Honda CB750 F1 SOHC engine rebuild. Part 1.

Looks like I have enough photos for yet one story.

That was a year 2010, when one friend of mine appeared in my former garage with Honda CB740 F1 (SOHC) engine. F1 (SOHC) is  the on of the latest and most powerful version of CB750 SOHC bikes, as I know. He bought this bike in winter 2009. The former owner claimed that engine was overhauled and in good condition except some question with gears shifting, so the bike was moved to the new owner by bus. In Spring the new owner rode the bike a little and it became obvious, that something is completely wrong with engine, so he gave the CB to nearest workshop. After mechanic separated crankcase halves, he said to my friend that repair could be impossible…

But this information needed confirmation, so initially I was involved in estimation of engine condition. Thus, the new owner drove engine to my former garage and we lifted it on workbench.

Honda CB750 SOHC engineFirstly I disassembled engine completely:

Honda CB750 SOHC engineWhat could I say about what I found. I have never saw engine in such a bad condition not before nor after. Exactly that was a mess from damaged parts and only few of them could be used further. And all I saw was a result of not exploitation but of previous engine overhaul and maintenance.

As we discovered, the former bike owner gave CB for engine overhauling to some regional so called “workshop”. He also bought on ebay used gearbox in good condition, and other parts. After those guys installed a gearbox, they rode the bike and as a result, drive chain crashed crankcase. I don’t know, maybe they continued ride after fix chain with liking oil or maybe that happened after they welded crankcase and tried to grind crankcase halves, but, as result, gearbox was mostly killed. It was overheated to blue shift forks, damaged gear’s teeth and so on.

Honda CB750 SOHC engine img_9987Crankshaft journals and its bearings were killed in the same way. Or maybe “workshop” guys installed new but wrong-sized bearings in addition. No matter, bearings and journals were rubbed to junk.

img_9981 img_9999Sometimes I think that guys had a complicated destroying plan and no one engine part had any chance to survive. Look at this piston condition (scratch marks on them were left by “mechanics” to). Is it what you expect to find in engine after its overhauling?

img_0103 Honda CB750 SOHC engineThose thoroughly destroyed parts should be placed on shelf, for history, not in engine. And consider “additional bonuses”, like drilled holes underneath piston bottom (to align pistons weight I suppose) and those part of oil ring shoved in top compression ring groove to decrease ring-to-groove clearance…

img_0471 img_0449Due to that guys “total destroying doctrine”, the camshaft was killed also. How could camshaft beds be set on their places and tightened properly, while they are abutted on threaded rod?

Honda CB750 SOHC engineOf course, both, the camshaft and its beds were unsuitable for further using.

Honda CB750 SOHC engine img_0048Nevertheless, the cylinder head and valves seemed to be suitable notwithstanding for their carbonized look.

Honda CB750 SOHC engine img_0126Ok. What we have got alive? Cylinder head and valves are still might be used, as well as some gearbox and clutch parts and piston rods and engine covers… Not good point to start repair, but I was already involved too deep and that CB750 is too rare in our country so deserved to be not in a junkyard, but ride the roads, so I decide to do all I can for bike restoration.

 

To be continued.

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