Topic: Restoring my K0 engine

[jebon]

Hi!
My CB750 has a leak that I can't live with and also it doesn't has as much pep as one would expect. I haven't checked compression yet but the leak alone is enough to justify it getting looked at from the inside. So I'm thinking I'll open it up over the winter and though I bought the 9 hour video from MatthewMCRepair (haven't watched it all yet) I'm gonna need all the help I can get. I'm thinking I'll have it bored up to next size, change bearings, cam chain etc.

First question:
Will I need new pistons when going up one size or only rings?

I'm going to have it done locally at a very renowned shop. I'm sure they can help with questions too, but do I get pistons/rings first and then they measure the exact size or the other way around?

Thanks,
Jess

[754]

Order pistons second ... measure first.

[newday777]

Yes you will need bigger pistons and rings... And yes measure your bore size you have first and the condition of your jugs to determine what size you need to order.

[69cb750]

I'm thinking I'll have it bored up to next size, change bearings, cam chain etc.

How many miles on your bike ?

[bryanj]

Its rare to need new bearings but cam chain and tensioners  (rubber will be very old and hard DO NOT buy pattern parts for these) primary cains and tensioner pluss seals and gaskets, i have seen a 120,000 mile motor where the bore was still OK and in spec but also a worn out 20,000 mile abused one so strip and carefull check is called for, including valves and guides.

UTTER PARANOIA ABOUT CLEANLINESS on rebuild is a must as is throwing away any rtv type sealant but the only "special" tools you need are the clutch centre nut tool and a roto puller, which can be a ht bolt of the correct thread

[jebon]

I'm thinking I'll have it bored up to next size, change bearings, cam chain etc.

How many miles on your bike ?

At least 30,000 I know it's not necessarily a lot but just did a compression test on it warm and only managed to get it up to 60-65 psi. However these may not be accurate numbers because the gauge didn't stay up when releasing the start button, but still..

[jebon]

Its rare to need new bearings but cam chain and tensioners  (rubber will be very old and hard DO NOT buy pattern parts for these) primary cains and tensioner pluss seals and gaskets, i have seen a 120,000 mile motor where the bore was still OK and in spec but also a worn out 20,000 mile abused one so strip and carefull check is called for, including valves and guides.

UTTER PARANOIA ABOUT CLEANLINESS on rebuild is a must as is throwing away any rtv type sealant but the only "special" tools you need are the clutch centre nut tool and a roto puller, which can be a ht bolt of the correct thread

Thanks!

[ekpent]

 Have a real shop check compression first using the correct technique and using good tools. More then a few people have 'rebuilt' and good engine using Harbor Freight junk testers according to their own reviews.

[jebon]

Have a real shop check compression first using the correct technique and using good tools. More then a few people have 'rebuilt' and good engine using Harbor Freight junk testers according to their own reviews.

Got it!

[754]

I would worry more about bore  size. COMPRESSION don't much matter if you are changing rings, and touching up the valves..

[bryanj]

You are going to have the engine out to fix the leak, at that point when head is off you have removed the clamping force on base gasket and O rings so false economy not to lift cylinders even if you are leaving bottom end alone.
At this point take them to your respected machine shop and ask them to check bore for taper and ovality, Mark (Hondaman) says in his respected book that this is common after a few years of the cast iron and aluminimum heat cycles. It is then down to results, your preference and cost as to what you do.

Informantion of the correct type (not hearsay) is vital in rebuilding these motors

[jebon]

You are going to have the engine out to fix the leak, at that point when head is off you have removed the clamping force on base gasket and O rings so false economy not to lift cylinders even if you are leaving bottom end alone.
At this point take them to your respected machine shop and ask them to check bore for taper and ovality, Mark (Hondaman) says in his respected book that this is common after a few years of the cast iron and aluminimum heat cycles. It is then down to results, your preference and cost as to what you do.

Informantion of the correct type (not hearsay) is vital in rebuilding these motors

Thank you. After checking prices on original bottom end parts I may well be looking at top only

[MauiK3]

Hondaman’s book is a must. Really.
You will be spared much expense and frustration if you use it as a guide.
Your entire motor is old. The metal in it may be pretty good but every consumable rubber part is old and stiff, including the primary chain and tensioner, all seals, etc. Don’t just do the top end, you will regret it.

[HondaMan]

Chances are very high that the valves are leaking and it needs at least a step-bore and you can get genuine Honda cam chain tensioner and chain parts from either Sound Sound Honda or JTMarks at eBay. When you talk with your "reputable shop", ask them one VERY pointed question: "What piston clearance will you use?". If they tell you anything more than 0.001" (that's one thousandths of an inch - it should actually be less than that...), then go find another shop, or you WILL end up right where you are now in less than 5000 miles, if that far. Almost half of the engines I receive for rebuild now were butchered by some "reputable shop" or another - which is why I wrote that book that MauiK3 mentioned.

[jebon]

Chances are very high that the valves are leaking and it needs at least a step-bore and you can get genuine Honda cam chain tensioner and chain parts from either Sound Sound Honda or JTMarks at eBay. When you talk with your "reputable shop", ask them one VERY pointed question: "What piston clearance will you use?". If they tell you anything more than 0.001" (that's one thousandths of an inch - it should actually be less than that...), then go find another shop, or you WILL end up right where you are now in less than 5000 miles, if that far. Almost half of the engines I receive for rebuild now were butchered by some "reputable shop" or another - which is why I wrote that book that MauiK3 mentioned.

Thank you!
They actually told me to bring the pistons. Is that clearance with rings on?

[RAFster122s]

South Sound Honda of Olympia Washington is where Mark was referring to above...

[bryanj]

There is a piston to bore clearance and a ring end gap spec

[jebon]

When I replace the chains, do I replace the sprocket(s) too?

[bryanj]

No especially as some of them are part of the crank!

[jebon]

No especially as some of them are part of the crank!

[jebon]

Started on the engine yesterday.
Was wondering... Do I need to pull the rotor to split the cases?
If I can save a little on tools..

[bryanj]

No but you do need to take the generator cover and pointsplate/advancer off plus starter out, plus lots of bits under the cover to rear of generator. BIG HINT take lots of pictures of the inside of that cover as its complicatedin there and also get a bit of card and put the screws where they come out of on a picture as they vary in length

Thats a nice clean looking engine but even in the picture i can see that the cam chain tensioner wheel is badly worn

[jebon]

One of the first observations after removing the cylinders is no O-rings on the bottom of the cylinder tubes and no visible honing marks

[bryanj]

Its been apart before then

[HondaMan]

Aye.
Someone has been in there before you: now you might find several other 'surprises'. A common one: the spring washer that holds the clutch housing onto the mainshaft may be backward (which makes for noisy, clunky shifting), the oil port on the left side of the engine, next to the countershaft bearing housing, may be gone, incorrect clutch plates might be installed (and boy, do I have a long list of other things I have seen!).

My favorite: on a CB750K5 that had just been ridden from Michigan to California (and came by here for me to see it and install a Transistor Ignition halfway thru his trip) was later sent to me for a full rebuild. It was actually MISSING its oil jets in the head! Yet it ran perfectly, the bottom end bearings were in excellent condition, no harm could be found anywhere inside. And the cam and rockers were gorgeous. The cam cover had been extra-sealed with some black goo, though, which probably was needed with the extra-deep oil it was getting!