Cb750 engine bore

[quadracer351]

Hey guys, I'm in the process of a 74 cb750 cafe project. I was able to find a mechanically sound bike that was maintained well enough and for a great price. This made me happy, I wanted a nice winter project but have limited funds since I'm still in school and only working part time. I know some of you guys will hate this but the original plan was to strip this bike down, send the frame, wheels and front brand out for powder coat, then upgrade the wiring put knee indents in the tank a new seat and suspension and since money is tight at the moment I was planning on tuning the engine up and saving for an 836 kit. Please don't hate! Only reason again I wanted to put off on heavy engine work was to save the money for the 836 kit.

When I pulled the exhaust off I found allllllot of oil in the exhaust on the second cylinder. Since the engine isn't out yet I haven't been able to take the head off and see what's truly going on but I'm pretty confident the rings need to be at the least replaced, but this got me thinking. Since I'm already going to have the engine out and again LIMITED FUNDS how much If at all can I safely have the cylinders bored out without needing resleeving? Also if increasing the cc in a 4 stroke does that mean having to change anything else?
Sorry to sound like such a newb this is my first bike build and my first time doing a 4 stroke engine. I've done plenty of engine work to my 86 Honda trx250R (now 265) but I know this is a totally different beast.


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[quadracer351]

Before it got stripped
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[Big R]

Technically, you'll need to know what size the cylinders are now. Because you should be able to find pistons and rings that are slightly oversize (+.25 mm and up). And they wouldn't require a re-sleeve.

 

[quadracer351]

Yeah I know If I just wanted to fix the problem that's what I would do not what If I just wanted to bore it bigger without needing new barrels? What's safe to go up to?

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[Big R]

Look at the total cost of parts plus labor for the big bore kit (with new cylinders) and the same for +1mm pistons (current cylinders). Either way, you are buying new pistons and rings. The +1mm bore is normally the largest you can go without new sleeves.

Or......just save up a little bit longer and get the big bore kit that you want to get anyway.

 

[rockcitycafe]

on the 750 it's pretty common to see 836 without a resleeve, I've heard of machine shops cracking the barrels with that size overbore as well though...

 

[AlphaDogChoppers]

The oil could be a bad valve stem seal. Do a compression test and determine where the problem really is. If the engine runs OK, and the only problem is a little oil consumption around a valve stem, then hold off on pulling the engine apart until you are ready to build the engine the way you want. So, it may smoke a little. So what? Ride that sucker and rebuild later.

 

[ronnie]

Re: Re: Cb750 engine bore

The oil could be a bad valve stem seal. Do a compression test and determine where the problem really is. If the engine runs OK, and the only problem is a little oil consumption around a valve stem, then hold off on pulling the engine apart until you are ready to build the engine the way you want. So, it may smoke a little. So what? Ride that sucker and rebuild later.

Is concur. Ride it so you know what it handles like stock, and get everything running as perfect as possible(brakes, suspension, carbs.. All calibrated to the t) this way when you're ready to splurge you know the only pandoras box you'll be opening is the engine.

And trust me, it will be a pandoras box.. Haha but I'm not discouraging, I hope you bore it and make it everything you want! That's what I'm doing with my 400f engine/350F bike.

Save up for the big guns and for future issues!

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[quadracer351]

Sounds perfect I was afraid you guys would rip me a new one for not wanting to do engine work now. Now that everything is off the bone and the 2 and 3 cylinder are easier to get to ill do the compression test now. When I got the bike home before I started tearing it down I tested cylinders 1 and 4 and they where both prefect so maybe it is just a valve seal which would be nice.

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