WHERE DID IT GO , IM AM WORRIED

[funky]

Hi every one hope you are well .
Got my new piston kit today and i checked the bore to skirt size and the pistons are about 1 thou to big . My question is... do I hoan the cylinders myself or do I get them machined at the shop. The tollerances should be 2 to 2.5 thou at the moment they are 1 thou...So each cylinder needs to be 1 thou bigger .Your comments and views will be greatly appreciated .
All the best Funky

 

[Bob Kelly III]

I will let 2M answer your technical question or other guys that are more knowlagable than me on these things....
I would hazard to guess that it's not that critical but it may well be in this beast I just don't know !
myself I would hone it and then put it back together and Hope it didn't seeze then it got hot ..... but there is better ways to go about it than that <GRIN>
I've not read where people have a trouble with the cylinders seazing on this machine.... so it's possable that only 1 thou is enough....
i'ed just hone it real good trying to keep it even from top to bottom of the cylinder and go for it !
but that's me..... and I have been known to take larg risks in engines ! <GRIN>
Bob...........

 

[kopcicle]

Another boring thread (cylinder)

 

[TwoManyXS1Bs]

Funky, this is one of those things best done at the machine shop.
Just take the cylinders, pistons, rings, and book specs to them...

 

[kopcicle]

^ ^ ^ ^ Good advice in any case. I'd just like to add that it must be a machinist with experience in Air Cooled Motorcycle Cylinders . In the link above I was talking about Hypereutectic Alloys with a much smaller coefficient of expansion . However the technique is the same . Coarse hone nearly to size and finish with a finer hone . This leaves a portion of the coarse hone pattern to retain oil for lubrication and the finer hone to contact the piston and provide ring seal. The only difference is the final clearance . Call me a dinosaur but I break in with Castrol HD 30wt .
Do take care to NOT elevate rpm abruptly until the oil is full hot . Ever wonder what collapses sump screens ? Believe it or not it's oil pump suction ...

~kop

 

[funky]

Ok thanks will do what To many says. I was going to hoan out thinking ist only a thou but my consern was getting an even contant clearance down the bore with a hoan on a drill. The fact that the last time i did this was 30 years ago and I have no way of being precise bar a feela guage leaves room for error.
Im sure the latest rebore equipment can take a thou out. or an experianced person can hoan to that tolerance.

 

[Bob Kelly III]

Good ! I think you are wise !
that should cure the Oil loss ... if it don't .....i think you may have a evil deamon that is drinking it out of the engine when your not looking !...they do that ya' know !
Bob.......

 

[peanut]

well at the risk of being shot down in flames I'm going to suggest that that amount of oil loss couldn't possibly be accounted for by loss past the piston rings .

Commonsense should tell anyone that if you can lose that amount of oil in a 50 mile ride with a brand new engine ........then how much oil would the rest of us be losing every 50 miles with our clapped out engines with wornout rings !...........

I recently rebuilt my 79 special ll engine. I lightly deglazed the bores and reused the original pistons and original piston rings . The bike had done 18,000 miles according to the speedo and I'm betting that they are original miles since the bike was new in 1979. It uses no oil and it loses no oil due to care and attention in rebuilding.

Looking carefully at the head gasket images it seems clear to me that compression was leaking past the head gasket into the cam chain tunnel and cyl head bolts. Look carefully at the blue and blackened gasket surfaces ringed in red.
It is clear that compressive gases have been leaking into the crankcase causing excess crankcase pressure resulting in oil loss from the crankcase breather . gggGary and Dogbunny had the right idea right at the beginning of this thread . If you had connected a catch bottle to the crankcase breather and tested the engine as suggested BEFORE stripping the engine, you would have been able to eliminate this possibility.

I have posted about this before and the first time that I became aware of this particular issue was when Anlaf had to rebuild his brand new engine 6x times yes thats six times ! before curing a smoky engine that was losing oil. See post #13 in this final thread for his solution http://www.xs650.com/threads/anlafs...ith-video-look-away-5twins.31182/#post-314752
If you want to eliminate the possibilty of compression leaks into the crankcase it is essential to apply liquid gasket around the camchain tunnel and cyl head bolts to both sides of the head gasket when rebuilding the engine. Resurfacing the gasket faces often doesn't seem to be enough with these engines .

 

[Bob Kelly III]

Hummm... you do have a very good point peanut !!!!!!
I would think that if the engine was burning that much oil in the cylinders it surely would have foweled the sparkplugs ! yet he said the engine ran very good !....
but to me that head gasket looks fine.... head gaskets I've seen that have leaked compression are black, permeated with carbon.... you cannot mistake where it leaked, There is a black line across the gasket to the relief point, but I see none of that, what I do see is areas where the gasket over hung the opening of the cylinder and the head and they were slightly discolored by the constant flow of oil..... this tells me the cylinder head was sealed well.
but perhaps he cought it before it had a chance to discolor the gasket it may have only leaked at higher RPM's and he said he hasn't driven it at higher speeds that much.... it is an odd one that is for sure !
I am ever so thankfull that I put my cylinder head on with gasket goop now after what you have said.... I may never get it off again but I'll cross that bridge when I come to it ! LOL....
Your absoultly correct however... that much oil say almost 3/4 of a quart.... would have killed the sparkplugs in short order.... if it blew it out as you suspect the breather tube would have taken it to the air box which should be covered in oil....
the question is .... is it ? if not... it had to go somewhere...... but where and how ? if it was a head leak the seals would be in jeperty because of the higher crankcase pressure.... the oil could have easily excaped from the sprocket seal and not be noticed because there is alwayse some oil on that side after oiling the chain..... it is the seal that gets the most dammage and usually the first to leak in my experience.
that much oil though it should show up on the tire as well and that you do NOT WANT !...... trust me !!!!!
....
How about it Funky ? any oil in the airbox ? or oil on the back side of the bike anywhere ?
......
Bob..............

 

[TwoManyXS1Bs]

Sir Funky. If you intend to use that copper head gasket, definitely have the head and cylinder milled, decked (skimmed)...

 

[funky]

Hi Chaps thanks form responses. Head gasket came with Piston kit so its not a copper one, there was no oil in the air box but there was in the intake ports so Penut could be very right. I went to my local machine shop and he said Hoan it out as 1tho was to smaller amount for his equipment. felt like he was not intrested and he said thats what he would do,,, so I bit the Biscuit and did it myself. It was good fun I took my time .... so i am at 0.0020 ish probably + or - 0.0001 I recon per Piston I am going to put in extra heat cycles when running in initially and see what happends. My worry is the the ring gap there at the high end of tolerance but being these are 12 over pistons it may be fine.

 

[Ed Zornes]

[
I'm not anywhere near an expert on this, but I think the clue to this issue was given early on: The OP stated when he " got on the gas" there was blue smoke out the exhaust. Typically, when an engine smokes at idle, that is the point where there is maximum vacuum in the intake path, and if there is a problem with the intake valve seal, it will smoke at idle, but not under throttle. (as the carbs are wide open, there is not much vacuum trying to pull oil past the intake guide seal) However, at full throttle, if the rings do not seal correctly, there is blowby past the pistons, and you will get excessive oil consumption, and blue smoke out the exhaust...Rule of thumb for ring end gap? .004 for every inch of bore.....Just my .02....

 

[funky]

Thanks for that I will appy that rule I got a feeling there over going to re check them .

 

[funky]

I have checked the end gaps and there too big top rings 0.0023 and 0.0025 second ring are 0.0012 and 0.0013 top rings the scraper ring the smaller of the two rings .Not sure what to do bar search for slightly bigger ringsset . Bores measure 78.03 and 78.02

 

[funky]

I just found this from XSLeo

It depends, Are the bores stock or first over? If stock you can use the stock rings with stock pistons. If first over you need the first over pistons and you will need first over rings.
The stock bore is about 75.010 to 72.016. At least in the engines I've torn down. The first over bore is 75.25. Rings have a certain amount of end gap. This is the gap between the ends of the rings when installed. the top two rings gap is .2 mm(.008 inch) minimum, .031(.8 mm) maximum, for the 70-77, the 78 up the maximum was changed to .040 (1.0 mm)
This applies to stock as well as oversizes. If you try using standard rings with the first over the gap will be too wide. Take the bore size times pi to find circumference of both sizes then subtract the standard from the oversize. Pi being 3.14.
75.010 x 3.14 = 235.5314, 75.25 x 3.14 = 236.285
236.285 - 235.5314 = .7536
So the end gap will increase by .7536
This would make the gap .7536 + .2 = .9523 mm to .7536 + .8 = 1.536 mm. The tension of the rings against the bore will be greatly reduced by a gap this large. Between the huge gap and very little pressure of the ring on the cylinders you will get tremendous blow by of compression prfessure past the rings.
What I might do is measure your bores to determine if stock pistons can be used or you need the first over. My guess is that they are there then you need the bore. If you need the bore you need the rings.
If you don't have the tools or skills needed to accurately measure the bore sizes take them to a machine shop. Most won't charge much. If they do the machining they may do it for free.
Leo

I wonder if this continued to the 1981 model
the top two rings gap is .2 mm(.008 inch) minimum, .031(.8 mm) maximum, for the 70-77, the 78 up the maximum was changed to .040 (1.0 mm)
I applied the math to my dimensions and came up with 9.4828,mm which is 0.0038 which means my end gaps are ok I think.
my bore size 78.03 x 3.14 = 245.0142
std bore size 75.01 x 3.14 = 235.5314
245.0142 - 235.5314 = 9.4828mm or 0.0038 thou
What I have read is that overbores need to have more gap due to the extra thermal expantion due to obviously greater heat. the end gap has to be right for these pistons as they are specific to my bore size 78mm even if i put new sleeves it I would have to fit the new pistons which will result in the same measurement. the stock clearance end gap cant be the same for pistons 3.03 mm over the maths prove that. Just saying like

 

[funky]

Also found this article
Piston Ring End Gap Recommendations


Most piston ring manufacturers recommend a minimum end gap of .004 inches times the bore diameter for the top piston compression ring. So for a 4 inch bore, the standard end gap would be .016 inches.

For the 2nd compression ring, the standard end gap recommendation for most stock engines is typically .005 in. x the bore diameter. So for a 4 inch bore, the minimum end gap on the 2nd ring would be .020 inches.




For a modified street performance engine that generates more horsepower and heat, the end gap should be opened up a bit to compensate for increased thermal expansion. The recommendation would be a minimum top compression piston ring end gap of.0045 to .005 inches times the bore diameter. For a 4 inch bore, the ring end gap on the top ring should be increased to .018 to .020 inches.

For the 2nd compression ring on a modified street performance engine, the recommendation is typically open up the end gap to .0055 inches times the bore diameter. For a 4 inch bore, the 2nd ring would be gapped to .022 inches.

For a nitrous or blown racing engine, the top ring end gap should be opened up to as much as .006 or .007 inches times the bore diameter. Now we're looking at a 2nd ring end gap of .024 to .028 inches in an engine with 4 inch bores.

For the 2nd ring on a nitrous or blown motor, the recommended ring end gap is even wider: .0063 to .0073 inchestimes the bore ( or 0.025 to 0.029 inches with a 4 inch bore).

The recommended ring end gap for oil rings regardless of the engine application is typically .015 inches.

Some racers believe that opening up the 2nd end gap even more (say an additional 10%) can improve overall ring sealing by allowing trapped gases to escape before they blow past the top ring and cause ring flutter at high rpm (say above 5000 to 6000 rpm).

 

[funky]

I just re read the bit about oil in my air box. Bob Kelly III. On the xs650 special 1981 the oil breathers are forward of the air filters the passage of air from the filter to the intake would keep the oil away from the air box and the filter thats why i had wet oily intake ports and dry clean air box and filter.

 

[Bob Kelly III]

Funky: if there is any amount of Oil ANYWHERE in the air box breather system you have a problem !
a fine oil mist is all that should be coming out of the oil breather tube... if it's enough to coat the entire intake system up into the cylinder
i think you found where your Oil was going ! and it explains why the plugs didn't fowel out because the Oil was atomized well by the time it was burned
..... I think allot of blow by due to the rings and a slightly leakly head like Peanut said was the cause.... (probably due to not re torquing the heads after the 1st. and 3rd run.... (not the rebuilders fault) ..... that is my guess ... putting it back together again should cure the whole problem I suspect.
but be sure to Re torque the head after a few runs !
I like to retorque after the first initial warm up and cool down..... then ride it a few miles let it cool and retorque again....
then after a 100 miles or so and that should be all that is ever needed...... don't over torque ! the nuts will strip or worse pull the stud out of the case below !
Good luck hope you get it running soon.... take a file to the ring gap and get'em installed ! (if you feel so inclined !) a few strokes with a good file is all that is needed. finish with fine sand paper.
.....
Bob.......

 

[jaybird51]

On fresh rebuilt engines we never use synthetic oil for break in. We use mineral oil for 500 to 750 miles to give the rings a chance to seat. Syn is way to slippery and we encountered trouble with new rings seating. It could be what happened in your case. The rings may have never properly seated. Just saying! After break in synthetic is a great choice.

 

[TwoManyXS1Bs]

I used to examine the wear patterns on piston rings, looking for anomalies. Many of the '70s Hondas had a very slight edge taper (1/2°-1°) on the top rings, and you could see the break-in/wear-in pattern on the upper part of the ring edge, and could tell how well they were seating, or if glazing.

As an example, this fuzzy pic shows a wear-in anomaly on the 2nd ring, caused by a bore job done without torque plates (different engine, doesn't apply to the XS650).

If you like, you could try to post exceptionally crisp/clear closeup pics of the wear edges of your old ringset, and we could try to interpret...