Piston Clearances

I apologise for being so inept but I am still struggling with the question of whether or not my '78 SE needs a rebore and 1st oversize pistons. I posted a photo last week showing a .009" feeler gauge inserted between my piston and the cylinder wall (pic 1). I had made my mind up to take it back to the shop and get them to measure it with the appropriate instruments but I decided to attempt to resolve the issue myself. Following advice from the forum, (much appreciated as always), I turned the jugs upside down and installed the pistons in the cylinders (pics 2 & 3). I now found that I could barely insert a .002" feeler between the piston and cylinder wall (pic 4). I realise the cost of the measurements will be insignificant but I hesitate to turn up at my local shop with a set of paranoic questions that leave me looking like a yabbie, (which is how I am beginning to feel). Obviously, I don't want to rebore and purchase new pistons if I don't need to. Have a look at the pictures and tell me what you think.
Sorry about the focus in pic 2 - looked fine without my glasses!

Wellll, the feeler gauge is going in, at least in the first picture, which is enough. So take the max measurement from the book and multiply it by two, to account for max on both sides, then, with no rings on, if you can cram a feeler gauge bigger than that between the piston and the wall, there's too much wear, technically speaking. I wouldn't feel bad about taking to a machine shop just to confirm, before he bores it out.

Take it to the shop and he can measure with a dial bore gage, the cylinder may be out of round. The only way to be sure. Just go to the shop and ask them to check it, leave it at that.

Took my jugs and pistons back to the shop and had them measured. Actually, he only checked the piston as he knew he had provided .0036" clearance with the hone. Piston measured in at 2.951" or 74.9554mm. Original spec was 74.959mm so I had lost .0036mm off the piston through wear. I was surprised at this number as it matches the clearance given in inches.
The clearance when new was .051". If I add this to the new clearance, I end up with a clearance of .0546" which would appear to be in spec meaning I can put the pistons back in using standard rings. But, if I take the new piston diameter of 2.951", add the original clearance of .051" and then add the honed clearance of .0036", I end up with a bore size of 3.0056" or 76.34224mm which requires a piston from the Titanic.
Interestingly, when I asked the mechanic to calculate the "finished bore", he told me that it was 2.947" or 74.8538mm - he wrote this down.
Those are the figures I have. Am I doing something wrong - it all doesn't seem to add up. The machine shop is reputable but they seem to feel that, given these figures, I should be able to calculate ring size accurately. Am I over thinking the numbers? There are some coincidences and contradictions here that serve to confuse the issue. I've read the Clymer, I've scoured the posts and I've been back to the shop twice and I still don't know whether to buy standard rings or oversize pistons and rings, or just oversize rings and file the gap, or a partridge in a pear tree, or........

I CAN'T tell what's what for your #'s, so what is your bore size and what is your pistons size and what is the ring end gap. If he honed and you have only .0036 piston to cylinder clearance you are good to go. That's what we set up new JE piston with... Ring end gap is generally figured .004 per inch of bore.. also Yamaha ring were always on the short side.. I usually get the next size up. So in you case min. would be .012, up to .020-.024 max.

I think you're mixing inch and metric sizes together and that's confusing you. Yes, the "new" piston to cylinder clearance should be .050-.055, but that's MM not inches. That would equal .0019-.0021 in inches. Stock, all these pistons were a little under 75mm and the bores a little over. If "959" is what's stamped on top of your pistons, the original size was 74.959mm. There should be a 3 digit number stamped on the side of the sleeves that hang out the bottom of the cylinder casting. To meet the "new" specs, you would want to see "009" to "014" there. That would give you an original bore size of between 75.009 and 75.014mm.

Hello hooser
The mechanic didn't use a bore gauge on the cylinder, he mic the piston and then pointed out that there was .0036 clearance - this is how he wrote it down,
piston size 2.951
clearance .0036
finished bore 2.947
As you can see, he has subtracted the clearance from the piston size. I would have thought that these two figures should have been added to give the finished bore size. Maybe he's a mug not me - feel better already at that possibility but still confused.

David, first off I'd suggest sticking with either MM or inches. Doesn't matter which you use but using both just clouds thing up. I'm gonna use inches here....

David Toll said:

Piston measured in at 2.951"

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And then you state...

David Toll said:

....asked the mechanic to calculate the "finished bore", he told me that it was 2.947"

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Based on that, your pistons are .004" larger than the bore. Since they came out of that engine, I think we can assume one (or both) of those numbers is wrong.
You need to either get accurate measurements of pistons and bores. or go back to the feeler gauge method.

Hello 5twins.
Yes, my piston was stamped at 959 and the bottom of the cylinder is stamped 010. So the original clearance was .051mm or .002". The piston now reads 2.951" so if the honed cylinder is still in the same spec as original, the clearance should be 2.953" (75.010mm) - 2.951 or .002" which is fab; however, I find it difficult to believe that the cylinder has lost none of its substance over 20,000 miles.
Regardless of this, the cylinders are within the spec you cite above 5twins so I assume I should stop procrastinating and order some standard rings.
I believe you recommend getting first oversize and filing the gaps, might try that.

David Toll said:

Hello hooser
The mechanic didn't use a bore gauge on the cylinder, he mic the piston and then pointed out that there was .0036 clearance - this is how he wrote it down,
piston size 2.951
clearance .0036
finished bore 2.947
As you can see, he has subtracted the clearance from the piston size. I would have thought that these two figures should have been added to give the finished bore size. Maybe he's a mug not me - feel better already at that possibility but still confused.

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How did he measure the cylinder? can't do one and not the other?
Did he do the cylinder before? so how did he know there was .0036 of clearance. as I said if that is the clearance you are go to go.

You need to either get accurate measurements of pistons and bores. or go back to the feeler gauge method. (Jim)

I tried the feeler gauge bit Jim. Could stick a .009 gauge into the top, (with piston at TDC) but only a .002 gauge from the bottom (with the piston about an inch up the bore). Took that measurement at 90 degrees to the pins. I was encouraged by this as I have read that the piston expands more at the top do to the heat generated by combustion. My wife tells me to take it back to the shop and have them measure the bores, she is tired of my temper tantrums, sobbing and heavy drinking. Perhaps she's right but I feel that the answer must be here.

xjwmx said:

Wellll, the feeler gauge is going in, at least in the first picture, which is enough. So take the max measurement from the book and multiply it by two, to account for max on both sides

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OK, page 27 of the Yamaha manual....


The clearance stated on page 64 is .050-.055 mm
Lest we confuse poor David here... that clearance is "per piston", not "per skirt." You most definitely do not double that number.

Yes hooser, I had taken the pistons and cylinders to the shop previously for a hone. They must have measured the bore then 'cause the number .0036 was written on the top of one piston in indelible pen when I picked them up.

Run them,
what is the ring end gap?

David Toll said:

...but only a .002 gauge from the bottom (with the piston about an inch up the bore).

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Top is not where you measure, you do it at the skirt and it sounds like your .002" was done correctly. Book tolerance is .002" - .0022". So, if your "skirt" measurement was .002".... you're golden

David Toll said:

I believe you recommend getting first oversize and filing the gaps, might try that.

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Not following your measurements, but if you have determined that standard rings will do...I suggest getting the standard rings from cruzinimage_co on ebay. First, they're the least expensive you'll find. Second, they press hard against the wall, and because of that you won't burn any oil. Mine burns literally no detectable oil between 1000mi changes.

Interestingly, I have read posts, on this and other forums, that suggest that the max clearance be doubled. No, I'm not quite that dumb Jim but please, don't hesitate to continue to simplify things for me, assume I think a spanner is a species of edible crab.

Thanks xj. Unhappily, cruisinimage do not post to Australia. Still sore about the Kokoda trail I suppose.

Little bit of excitement in the woods is nothing compared to what we did to them. More likely your customs guys are holding stuff for six weeks and he doesn't want the bad feedback for it Or could be you make him fill out reams of paperwork and he's just in it for a hobby.

You got a point xj. Maybe he just doesn't like our accent. Either way, he doesn't ship here. Shame 'cause at $25 (US) for a set of two they are really cheap.