximum hence going ahead with the work.
The chap I engaged to do the work was recommended by a motorcycle mechanic I know, he did ask if I wanted to rebore to a new piston but also said the clearance wasnt far off and suggested honing the barrel and knurling the piston.
Steve, the followers were the other way (flats facing up) when I dismantled the motor but the book I've got shows them facing down !
Cheers Stu

email (option): stuartcameron66@hotmail.com

Oh Dear, I don,t like the look of that piston. Surely NOS pistons arn,t that difficult to find? Also "resleeving the valve guides" whats that about, NOS guides are available. Bodging is only a last resort if all other options are dead.

email (option): ronpier@talk21.com

Ron,s diagrams are quite correct. If you think about it if it was the other way round hardly any of the flat large area would be in contact with the pushrod. It does matter too, Steve because valve timing and dwell would be very different depending which way up you have.

I would also double check your valve timing, it may not be quite correct. Do not rely on the marks on the camwheels, they cannot be relied upon. Use a degree disc.

Helpful hints re: Norton cam followers so thank you chaps.........

I was aware that 16H and Big 4 cam followers differed, but owning the former rather than the latter I was unaware of what the difference was..............

The Big 4 was also a slightly later engine than the WD version of the 16H so small details such as the followers may have been improved upon.........

16H and presumably Big 4 valve timing does indeed benefit from a degree disc......I found out when rebuilding mine that not all cam wheels are marked at the correct timing point, the WD manual in fact mentioning this..........mine are since marked after confirming the correct valve timing with a degree disc............easy and simple to do.......

Worth mentioning that 16H cam followers are difficult to obtain new today, and most engines feature these worn to a degree..........the obvious remedy is to get them welded up and re-ground to the correct profile...........I was however fortunate enough to find a NOS pair at Popham autojumble only 3 - 4 years ago now for an unbelievable £5............they are now in my spares stash for a rainy day.....!

My camwheels were incorrectly marked as well but I only found out after I'd put it all together, as you do

Thanks Chilly for the guide info I wasn,t aware of K-Line.

I have had my mitts on a total of only 3 Norton engines...All Big 4s. I have yet to see any timing marks...I guess I must be unlucky...

I'm not quite sure why Norton seemed to be incapable of making everything accurately enough to use reliable valve timing marks.... .. ..Ian

email (option): ian@wright52.plus.com

It wasn't that the parts were inaccurate it was that they put the marks in totally the wrong places prob to confuse things if it fell into enemy hands!

email (option): horror@blueyonder.co.uk

Thanks for that Horror, and the 16H has the earlier cams presumably, if not different to earlier Big 4 (civvy) ones too...?

One set of cam wheels in my boxes are marked, but I would agree with you that these were probably owner or workshop applied as they're very crudely done..........