Top end rebuild on a budget.

webbie

webbie

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Hey all, first time posting to the forum - though I've been around on the 650garage and a few others.

I'm not new to the xs, however. Mine (1982 Heritage Special) has been in a perpetual status as project / PITA for a few years now. Bought it straight out of a muddy ditch next to some guy's house in Queens, and I've been slowly nursing it back to roadworthiness since.

I've not had the time or money to do a full rebuild prior to now. Finally, I decided to make a proper attempt, and I've stripped the bike to the frame for cleanup, painting, wiring, etc. I've got a PMA + CDI kit from Hugh's ready to install upon reassembly, and I'm looking forward to a running bike. I also decided to freshen up the top-end with gaskets, valve-lapping, etc. I was under the impression that the mileage was low, but I'm not so sure now, based on the number of wear-items.

I should mention - this is my first engine rebuild. I haven't gone deeper than replacing the head prior to this, so I'm trying to learn quickly.
Took the jugs+pistons to a machine shop today and the verdict (on the worse of the two cylinders) is:
Min diameter 0.002" (0.0508mm) over spec and 0.0015" (0.0381mm) out-of-round. Stock 75.01mm.
Associated piston is 2.950" (74.93 mm). Stock 74.959 mm.
Giving me an estimated PTW clearance of approx 0.0051" (0.13 mm), compared to a stock number of 0.002" (0.051 mm) - if I'm not mistaken.

In summary, I think I need to go with a bore and oversize pistons. The machinist recommended the same. I want to do everything *right* while I have the engine out and in pieces, I also want to keep this on a short-ish budget (most of which went into the PMA+CDI conversion). Would I be committing some grave error if I sought out some used oversize pistons and bought them at a discount? I'll need new rings, of course, but the machinist suggested bringing him a set of pistons and a desired clearance so he could give me well-matched bores. Any reason I couldn't bring him a set of minimally-worn pistons?
 
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the best thing to do is find a piston set and bring it to them. they will bore and hone to match each piston to each bore. I know a lot of people that have used used pistons. Also I would suggest buying a set of new pistons. If it were me, id buy a set of new oversized. You don't know how hard of a life the used ones had.

*edited for future searches. I was wrong about forged pistons and like stated below by Hooser, trying to rid misinformation.
 
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meanEG said:
if it were me, id buy a set of new oversized. You don't know how hard of a life the used ones had.
Click to expand...

Yeah, that's a good point. I suppose, if I were lucky, I would find used O/S pistons for maybe $50 shipped. That's only about an $80 savings compared to new pistons, and with the cost of the bore and rings, it probably wouldn't be worth it.

As another alternative, what about re-sleeving the cylinders? Do the ones on Mikes come with a stock bore, or would they require boring, even for stock pistons? If they show up ready to accept stock pistons/rings, that might actually work out to be a cheaper option.
 
5twins said:
That "associated" piston at 75.93mm can't be stock, it must be an oversize. Pistons don't wear bigger, they get smaller. Bores wear bigger. There is a reasonably priced aftermarket piston supplier on eBay now. You can get .5mm and 1.0mm oversize sets for not much more than $100.

http://www.ebay.com/itm/74-76-YAMAH...S-2-/162127412217?vxp=mtr&hash=item25bf8bfbf9
Click to expand...
Whoops, my mistake. Typo -- I meant 74.93 mm.

I saw those on ebay. Are they of comparable quality to OEM or the ones Mikes sells? They look to be made in Japan, so there's that. I'm just wary of seemingly-generic and cheaply-priced components.
 
Quite a few of us here have bought them and there are no complaints yet that I've seen. I have some but haven't used them yet, but I foresee no problems when I do.
 
Great, I appreciate the links. After reading through both of those, I'm ready to go for the ebay pistons. I contacted the seller, and he's got only 4th oversize available (no mention of future stock), so I suppose I'll be skipping oversize 1-3 and going straight to a 1mm over bore.
 
The availability changes frequently, tomorrow or next week they may have the other sizes in stock. It's a crap shoot if you wait.
 
Ahh, well I already pulled the trigger on the purchase. No harm in boring 1mm over instead of 0.25 or 0.5, I figured. If I end up needing to re-bore a few years down the line, I can re-sleeve or go to a big-bore.
 
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meanEG said:
the best thing to do is find a piston set and bring it to them. they will bore and hone to match each piston to each bore. I know a lot of people that have used used pistons. Also I would suggest buying a set of cast pistons. Forged pistons will run looser in the bore and not last as long. The pistons will rattle a lot, especially when cold. if it were me, id buy a set of new oversized. You don't know how hard of a life the used ones had.
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What is the clearance on your cast pistons.. My JE Forged are .0025, never rattle. Ran them for 5 season of racing. Gary
 
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I was speaking g from automotive experience only, but theory was applied regardless of manufacture. .0025 is pretty tight for a forged piston IMHO. Usually .004 in smallest I have ever seen. Also your rebuilder might charger a fee to rebore a second pass.
 
meanEG said:
I was speaking g from automotive experience only, but theory was applied regardless of manufacture. .0025 is pretty tight for a forged piston IMHO. Usually .004 in smallest I have ever seen. Also your rebuilder might charger a fee to rebore a second pass.
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Je recommends that usually Roger goes .003, what is IMHO, if that is opinion we know what that is like. Go with facts not opinions. Gary
 
Wow, hard to judge tone in A forum thread, but the point was .0025 was tight and you just proved that with your "fact". I also stated already that I was speaking from automotive experience. Also in a forum thread you are going to get opinions. That's the purpose of them. I am not the only one to talk about rattling/diesel sounding forged pistons. Man, I hope I was just misunderstanding your demeanor.

*edit for future searchers. I misunderstood above Hooser's post regarding what JE was stating as spec due to punctuation error. I see where some of the confusion was. Regardless I eat my words regarding piston slap with the forged pistons.
 
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I am sorry for being hard but I see so much miss- information on here that is called opinion. In this day with the quality of Forged pistons I cannot think of any reason to go with cast other then they are cheap.. I don't know of any modern race program running them. Now here is an opinion I think JE are the Best piston you can by, some think Wiesco. Sorry again if I go over the top.
 
I really didn't give any misinformation. if I was going to run a forged piston I would use what the manufactures specs were if you took a post from some forum to your builder and said use this ptw clearance, you would be laughed at. what I like to see is a discussion regarding running a tighter ptw, like .0025, and with 5 race seasons the thing never seized up. You may have some scuffing on the sides, but you can get away with it. that is real world information, nice. also he isn't building a race motor so most of this discussion doesn't apply. maybe in a car the pistons are larger and will expand to a larger oa dimeter, that's why the larger ptw clearance. I am not an engineer, just a mechanic.
 
My son went with a second overbore kit off ebay, and the whole job with quality gaskets and boring set him back about 250 bucks. Runs like a top.
 
Gary, why did you apologize to this twit? Congrats, Mr. Cage Wrench, you've been smarting off to a serious professional tuner and engine builder--the proprietor of Hoos Racing. To avoid embarrassing yourself again in future, you might consider 1.) following the suggestion to base your comments on experience with air cooled motorcycle engines at a minimum and preferably with the XS650 engine in particular, and 2.) hanging around long enough to find out whose advice you're thinking about challenging before you post. My experience doesn't compare to Gary Hoos' by any stretch of the imagination, but I've built and run XS650 motors with both Venolia and JE forged pistons on the street in daily use for the last 15 years with no piston rattle or ill results. Whatever you may know or think you know based on hearsay about 4 wheeled vehicle motors, you're a rookie here and would be well advised to express yourself accordingly.
 
Here is what you said "Forged pistons will run looser in the bore and not last as long. The pistons will rattle a lot, especially when cold. if it were me, id buy a set of new oversized. You don't know how hard of a life the used ones had."
It is not true they do not run looser, they last a long time (I am running 8500 at the end of a straight. They do not rattle. So yes you gave mis-information. Of course you certainly know more the JE Pistons . Also no where in that statement above do you say in my opionion.
 
grizld1 said:
Gary, why did you apologize to this twit? Congrats, Mr. Cage Wrench, you've been smarting off to a serious professional tuner and engine builder--the proprietor of Hoos Racing. To avoid embarrassing yourself again in future, you might consider 1.) following the suggestion to base your comments on experience with air cooled motorcycle engines at a minimum and preferably with the XS650 engine in particular, and 2.) hanging around long enough to find out whose advice you're thinking about challenging before you post. My experience doesn't compare to Gary Hoos' by any stretch of the imagination, but I've built and run XS650 motors with both Venolia and JE forged pistons on the street in daily use for the last 15 years with no piston rattle or ill results. Whatever you may know or think you know based on hearsay about 4 wheeled vehicle motors, you're a rookie here and would be well advised to express yourself accordingly.
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griz1 easy all I know I learned from Shell Thuet , Roger Johnson and Mike Anderson just to name a few. So griz1 if you could remove the BS on that I would thank you. I wish I was all that but I am not.Just trying to keep some good products on the shelf. Rogers the real builder/machinest. Gary
 
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