I'm in the process of building a little rat bike using mostly CL stuff that I've got laying around. Having a bunch of bad 305 sleeves that wouldn't clean up at anything less than 2mm over, I did some research and came up with a really cheap 63.5mm piston assembly ($20 ea). The vendor says they're for a CG200 , whatever that is. Probably some Chinese ATV or something. Anyway, the pistons are 63.5mm with a 15mm pin. Came with rings, pin, clips , and something that I don't have a clue but looks like valve guide seals. I've got them installed in a stock length, non-decked cylinder and the top of the piston looks to be down about .040 at TDC. I really don't want to deck the cylinder but I'm afraid that if I use a .040 thick gasket the squish is gonna be too much and even with low compression I might get heat at the bore. Has anyone used a .020 gasket and if so will the standard o-rings take the extra squeeze?
Thanks,
Mark
Poor Boy Big Bore KitPoor Boy Big Bore Kit
flathead,
I really don't have answers for you, just questions. What you are doing is something that has crossed my mind. I see those cheap pistons and 26mm Chinese carbs on eBay and I'm always tempted to experiment. Also, I am considering putting the 3mm oversized pistons in my cb since I have to rebore the cylinders -- some of what you are doing seems to overlap with what I anticipate will be necessary to accomplish that. What do you mean by decking? Milling the top of cylinders? What brand/type of thinner gas gasket are you thinking of using? (The reason I ask is because I am going to order a copper head gasket and I am wondering about what thickness sheet it should be made from -- is .040 standard?). Did you bore out stock sleeves to fit a 63.5mm piston? If so, did that leave them thin-walled? Aren't you going to wind up with low compression given all that space in the dome and the flat-topped piston (even if you deck the cylinders)? Anyway, it looks like you have an interesting project. nander Nander,
It's loads of fun doing this stuff isn't it!!! I really dont know what the compression ratio is gonna end up at. I haven't CC'd a head to to do the math and I really don't know if that's gonna tell me a whole lot, we'll just have to wait and see how the things gonna run when it's all together. I do know that the displacements out to 342 CC's . At stock 8.5/1 , 60MM x 54MM = a volume of 17.99 CC at compression. We're adding 18.33 CC's of displacement and not changing the stroke. My guess is the volume of the head is maybe 25. If it was a perfect half sphere, which its not, at 54mm dia it would be 41.2 . So at that and assuming the pistons are flat the compression ratio should be at around 6.8/1. I kinda think it's gonna be a little more, but it really does'nt matter, it'll either run or it won't. On the 305's and 250's , I always just bore the sleeves on a lathe when there out. Some folks don't like that but I've never had a problem. I leave .003 +/- and hone after putting them back in. Use a good hone with 2 stones and wipers. The wall after boring to 63.5 was around .08 if I remember. The Chinese pistons look pretty good. They're cast and do have a little bit of a dome. Quite a bit shorter skirt and lighter but then again the proof will be when we get it all together. I'm gonna probably use a copper head gasket. I've got a few pieces of .02 laying. A smaller dia cross section O-ring may have to be used if the stock ones won't squeeze down. I was hoping someone could comment on the thin gasket with the stock O-rings. Another thing is with the pistons being below flush(roughly .04) a stock fiber gasket may work. It's only about 1mm smaller in dia at the steel ring and doesn't look like there would be any interference so that may be an option. As far a the cheapo 26mm carbs go, I tested one of those on a Triumph Cub race bike that I restored a few years ago. I've never ridden the bike or even started it off of the lift, so I could'nt comment how friendly they are. I have heared that jets and other parts may be a big problem. Also I don't know where the measure those things but I couldn't find 26mm on it anywhere,,,, bore, slide, length, heigth or width. They must do some creative measuring over there. They actually look to be more in the 22mm range compared to a 26mm keihin. Hope some of this helps. I'm still pretty ignorant about what the end results are gonna be but then again it'll either run or it won't and if it does it'll either be OK or not OK. I'll keep things posted. Mark
I've been thinking about popping the liners out and boring them on a lathe. Seems reasonably straightforward with a big enough chuck and a boring bar.
I can see why people warn against boring the sleeves when they are removed from the block - the aluminum cylinder housing could theoretically distort the sleeves when they are pressed back in. So it would be best to do them in place. However, centering with that offset setup on the lathe sounds like a nightmare. I can't find anyone locally that has a good reputation (and I am in L.A.). I read somewhere on here about a place called Q and E in anaheim. They have a nice website, I might have them do it but that's a long drive for me. Did I understand you correctly in that you think a standard gasket will fit the oversized pistons? - i.e. there won't be interference with the flame ring? I've mocked it up on a set I have and it looks like there isn't enough clearance. nander I got a little more done today so I thought I'd post a bit.
Nander, I've not had a problem with the the bores getting out of round when they're bored in a lathe. First off don't bear down when you chuck the sleeve. Use a test indicator to check out of round and concentricity before you take a cut. I usually take .01 max per pass (.005 per side) and leave at least .003 to hone after the sleeves are istalled. DON"T press them back in. Put the aluminum casting in an oven, heat it to 350f, take it out ,and let the sleeves drop into place. Worst case you may have to "tap" the sleeve to get it to seat. MAKE DADGUM SURE EVERTHINGS CLEAN BEFORE YOU START. My heat treat oven is pretty close at 350f +/- 5 but I think any kitchen oven will work......... as long as its girls nite out. Another thing is, I use to fight getting a bore to size until I invested in a decent hone. I use a Lisle with 2 medium stones and 2 felt wipers. It's a piece of cake to take a bore running .002-.006 out and get it dead round and on size in a few minutes. Here's a few pic's of the progress so far. The pistons are installed. I made up a .021 copper head gasket. The bore is at 63.5mm which is pretty much 2.50". I had a 2.625 punch so I just used that for the bores. The stock head gaskets are oversize so I figure 1/8" over probably will be OK. I also made up some .032 thick teflon washers to use in lieu of the orings. I could'nt come up with an oring that I felt comfortable with squeezing down to .02 so i thought we'd give the teflon a try. I think its probably gonna leak but we'll see. I've got a really crappy head that I'm trying to salvage so we're on to that next. I cut about .015 out of an exhaust seat last nite and still have a big old rusted out spot that I could'nt get to lap out. Mark Oh BTW this one a TWINGLE also.
Nander,
I've got a Strippit Super 30 Fabricator here at the shop. It's a single station 30ton x 3 1/2" dia capacity punch press with drop in tooling and micrometer gaging. 1950's technology. Sat down at the computer, measured a head gasket, made a cad drawing of the holes , and punched out a couple of gaskets. I just traced the outline with a sharpie and cut the ouside with a pair of snips. Took about an hour. I don't like to use hardware store o-rings for the simple reason that you really don't know what material you're getting. I've seem o-rings deteriorate sitting on the shelf for a couple of months. I did locate a 1mm x 10mm and a 1mm x 12mm in Viton at McMaster Carr. I think one of these will work so I'm gonna order up some of both to see which looks the best. Mark
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