Determining Condition of Pistons, Rings, and Cylinders

[abyssmaltailgate]

Certainly not a new topic, but I'm trying to appraise the condition of my pistons, piston rings, and cylinders.

First, I want to replace my piston rings, but am unsure what size/dimensions I need. The local Yamaha dealer wasn't much help since their computer database archives nominal sizes i.e. "standard," "1st oversize," "2nd oversize," etc. Based on the info outlined in the Haynes manual and that stamped on my engine's parts, the diameter of my pistons are 74.954mm, and the diameter of my cylinders bore is 75.007mm, which means there is a 0.053mm clearance. This isn't much help since Haynes doesn't indicate if this is standard bore or not. I assume it is but I want to know for sure. Hopefully someone out there can help me clear this up?

Additionally, I did the poor man's test and shined an LED light upwards from the bottom of my cylinders with the pistons and rings in place in pretty much total darkness. No light shone through. What's more, the rings were within the measured tolerances outlined in the Haynes manual. Based on these findings, everything should be in good, running condition; however, this is a 40+ year old engine with unknown mileage, and I believe most of the internal components to be original. I also removed the rings to polish the piston heads. I figure I should replace them anyway as a measure of cheap insurance.

As far as the pistons and cylinders are concerned, they all seem to be in good condition more or less. One piston has two, small, shallow scratches on the dome since I dropped it while polishing it by hand. I can't imagine this would create any problems, but then again, I really don't know. Finally, the cylinders all feel smooth on the inside but have some transverse staining (probably from the pistons sitting in place since it was last ran) and what is potentially light scoring. Will someone please appraise the condition of these parts and point me in the right direction?

I've considered having the cylinders bored and honed to 750cc, but I also want to get this Yammie back on the road before the end of the season. This will obviously increase my spending and the time the bike spends off the road, but if the the rings and pistons need to be replaced and the cylinders honed anyway, maybe it's worthwhile since the engine is completely disassembled now?

 

[gggGary]

those "stains" look like rust pitting from here. The largest you can bore the stock cylinders is 706CC or 78mm My engine guy is whining about how thin the unsupported sleeves are below the castings at 78mm. Takes several thin cuts to get it opened that big without distorting the sleeve during the cut. This is above my paygrade, just reporting what he says. Pistons and cylinders do not wear evenly they wear most near the top of the ring travel and more at the front and rear than sides. how many miles how hard are you going to run the bike? piston sets from japan are sub $100 2 bores $50 to $140ish a safe bore to clean up the rust and for long life is prolly 75.5 or second over.

 

[abyssmaltailgate]

how many miles how hard are you going to run the bike? piston sets from japan are sub $100 2 bores $50 to $140ish a safe bore to clean up the rust and for long life is prolly 75.5 or second over.

All good to know, gggGary. I'd like to make this bike a daily rider; however, I live in the Chicagoland area, which means she'll hibernate for half the year. I'm 145lbs, so the bike won't be hauling a ton of weight unless I take it camping and lug some gear or my girlfriend hops on the back.

Can you recommend some good places to purchase quality aftermarket pistons and piston rings? I've seen the name Wiseco thrown around a lot...?

Lastly, I assume if I bore the cylinders to 75.5mm and go 2nd oversize, the piston size will stay the same, but the piston ring size will change?

 

[abyssmaltailgate]

I see in another thread that you approve this brand, which along with the opinion of DaddyG, is good enough for me; however, my bike's a '74 or '75 XS650B, not a '70 thru '73 XS1... I'm wondering if this 75.5mm set will fit? Anybody know?

(2nd oversize is +0.50mm, my cylinder is 75.007, and 75.007mm plus 0.50mm equals 75.507mm, or approximately 75.5mm)

http://www.ebay.com/itm/70-73-YAMAH...ded-/161019611496?vxp=mtr&hash=item257d844968

 

[5twins]

No, that piston set won't work. They're for the '70-'73 "256" motor. You need '74 and later "447" pistons. I don't see a listing for them in the .5 over size at the moment. The seller must be out of them. He does usually have them.

Your pistons and bores look to be original size. The original clearance spec was .050-.055mm. But your pistons and bores are used now so they probably have some wear. They will have to be measured. If still within spec, you could just replace the rings. You would just hone the cylinders if you did that.

 

[TwoManyXS1Bs]

Quick answer, and shooting from the hip, it doesn't look like those bores will cleanup with a hone and remain within diametrical spec.

As 5twins said, you want the '447' version of Cruzinimage's 75.5mm piston kit. You can contact him thru eBay's message system, see if he's got any.

Else, you could get (2) each of these:

http://www.mikesxs.net/parts/yamaha-xs650-447-piston-2nd-oversize-0-50mm-74-84-oem-447-11636-01
http://www.mikesxs.net/parts/yamaha-xs650-ring-set-1-cly-2nd-oversize-0-50mm-oem-447-11610-20

You can also look into 650central.

Then, take the piston kit to the selected machine shop, show them the piston/bore clearance specs, and ring gap specs, from your service manual. They'll need all that because cylinders have to be bored to precisely fit the pistons, and they'll set the ring gaps.

 

[gggGary]

cruisin' has a 76mm set for the 447 motors on fleabay. this fits all 447 engines 74 to 84

http://www.ebay.com/itm/74-76-YAMAHA-XS650-1-0mm-OVERSIZE-PISTONS-SET-2-PISTONS-447-TYPE-10-XS650PS-2-/131160014127?hash=item1e89befd2f:m:mr060Ky-5ycW3r_i5qK38xQ

No reason you can't or in the USA shouldn't, bore to 4th oversize....

Generally you need to keep ring and piston size matched, there is a "mis-match" trick but it's kind of advanced to get done.

Commercial message warning;
FYI I do have a 447 .5 over piston, rings and bored to match cylinder set in stock...

 

[abyssmaltailgate]

Thanks, 5twins, thought it was worth a shot.

I was going to ask about Mike's pistons. I figured if the Jap ones are good, so are the ones he carries. Thanks 2M. Not that it'll save me any money, but I'm wondering if the pistons Heiden Tuning carries are the same as Mikes? The descriptions are mighty similar... just curious: http://www.heidentuning.com/p/28/22...9-piston-2nd-oversize-447-type-price-1-piece-!

or http://www.heidentuning.com/p/28/3770/mo8-cg|txt=/#05-4472-piston-kit-7550-2nd-oversize-us-447-type-

One final thought: what does boring the cylinders to 76mm or 4th oversize do to the displacement? Still just curious, but I have to imagine it's going to bump it up from the stamp 653cc a little...

 

[gggGary]

quick math sez 670cc or 2.7%

 

[5twins]

The two quick and dirty checks for wear are fitting some of the old rings in their respective bores and checking their end gaps .....

A little too big could be just ring wear but really big is probably a combination of ring and cylinder wear.

The other test is to insert a bare piston (no rings) back in the bore and see how big of a feeler gauge you can jam down in between it and the bore. Any bigger than .004" is too much. The original clearance was about .002".