Hi Guys, So, I got caught speeding the other day. It was a stupid thing to do and I got away with a light punishment all things considered (1 month of motorbike and car... rest, shall we say) Anyway, i've decided that because I will not be able to drive or ride, and my bike has a serious oil leak through the exhaust and spark plugs; that I will remove the engine and fix this leak. (1980 GSX250) Last time I pulled the heads off I checked the valves and clearances, all good there. The cylinder walls were a little smooth, but no worn ridge or anything like that. I sanded back the pistons and the valve heads to make them a little cleaner. Its a DOHC Engine, so the oil must bypass the cylinders somehow and i think it may be blocked. Also I'm sure that new rings wouldn't go astray, lots of places sell 0.5 oversized, would this be a smart move seeing the cylinder wall will be quite smooth??? Or should I get the cylinder lapped and a new piston? Thanks for your help ~B
Hmm. It depends what you're seeing exactly. When you say oil is bypassing the pistons I assume you mean that oil is accumulating on top of the pistons and therefore around the spark plugs? If that's the case you probably need new rings (oil rings specifically, but you'd do the whole lot at once). There are a couple of ways to tackle it, but to save yourself time (and money) measure everything up first (pistons, ring gap, bore size) and check against spec. 1. If pistons and bore in spec, replace with STD rings and you should be fine. 2. If pistons or bore are out of spec, buy oversized pistons/rings - get the bore professionally expanded to oversize, fit and off you go. I personally am against this whole "bore honing" process. If the bore is glazed, just get it cut back to oversize - don't waste your money trying to remedy it for the sake of fitting standard size rings. The reason is in the basic engineering, it's the clearance between the bore and the other parts internally that is important. Honing will increase that clearance, if even only slightly, probably resulting in more oil consumption and/or increase in blowby. Sure some people may have other thoughts, but why the heck would you want to set your clearance somewhere between std and oversize? Just go to oversize and be done with it. My 2c anyway. Good luck.
Thanks Stu, Yes, there is clear new oil coming from the exhaust and the spark plug. There is clearly too much positive crankcase pressure going by one of the pistons. I don't have a gauge to test what percentage of blow by it but its a pretty obvious problem. So you are saying, after I measure the cylinders if they are out of spec then to take it to an engineer to hone out the cylinder, buy an oversized piston and new rings and put it all back together?
Be careful differentiating between "re-boring" the cylinder to oversize and "honing" the cylinder. The first is a cut that moves the cylinder clearance to the "oversize" specification, the second is a (IMO poor) abrasive technique to remove "glazing" from the bore. Both will widen the bore, the first more than the latter. If you've already got blowby and oil leaking problems, honing the cylinder carries a very real risk of making them worse - even if you put new rings in. At a guess I'd say your rings are just done. But it always pays to check everything to be sure. By the way, do all four cylinders at one - especially if you have to take 1 to oversize.
Buy the new oversize pistons and rings and take to a professional engine reconditioner and get the cylinders (both of them) rebored to suit the oversize pistons. Get the pistons first so the shop will know what size to bore to (they should measure individually and bore to suit individual piston), and this way you will be starting with a fresh bore/piston which will save you both time and aggravation later on.
