Barrel and Piston Query

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Re: Barrel and Piston Query

Postby SPRIDDLER » Thu Aug 02, 2018 9:11 am

Hmm. Hmm............
You've only done 50 miles on the bike before this problem occurred which triggers concern to me. Do you know or were you told any history of the bike? Has it been 'restored'? Realistically there's more to consider than the damage to the oil pump.
A worn timing side bush can allow the axle to 'hammer' causing the splines/worm meshing depth to vary and the splines will quickly wear, jam or 'skip' out of step with the worm.
Plungers wear when the timing side bush is worn but they rarely seize (and have bits of the splines crack off as you mentioned). Your new plunger (if you have the correct two-start worm) and the old one should have 2S stamped on the end. The annular groove in the plunger for a post 1947 engine should be 1/4" wide and the oil pump guide pin should be a close fit in the groove. (The plunger was changed in 1947 from a 3/16" annular groove and guide pin and the worm was a one-start). If your timing side axle has (for whatever reason) the earlier one-start worm the new 2S plunger will not function correctly and damage will occur.
I don't want to spread doom and there's more opinion that I could add, but if debris from the pump has spread it could/will also have been transported by the oil to the big end, main bearings, valve gear etc. Bits may well be lodged in the relatively soft phosphor-bronze timing side bush and you need to examine the oil pump plunger housing in the alloy crankcase for scoring.
You must at the very least drain the oil and empty the crankcase (looking for steel and bronze coloured debris) and filters (2) and flush the oil tank and the engine to dislodge any debris in crevices.
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Re: Barrel and Piston Query

Postby aobp11 » Thu Aug 02, 2018 2:15 pm

The piston for sure is (or was...) a GPM 6916. All my GPM pistons have the size in mm stamped on the piston crown. However all these piston where oversize pistons, it could be that the standard size has no stamp. A 6916 +40 lying here is stamped "70".

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Re: Barrel and Piston Query

Postby HarryD » Thu Aug 02, 2018 9:31 pm

Hi Spriddler, Thanks again for your note.
The bike is a lovely old oily rag. The frame still has the original enamel. Other bits here and there have been painted or sprayed including, for example the handlebars and wheels. But I love it!

It didn’t come with any great history but was running OK when I got it. The root cause of all the problems was that the guide pin and the screwed body holding it in to the plunger was a ‘bodge’. The correct pin hadn’t been used but a home-made version had been fashioned. Essentially, that caused the plunger to seize and, it’s thought, led to the starvation of oil causing the damage to the piston.

As I mentioned, I have taken the engine apart; following some interesting discussions at one of the Club meetings, I decided to have the big end renewed which was undertaken professionally.
The bushes and bearings have all been replaced on the basis that for the cost, they might just as well be done while the engine’s apart. All the parts were replaced ‘like for like’ and sourced from the Club Spares Shop.

I am in the process of cleaning the oil tank but am having a job releasing one of the nuts that holds the metal gauze oil feed filter; I have also bought the filter for the return side.

One other small problem that I have run up against is that I have a bolt that has a sheared thread and the Shop hasn’t got any – so I’ll get one made up.

It may sound a bit daft, but in a way, I’m not disappointed that I have had the problem as it has made me learn so much more about the bike. I’m even able to convince my friends who know absolutely nothing about the bike that I know something!

Hi Albert, Thank you for the note. At least I now know the make although some of the comments on line are not particularly complimentary about them.

Thank you all again, H

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Re: Barrel and Piston Query

Postby SPRIDDLER » Thu Aug 02, 2018 9:44 pm

HarryD wrote:As I mentioned, I have taken the engine apart; following some interesting discussions at one of the Club meetings, I decided to have the big end renewed which was undertaken professionally.
The bushes and bearings have all been replaced on the basis that for the cost, they might just as well be done while the engine’s apart. All the parts were replaced ‘like for like’ and sourced from the Club Spares Shop.

Proper job, Harry! You've dealt with all the things I was loathe to suggest for fear of being a doom-monger.

It may sound a bit daft, but in a way, I’m not disappointed that I have had the problem as it has made me learn so much more about the bike.

Nothing at all daft about that. Getting up close and personal is the key to a long and rewarding relationship. Or so I'm told ;)
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Re: Barrel and Piston Query

Postby HarryD » Sun Aug 12, 2018 11:47 pm

Cylinder Measurements
The result of the measuring of the cylinder bore is that it varies from a minimum deviation from standard (2.7187”) of +0.0009” to a maximum of +0.0024” - a difference of 0.0015” over the length of the cylinder through both axis’.

I understand the allowance for honing may be another 0.001”.

The manual suggests that a re-bore to +0.020” should be done where the wear reaches 0.008”.

Piston Measurements
The standard piston is 2.7143 +/- 0.0005.
Therefore, the clearance between a new standard cylinder and piston is 0.0044”: the extremes would be 0.0034” and 0.0054”.

So;
I have concerns regarding the variations throughout the cylinder bore:
A standard piston could result in a gap of 0.0078 ish (after honing) which is mighty close to the limit:
I have been advised that it would be better to fit the bore to the piston and not the piston to the bore.

My head is telling me to buy a new piston and have the barrel bored to fit.

I would be grateful of observations!!!!
Harry
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Re: Barrel and Piston Query

Postby SPRIDDLER » Tue Aug 14, 2018 4:28 pm

HarryD wrote:I have been advised that it would be better to fit the bore to the piston and not the piston to the bore.

Fitting the piston (by machining it?) to match the bore is not something I've heard of. Besides, pistons are designed to have some ovality when cold which (I assume) would be lost by machining process. I don't know how much that would matter in reality, but it's something to be aware of.

My head is telling me to buy a new piston and have the barrel bored to fit.

That's the usual way.

If it was mine and in view of having already replaced the big end, main bearings and bush, plus presumably the timing side axle and pump and the fact that the piston has some damage it wouldn't make sense in my opinion not to rebore and fit a new piston. At least you'd be starting from a known position.

(I'll probably be offline for a week or so as off toward the Jampot venue early tomorrow via a night stop en route).
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Re: Barrel and Piston Query

Postby HarryD » Tue Aug 14, 2018 8:36 pm

Sorry Spriddler,
What I was saying was that I should buy the piston and then get the bore sorted out to match the piston; as opposed to simply getting the rebore and then buying the piston.
Sorry if I confused you but thank you for the observation - I'll buy the piston and get it re-bored.
H

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Re: Barrel and Piston Query

Postby SPRIDDLER » Tue Aug 14, 2018 9:52 pm

HarryD wrote:Sorry Spriddler,
What I was saying was that I should buy the piston and then get the bore sorted out to match the piston; as opposed to simply getting the rebore and then buying the piston.
Sorry if I confused you but thank you for the observation - I'll buy the piston and get it re-bored.
H

Well I'm old school, but I did wonder if there was a Hi-Tech piston reclamation/machining process of which I was unaware. :?
The cylinder is bored to match the piston because the borer will (should) want to know how much piston to cylinder clearance to allow when he bores the cylinder. The suitable clearance can vary between piston manufacturers/materials. The piston oversize selected will also be determined by the amount of wear or scoring that has occurred to the cylinder liner. But you knew that I'm sure :oops:
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