1986 yamaha 70 hp top end rebuild ???

[rdm429]

picked up motor recently ,low compression top cylinder,rings are stuck scuff marks all the way around plan on all new pistons,hone and or bore,top cylinder 24 lb lower than middle and bottom..Seems there are couple options on pistons oem,wiseco,sierra...know people have there faves but for longevity which way to go...by the way dont know how they did it but thermostat was in on angle...crank looks very good..bottom bearing on crank (large ball bearing)has some slight rust marks...gonna replace top roller bearing along with bottom on crank..anything special too look out for ...thanks...

 

[bodogg]

Re: 1986 yamaha 70 hp top end rebuild ???

Hi, I would do everything possible just to crosshatch hone the cylinders because boring them can cause overheating in higher rpms and if you can try to use oem parts due to longevity,but if cost is a major issue use wiseco! The thermostat could possibly be from a 40 horse or forced in generic. AS far as the bearings if you are in doubt with a rust issue you should replace it also,remember this motor will be ran most of the time at wot so check clearances on your rods and rod bearings also on a two stroke make sure that every possible part gets oiled properly and when you finish always use ring free!

 

[rdm429]

Re: 1986 yamaha 70 hp top end rebuild ???

Thanks for the reply,cost between oem and wisecos is not much probably go oem unless wisecos are actually better quality, noticed there are caged and cageless top end bearings,any benefit too either...motor will be going on 17 foot deep v aluminum boat that will be used for fishing.. thanks

 

[bodogg]

Re: 1986 yamaha 70 hp top end rebuild ???

hi, I would go with oem rings just sheck the piston that seized rings for galling in ring grooves,on the upper bearings i would use caged,less chance for error. Just remember on two-strokes use plenty of assembly lube and always use ring free. Good luck!