qtws Posted April 6, 2010 Share Posted April 6, 2010 My sr125 had become very hard to start, was oiling the plug, bogging on max throttle and blowing a lot of blue smoke - a compression test showed 75psi (!!) warm, rising to 115 with a squirt of oil in the cylinder. So the rings are suspect. Stripped it down, and made some measurements... Bore: 57.02mm top and bottom, all directions.Fair enough, std is 57.00-57.10. Piston: 56.48 at the top, 56.88 at the bottom. Std is 56.945 - -oops! it gets better.... rings: end gap, top ring >0.5mm. Std is 0.15-0.35; 2nd ring 0.45mm, std is 0.15-0.35mm. Totally phooked, it would seem. Piston bore clearance is a thumbnail, greater than the acceptable 0.065mm. The bore has a series of vertical scores in it, which match wear points on the rings. These scores are ~2mm wide, spaced evenly (~7mm) all the way around the bore, and are **just** detectable with a fingernail. My question is, should I rebore to say, 0.25mm oversize, and install a piston/rings to suit, or simply take the view that the original piston is worn out, the bore is fine, and replace piston/rings with the standard size, no rebore? Happy to hear recommendations for either option; given the bike cost me £150 with 40k of the current 42k on it, its not been expensive so far, although a rebore will probably be as much again as a piston kit (~£30) and new gaskets (£30) put together, assuming I can find a place that will bore/hone AND supply a suitable piston locally (N. ireland). Cheers, Nigel Link to comment Share on other sites More sharing options...
Moderator Airhead Posted April 6, 2010 Moderator Share Posted April 6, 2010 I would buy new piston and rings and just run a bit of wet / dry 800 grade round the bore in circular motion to key it for the new rings. doesnt seem that the bore is too bad. new piston & rings, top gaskets set, job done. ...paul rings should be changed every now and again, i bet yours are original Link to comment Share on other sites More sharing options...
qtws Posted April 8, 2010 Author Share Posted April 8, 2010 Further inspection reveals that ring end gap increases as the ring is pushed down the barrel. So its, er, barrel shaped... However, an oversize+0.5mm piston kit inc. rings, plus free rebore to that piston, sourced for £35. Sorted. Now I need both inlet and exhaust valve stem oil seals from somewhere that won't charge £6.99 for delivery to N. Ireland, or insist upon selling me 10 off each and then gouging for offshore postage on top of that!! I mean, they are a quid each, £1.50 max! "Full gasket sets" don't have these, it seems... Any tips for getting the cam follower shafts out? Cheers, Nigel Link to comment Share on other sites More sharing options...
Moderator Airhead Posted April 8, 2010 Moderator Share Posted April 8, 2010 Dont you have any friends / family in GB who can post them to you? Link to comment Share on other sites More sharing options...
qtws Posted April 8, 2010 Author Share Posted April 8, 2010 Dont you have any friends / family in GB who can post them to you? Y'know, that's a damn good idea!!! An so bleeding obvious, with the sister in Preston an' all.... I'll try fowlers tomorrow for the seals as well - as I recall, they don't stick the arm in. Link to comment Share on other sites More sharing options...
qtws Posted April 27, 2010 Author Share Posted April 27, 2010 ok, a certain shop "at the end of the world" came up trumps. Turns out the inlet and exhaust valve stem oil seals are the same size... Valves out, a LOT of carbon on the exhaust, some in the inlet, the seats look like a polish would be in order, but would I be better lapping them in a little until there is an unbroken grind line on both the valve and the seat? If so, could I use autosol, or must it be proper grinding paste - I'm really only talking about tidying up the mirror finish on valves/seats - they are relatively shiny, but with some dulling here and there. Cheers, Nigel Link to comment Share on other sites More sharing options...
Moderator drewpy Posted April 27, 2010 Moderator Share Posted April 27, 2010 ok, a certain shop "at the end of the world" came up trumps. Turns out the inlet and exhaust valve stem oil seals are the same size... Valves out, a LOT of carbon on the exhaust, some in the inlet, the seats look like a polish would be in order, but would I be better lapping them in a little until there is an unbroken grind line on both the valve and the seat? If so, could I use autosol, or must it be proper grinding paste - I'm really only talking about tidying up the mirror finish on valves/seats - they are relatively shiny, but with some dulling here and there. Cheers, Nigel buy grinding paste ya tight git Link to comment Share on other sites More sharing options...
Speedshop Posted April 28, 2010 Share Posted April 28, 2010 The 0.5 kit is the way to go, 0.25 is rarely big enough to take out all the wear in the bore. Link to comment Share on other sites More sharing options...
qtws Posted May 2, 2010 Author Share Posted May 2, 2010 buy grinding paste ya tight git ...rumbled! Tight git has sorted out out said paste and a lapping stick this evening. The autosol didn't make any difference, so the proper kit is in order...just waiting on the postman now. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted May 2, 2010 Moderator Share Posted May 2, 2010 ...rumbled! Tight git has sorted out out said paste and a lapping stick this evening. The autosol didn't make any difference, so the proper kit is in order...just waiting on the postman now. Link to comment Share on other sites More sharing options...
Moderator drewpy Posted May 3, 2010 Moderator Share Posted May 3, 2010 Link to comment Share on other sites More sharing options...
qtws Posted March 21, 2011 Author Share Posted March 21, 2011 ok, a bit of a bump up for this thread as I delurk once again. Just finished rebuilding the sr125 engine (I'm a master procrastinator, and the garage is unheated...), everything timed and turning over nicely, new gaskets all round, a 0.5 piston and matching rings, valves set, fully charged battery, 2pints of oil in, clean spark plug (which produces sparks!) and new fuel. The bu66er won't start! Leak down test on lapped valves was ok - nothing coming through within a few hours. Cold, I'm getting ~90-100psi from the cylinder; would expect more when hot (its currently more than I was getting warm, pre rebuild! (75ish psi), if I could get it going. Got compression, fuel, air, spark.... Any thoughts? Last time the engine ran was over a year ago, burning dang near as much oil as petrol. There are no obvious leaks that I can see or hear, starter spins fine, carb is clean... I'm outta ideas, at least until the starter motor cools down again so I can have another go Link to comment Share on other sites More sharing options...
Moderator Airhead Posted March 21, 2011 Moderator Share Posted March 21, 2011 Easy start, or bump it. But surely if your starter motor is getting hot, then the plug must be wet through? if so pull it out, procrastinate for another day or so and try again Link to comment Share on other sites More sharing options...
qtws Posted July 31, 2011 Author Share Posted July 31, 2011 Moving with almost glacial speed (and having torn down the diversion for an engine out ground up rebuild/paint, fettle etc - MoT'd yesterday ) on the SR125, we could not get it started at all - low compression, so despite spark, fuel, air etc, I have torn it down again. Seat off, tank off, carb off, exhaust off, cracked the bolts and head/barrel off (o karp - must run to xj600 MoT.....10min late ). Anyhoo, some measurements this evening were illuminating, to say the least: 0.5 over piston is actually 57.2mm (one might expect up to 57.455mm). Supplied piston is thus more akin to a 0.25mm oversize. Barrel is 57.4mm at the top, 57.46 at bottom (!!) Top Ring end gap is more or less 0.01", rising to ~0.015" top to bottom of barrel. 2nd Ring end gap is more or less 0.0145-0.016" top to bottom - a bit too big, limit is up to 0.013", as for top ring. Suggests rings are correct for the piston, were it in a bore that wasn't 57.4mm. Not a bit wonder there was no bliddy compression!!!! I did wonder at how easily the piston&rings pushed into the bore! A 57.355mm diameter piston would be the thing now, if it can be got. i.e. piston/bore clearance of 0.045mm. On the upside, the xj600 looks great. Link to comment Share on other sites More sharing options...
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