cb650 engine rebuild Step 1 of 4

I’m rebuilding my 79 cb650 engine.

The engine leaked a shit ton of oil from the rubber o-rings around the head gasket. (Common 4cyl problem on old Hondas.)

My engine fins have always been completely covered with caked on oil, grease and dirt. If you didn’t already know, the engine fins are what cool your engine. So to say the least, my engine prolly wasn’t properly cooling itself as much as it should have been. No matter how much I cleaned it, 2 weeks later it was gross again.

In my plan to reduce overall engine temperatures, and efficiency, I’m doing a new gasket kit, having the cylinder jugs measured, and having the cylinder head pressure tested to see if everything is still within spec. Possibly installing a cb750 oil cooler.

Step 1 – Get the engine apart
Step 2 – Clean everything
Step 3 – Have cylinder jugs and cylinder head tested
Step 4 – Reassembly with new gaskets, new piston rings, possibly oil cooler

13 responses to “cb650 engine rebuild Step 1 of 4”

  1. admin says:


  2. admin says:


  3. Kyle says:

    Great Site! Very excited to see that there are other vintage bike enthusiast out there.

    What is required to separate the two halves of the engine? I got to replace a gasket to stop an oil leak. Thanks!

  4. admin says:

    Just to clarify, I never split the crankcase. We only took the head and cylinders off, which was super easy. Once you get all the bolts out, just use a rubber mallet to tap the cylinders and head loose.


  5. Salzig Hund says:

    Thanks a bunch for the insight and photos. I’ve had my engine (650) on the stand for months because I wasn’t sure what to do. Your photos and quick run though of what you did was helpful. Going down stairs to deal w/ it now. Please tell me did you:

    1. Hone the jugs/ cylinders
    2. replace crank case gaskets? its sat for 21 years

  6. Rick says:

    How much does it cost to rebuild a cb650 engine

  7. ed says:

    It depends. Below are super generalized estimates.

    If you’re just doing gaskets and cleaning. $100-200

    If you’re doing pistons / head work / machine shop work. $500-800

  8. jeff says:

    hey gasket kit did you use? and feedback on quality/fitment? thanks!

  9. Neal says:

    I was wondering…on this rebuild, did you address main and con rod bearings?

  10. Joe says:

    I have just started a tear down of my 82 CB650SC. I have owned it from 1984 and the last time it was on the road was 93. I have taken it down to exposing the pistons above the crank and everything looks good. Very little scoring in the piston sleeves and the valves look extremely clean. Will flip over tomorrow and expose the crank shaft to see how much rust dust it has collected. I have only found very light rusting inside and it was easy to take apart.

  11. Terry says:

    I have a 1980 CB650C also. I have a couple of small very minor problems with the engine until now. I sheared one of the valve steam adjuster screws and need to replace it, probably need to take the engine out to fix this. Do you think it would be worth it for me to do a total engine rebuild while I have it out? I don’t have any leaks or issues, so maybe this is entirely unnecessary.

  12. mike morris says:

    where does it tell me / show me how to get the engine apart?

  13. David says:

    I have an 82′ cb650sc and is only firing on cilynder 3. any ideas?

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