Need piston advice for a 750

[pearljam724]

I'm very close to putting my motor back together. I am debating new pistons at that moment for a couple of reasons. Before I tore the motor apart, the bike ran fine. The piston tops do need cleaned. Being the bike ran fine before the tear down. I would like to avoid having the cylinders bored for new pistons. I understand that is not always an option. But, being mine ran fine. Would it be a safe bet to just buy ordinary sized pistons if I replace them within a year or two ? Second question, what negative effect does carbon build up have on the piston tops ? Also, what should I do to help prevent a dry start up, when I fire it up for the first time or should that not be a concern as long as the pump is working fine ? I was thinking of mixing the fuel for that initial start.

[GT750Battleship]

Hi,why did you tear the motor down? were the seals the problem,or was there a crankshaft issue with the bearings as well? New pistons & rings would be o.k. to fit,if that's the way you want to go,however how do the bores look,are they standard or oversize? Is there any signs of a lip developing at the top of the bores,are the bores still within SPECIFICATION, IE: have you had them measured? These old girls are renowned for their ability to do BIG mileages without a problem! Carbon build up on the piston crown would not be a problem as such,as I can not see it being an issue with the use of modern day fuel & two stroke oils,but yes by all means clean your originals if you are going to reuse? Did you notice any "blow by" on the pistons or any "scoring" that may be considered excessive?
On the subject of lubricants before everything goes back together,just use a coating of two stroke oil on all the moving parts,DON'T start her dry!!On INITIAL START hold the pump arm in the fully opened position with your fingers at idle,until she really starts to smoke!! DO NOT mix TWO STROKE with the petrol IN THE FUEL TANK, as this creates all sorts of problems? TRUST the oil pump!! Check for air bubbles in the oil lines,if you see any bubbles continue to hold pump open until they disappear,you may have to bleed the system at the pump.Hope all this has been of help....
Regards,
GT750Battleship.

[pearljam724]

I did it because, I wanted to restore it proper. The bike is 37 years old and was poorly taken care of before I bought it. I understand that the carbon build up was from many years ago using standard motor oil in the pump tank if I were to bet. As oils from the 70's were high in ash content from what I've read. With the dry start, I was also considering removing the SRIS hoses in the front of the cylinders and squirting in some 2 stoke oil through the SRIS holes where the nipples are inserted. I understand that you're not likely to get that kind of carbon build up using modern 2 stroke oil. But, I wanted to be educated on what it causes. Being that Suzuki suggests cleaning the tops once they build up. I imagine it causes pre ignition. But, that's my guess.

I'm not being unappreciative. I can't imagine what possible harm mixing fuel would cause. If you don't use oil in the pump for the bearings, sure I understand that. But, can't see any technical harm in using both as a precaution for the cylinders. Suzuki warns of mixing fuel for the sake of someone not using the pump and damaging the crank bearings. Unless, there's something else I'm unaware of. Please, share. I have no rounded lips on the edge of the cylinder bores and no scoring on the walls. I felt no need to measure the bores as I intended to reinstall the old or new pistons of the same size being it ran fine. The bike has the original pistons. Being that so many people have restored or rebuilt these bikes, I was wondering what they came across and what they decided to do when it came to this. With the exception of the guys that just wanted more compression.

[GT750Battleship]

Hi,now I know a bit of the history of the bike,yes de-coke the head & pistons,also the exhausts are probably full of carbon & sludge,you haven't mentioned them,so take this as a suggestion!The reason I mentioned the two stroke in fuel tank is that the oil pump is variable,it will pump more oil as required with increased RPM to the engine,two stroke mixed with fuel in the fuel tank remains constant,the needs of the centre cylinder are far greater,the water cooling really helps,but a proper supply of two stoke whilst the machine is under load or accelerating is critical.Suzuki did their homework on this one,have lead the other Japanese Factory's efforts on oil pump design & operation since day one,(Trust the Pump!)they also say that the GT750 will run 30% cooler than an air cooled of similar design,(think they meant Kawasaki H2?)....When I rebuilt my engine 5 years ago I had no choice but to do everything,seals/pistons/crankshaft/rods,now running first oversize Wiseco pistons & rods,expensive yes!! But will last me a long,long time,& will provide trouble free motorcycling!

Regards,
GT750Battleship.

[argo1974]

I've made the experience on T500, pistons are wearing out twice as fast as the bores, so replacing pistons+rings only will work fine in some cases. When reboring, another option is GPM pistons available in 0,2mm steps, so you won't waste a full bore (0,5mm steps) in Suzuki's meaning.

[tz375]

The only way to determine if the old pistons can be used is to measure the pistons, bores and ring end gap. Visual inspection is good to determine if the surfaces are damaged, but doesn't help with clearances. My eyeballs are not calibrated to the thousandth of an inch

A quick and dirty way is to insert a piston into the barrel and slide feeler gauges between the front (or rear) face of the piston and bore and see what the clearance is. Pistons are oval shaped, so the clearance at the sides will be more than back to front.

Then remove a piston ring and slide it into a bore and measure the end gap. If everything is within spec, you can probably re-use the pistons, but glaze bust the bore first.

Battleship's reference to not using pre-mix is because the main bearings are shrouded and people that remove the pump to use pre-mix will destroy their cranks unless they are modified. There is no harm in adding a little oil to the first half tank of gas just to be sure. Don't use a premium grade synthetic though. Cheap non synthetic oil is best.