Barrel and Piston Query

[HarryD]

I am a bit confused regarding my cylinder bore and the current piston (that I need to replace).
So, it's a 1951 AJS 16MS (350cc single). I have travelled less than 50 miles on the bike since buying it.

The manual says that the standard bore is 2.7187'' +/- 0.0005''
As best as I can, I make the cylinder size 2.712'' – which is about 0.006 smaller than standard. However, that is near the rim of the barrel using digital calipers – there does not seem to be any carbon build up or burring at the bottom rim. The cylinder barrel is stamped 013791 and looks too new to be 67 years old.

The piston has nothing on the crown to indicate its size; on the inside is ‘6PM' and ‘6916. The piston fits very snugly in the barrel and too tight in places – not surprising as the piston size, in places, is as much as 2.712'' which according to the manual is correct for a standard piston (Top Skirt 2.7132'' +/-0.0005''). The piston and, to a lesser extent, the barrel show signs of a minor seizure. The piston looks to be the same as shown on the AMOC site as ‘800313/S 350CC PISTON STANDARD COMPLETE'.

The barrel has a shiny surface with little signs of honing.
However, in view of the narrowness of the cylinder, should I have it lapped or honed? I have also seen an article that suggests the clearance should be 0.005'' as opposed to 0.0035''.

So,
Should I buy a standard piston and then have the barrel customized to it?
Is the greater clearance preferred?
I apologise if this is all a bit basic, but it's my first effort and any advice would be gratefully received.

[g5wqian]

hi harry ;
firstly you have used the wrong measuring tool , and measured in the wrong place .

you measured the ridge of the bore but you really need to measure the bore lower down in the "worn" area where the rings pass through , good idea is to got 1" down the bore and take a measurement there and then go to middle and bottom of bore and measure there also .

you need an internal mic to measure the bore , you can also get away with snap gauges and then measure those using the vernier or external micrometer but it may not be as accurate .

a bore gauge is used to measure bore wear , you need a bore master as well though to set the slips at the bore size and then set the bore gauge to zero on them , then you can go down the bore with the bore gauge to see the differences , it will show very small differences as the gauge is only around .010" full scale movement .

usually an internal mic set will include adapters to suit a range of bore sizes so if you get one make sure it has all adapters , plus it will most likely have an half inch adapter to go in between , ie it runs in inch increments and then you can also use the half inch adapter to run in between these .

if the engine is well run in and used for a long time then you may not see any honing marks , the honing marks are put in at first to allow lubrication of the new piston and rings so it can all bed in , but they wear away over time and may not be visible .

of course there could be an instance of someone doing the job wrong and honing incorrectly or doing a bodge job .

the honing marks would be at 45 degs .

the bore running clearance depends on type of piston being used , if its an original 50s or 60s factory type then it will need less clearance but if its a modern type ie GPM or JP etc then you will need the .005" to allow for the extra expansion of these modern pistons .

unfortunately the manufacturing processes and materials used today are not same as they were when the bikes were first made , so we have to allow for the differences in quality and design .

a GPM piston will expand around .003" ,so youll need another .002" over that for adequate clearance when its hot , ie .005"-.007" is usually recommended .

i used .006" on my AJS twin , it does have a bit of piston slap but has not seized up and will run flat out revs .

if you have a standard bore cylinder barrel , then you can simply hone out the bore to suit a new piston if required and it should clean up easy enough .

you could hone it by hand or get a shop to do it .

the running clearance will be the standard bore size plus the clearance needed , ie 2.7187 [2.719"] +.005"

so around 2.7237" [2.724"] .


if you can use or reuse an original piston and find new rings then a simple hone by hand of the bore with something like a flexhone will do the job so long as it hasnt had a problem like seizing . .

you didnt say if youre engine had seized or if it runs fine and you are just intending on renewing the piston and rings .

most of the old bikes dont do enough mileage these days to warrant new rings and pistons , in fact theyd probably last a lifetime on one set .

regards
ian

[Rob Harknett]

I am sure pistons are oval, they expand round when heated to running temp.

[dave16mct]

The manual was probably referring to a wire-wound piston which had very small tolerances. Your piston may be a GPM.
Dave.

[SPRIDDLER]

Agree with all above but you did say this is your first effort (and we all started there) but are you familiar with signs of seizing? All pistons will have discolouration/blackening on its sides/skirt but has the piston 'picked up' on the skirt (physical damage that you can just feel with a finger nail) or has piston material been dragged across the ring grooves or is there scoring of the bore, (however slight), that you can feel? (A photo would help).
Are you in touch with Kent section as there is surely someone there with the kit to measure the wear accurately in the right places?
It's likely that all it needs is honing.

[g80csp11]

try a feeler gauge in the bore between bottom of skirt ( not in pin axis) and if you can just get a .004" feeler in then clearance is a tad bigger ie .0055" a good approximation of the clearance
as Neville says a picture will help

[HarryD]

Piston 1.JPG

First of all, many, many thanks for the comments and advice regarding my ‘little' problem.
It's very clear to me that my learning curve is more like a vertical line at the moment!

As I haven't got the appropriate tools and from the information you have provided, I thought the best first course of action was to get someone who knows what they are doing to measure the bore.
(I do go to the Club nights when I can and everyone is genuinely helpful; I just don't want to be a bother every time I come across an issue.)

So, I spoke to a local specialist who will take the measurements for me.
In the course of our conversation, I mentioned that the reason I have taken the engine apart was because the oil pump seized and I wanted to ensure that there was no chaff left after the problem. That, he said, may well have caused the/a minor seize in the cylinder and why the piston was marked. In hindsight, it would have been a good thing to have mentioned here but didn't think to do so. (Another lesson!)

I'll try to add some photos of the piston for information and if I can, you will see that there is longitudinal damage (that can be easily felt).

He also noted that the barrel has been sleeved; it was a really good job because even when he pointed it out, I could barely see the join. Nevertheless, another reason why such an old bike should have a standard barrel size.

So, I await the measurement and hopefully, honing will be all that is required (plus the piston of course that I'll buy from the Club website and which, I'm assuming' is of the new type.)

Again, I am very grateful for the help and guidance.

[SPRIDDLER]

Yes, that has seized and also dragged piston material across the oil control ring groove. It appears that the ring lands above have also suffered scuffing.
It's a bit concerning that the oil pump 'seized' as it is driven mechanically by a worm on the timing side axle. If it did actually seize the splines on the pump plunger are probably damaged. (The pump drive worm on the timing side axle may also be damaged but we won't go there, yet). Look at the very end of the plunger for any factory marking. It may have '2S' or similar stamped on it.

CIMG0004 (640x480).jpg

Here's one with knackered splines........

Oil plunger damage.JPG

[HarryD]

Re the oil pump, you are absolutely correct, the damage was exactly as you say, the splines were pretty bad. In fact, the reason I stripped the bike down was to get rid of all the bits that had come off the oil pump. I have replaced it and it does seem to 'oscillate' when i manually turned the crankshaft. I'll keep my fingers crossed that it has enough on the worm to keep it going.

[alanengineer]

If your oil pump looks anything like Spriddlers photo, i would not hesitate to strip the engine back to inspect the worm drive. Maybe, knowing bits of engine might still be floating around, strip the whole engine. Not such a big task really.