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77 GS750 Help & Ideas

  • Thread starter Thread starter speedyneo
  • Start date Start date
S

speedyneo

Guest
Ok i have a 1977 GS750 when i got it it was in pieces. The PO had removed the cylinder head and then for some reason they then parked the rest of the bike outside with only a blanket covering the engine The cylinders are rusted so i know im going to have to bore it out. Im wanting to go with 850 pistons and if i understand everything right i can do this with no other modifications am i right??? There is some rust on the crank but i think its only light and shouldnt be a problem once i get it cleaned off. The engine has about 50 thousand miles on it im not sure how far i want to go with it everything feels tight if money wasnt an issue i would go all out and replace everything but im on a budget. Should i replace the crank and rod bearings?? If so how hard is it to do or is that something a machine shop has to do? let me know what you think
Thanks


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It's not how the crank looks that might be a problem, it's the bearings. Unfortunately, they can not be seen with everything assembled.

If you do choose to use the 850 pistons and cylinders, you will have to restrict your search to a set from a '79 850.

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Would i have to replace the cylinders of could i just have them bored out ?
 
I'll have to let someone who has done that answer that one for you.
I have only read about how easy it is to drop a set of 850 cylinders on there.
I don't know if the 750 cylinders can be bored that far.

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Continue your strip down before making a decision on those 850 pistons.

One of your pics shows heavy rusting right up to the big end. If moisture has carried on down into the big end, the bearing could be pitted. If that has happened, you will be up for bearing replacements which involves a specialist machine shop pressing the old pins out and fitting up new ones. You could oil up the big end areas and try rotating the crank. If you feel any restriction to free movement, you will definitely need to replace the all the bearings.

Your 750 sleeves can be bored to take the 850 pistons, but the sleeve thickness is reduced to 1.5mm (0.060"). Personally, this is below the minimum that I would go. I have left 2mm on my 850 re-bore. Having said that , others have gone to 1.5mm. I haven't heard of problems, but you need to ask around. Inadequate heat dissipation and bore distortion are the enemies when you get too thin.
 
Boring a cylinder is expensive, it's far cheaper to find someone with a decent used 850 cylinder and hone it instead. Regarding that crank, if the bearings turn smoothly you might want to take a chance on it but used 750 cranks can be had pretty cheap (in the $30 range) so you might want to check around. Only real problem with getting a used crank is paying the shipping cost since the dang things are very heavy.
 
Look up Tkent02, he has a kit I'm sure he would sell cheap. I did a trade with him for some engine parts a while back and he was good to deal with.
 
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