Topic: CB750F2 cafe racer rebuild

[NalydStrea]

Hello everyone

As i already mentioned, my name is Dylan, 25 years old, from Belgium, and I bought a CB750F2.
It burns oil, it leaks oil, and it looks like crap.

So i'm going to rebuild it completely.

Here's what I'm starting with:







Also, the seat was mounted beatifully by the previous owner.
I already cut it off because I won't need it anyway.





I took off the carburators, split them, and took off the exhaust.
I also drained the oil. Isn't there supposed to be 3 quarts of oil in this thing?
It barely drained 1 quart. Most likely because of the oil burning/leaking.

[Jnel]

Looks like it has good bones.  The photo below should make you feel better.   Its where i started.

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[seanbarney41]

You should not have much problem selling that seat for a reasonable price based on condition....  Very rare in the U.S., less so in the U.K. and Europe, but still quite popular.

[NalydStrea]

Looks like it has good bones.  The photo below should make you feel better.   Its where i started.

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That actually does make me feel better! 
You made it into something beautiful though.

[NalydStrea]

You should not have much problem selling that seat for a reasonable price based on condition....  Very rare in the U.S., less so in the U.K. and Europe, but still quite popular.

Oh, I didn't know that.
I cut it off at the welds, so the mounting plate is in reasonable condition.
If anyone is interested?

[70CB750]

Welcome!

Depending on the mileage, you might need to replace valve guides and valves. You will see when you open her up.

[seanbarney41]

You should not have much problem selling that seat for a reasonable price based on condition....  Very rare in the U.S., less so in the U.K. and Europe, but still quite popular.

Oh, I didn't know that.
I cut it off at the welds, so the mounting plate is in reasonable condition.
If anyone is interested?

yep, just google Giuliari seat, lots of old threads on this forum and current ebay sales for similar seats.  Giuliari made seats for everything from Vespas to Harley Davidson so you have to wade through.  I think yours was probably meant for a K7 model.

I would happily give you an offer, but I won't be working on a k7 for some time and shipping to U.S. is probably too expensive.

[NalydStrea]

Welcome!

Depending on the mileage, you might need to replace valve guides and valves. You will see when you open her up.

It has 66000 km's on it, so about 41000 miles.
The plan is to give the engine new bearings, gaskets, seals, piston rings and valve guides.
After that cut the valve seats and valve lapping.
Will see if I need new pistons, valves, camshaft, ...
I will measure those to see if they're still in spec or not.

[andy750]

You should not have much problem selling that seat for a reasonable price based on condition....  Very rare in the U.S., less so in the U.K. and Europe, but still quite popular.

Oh, I didn't know that.
I cut it off at the welds, so the mounting plate is in reasonable condition.
If anyone is interested?

yep, just google Giuliari seat, lots of old threads on this forum and current ebay sales for similar seats.  Giuliari made seats for everything from Vespas to Harley Davidson so you have to wade through.  I think yours was probably meant for a K7 model.

I would happily give you an offer, but I won't be working on a k7 for some time and shipping to U.S. is probably too expensive.

Sean I think the seat is a Guilari copy (German make instead of Italian) -I got mine from German Ebay for $120.  - same crappy mounting points - still love it though. And yes they were expensive in the US -not sure what they go for now. Guilaris had the gold writing on the side.

Looking forward to your build!

[NalydStrea]

A quick question before I order my transmission bearings from SKF.

Referring to this thread: http://forums.sohc4.net/index.php/topic,60026.0.html
There is only one bearing, 6304NR, that I have to order with C3 clearance?

So the 6205NR bearing on the same shaft doesn't need the C3 clearance and I can just order a normal one?

Thanks guys.

[NalydStrea]

Today I pulled the engine out of the frame.



Then it was time to lift it onto my workbench.
Damn, that f*cker is heavy to lift by yourself.



Anyway, I took off the valve cover and cam chain tensioner and everything looked good at first sight.





But immediately some bad news.
The 4 center bolts of the rocker holders were different.



And the bolt for EX 1&2 holder was loose and poked out 2mm.

I bolted it back down but it didn't grip, so the thread was already ruined.





That's where I called it a day and covered everything up.

Can this be fixed with a Helicoil?
Or some other suggestions?

[seanbarney41]

yes, and it is actually pretty common on cylinder heads that have been worked on

[70CB750]

Look at the top end of exhaust valves.  My black engine had them worn out - there was a dimple on top of the stem from the adjuster.

[seanbarney41]

Mine too...within 28,000 miles.  ,,,and the valve guides are pumping more oil than the oil pump does

[NalydStrea]

Will definitely check that.

Any answers on the bearing question?

[web]

I tend to prefer Time-Sert for repairs like this, but helicoil should also be fine when installed with due care. Less investment if you don't encounter a lot of these.

[NalydStrea]

More stripped threads...



I don't know if it's possible to put a helicoil in the bottom one, because of the positioning pin that goes in that hole?



Here are some photos of the parts that I took off.



















Everything looked fine except for 2 of the intake rocker arms which have some kind of damage to the sides?





I could only measure the outer diameters of the parts because I have no inside micrometer for such small holes.

Rocker shaft measurements: (11.94mm is the wear limit, so should I replace them?)

EX1: 11.94mm (0.4701)
EX2: 11.94mm (0.4701)

IN1: 11.94mm (0.4701)
IN2: 11.96mm (0.4709)

EX3: 11.95mm (0.4705)
EX4: 11.96mm (0.4709)

IN3: 11.95mm (0.4705)
IN4: 11.95mm (0.4705)

Cam shaft measurements:

Journal 1: 21.94mm (0.8638)

IN1: 36.05mm (1.4193)
EX1: 35.60mm (1.4016)
EX2: 35.55mm (1.3996)
IN2: 36.10mm (1.4213)

Journal 2: 21.77mm (0.8571)

Journal 3: 21.79mm (0.8579)

IN3: 36.05mm (1.4193)
EX3: 35.66mm (1.4039)
EX4: 35.62mm (1.4024)
IN4: 36.10mm (1.4213)

Journal 4: 21.96mm (0.8646)

I measured with a calibrated micrometer and measured a couple times to double check.
The manual says that the wear limit for the intake cams is 35.86mm and the exhaust cams 35.36mm.
Also the base of the cam should not be smaller than 27.93mm. In my case they were all between 28.05-28.15mm.

The outer journals are bigger than the inner journals, with a wear limit of 21.74mm for the inner ones and 21.89mm for the outer ones.

So the cam is in spec, but I did not measure the bend in the cam yet.

I have ordered Hondaman's book so I don't have to ask every single question I have here.

[NalydStrea]

I got the head off and everything looked fine except for some carbon buildup.
Also, I only found 4 cylinder stud seals? Not 8?
Edit: this type of engine only used 4 instead of 8, and with HD cyl. studs they can all be left out eventually.





Did a little valve leak test by filling the head with brake cleaner, and nothing leaked.
But the top end of some valves are smashed, and the valves have so much play in the valve guides (especially the exhaust valves) that they just have to leak.







I then took off the cylinder and inspected the bores and pistons.
The bores look fine but the pistons have some scuff marks.
I still need to measure them. (Or do they belong in the thrash?)













And I started repairing the stripped threads in the cylinder head with helicoils.
I also put some loctite on them.

[Jerry Rxman Griffin aka MuthaF'er]

We see HONDA on one side of the piston. Is there a number on the other side ie 300, 392, 410?

[NalydStrea]

The other side says '410  3'

I measured the pistons at the skirt:

Piston 1: 60.93mm
Piston 2: 60.94mm
Piston 3: 60.92mm
Piston 4: 60.93mm

Service limit says 60.85mm, so that's good.
But what about the scratches?

I also need to take the cylinder to a shop to get them measured, because I only have a 3-point inside micrometer.

[NalydStrea]

Apparently I had the right measuring tools for the cylinders at work, after talking to some guys.



I measured the cylinders on the top, center and bottom, both in X and Y.
The wear limit is 61.10mm or 0.05mm difference in taper or roundness.
Looks like I have both...



So, what should I do...
Bore the cylinders to fit the next oversize pistons, or go straight to an 836 kit...
I would want an 836 kit, but I'm not planning on changing my camshaft, so the hp gains won't be that big.

Decisions, decisions...

[NalydStrea]

More bad news, awesome!

Found a lot of crap on the oil pickup screen (sorry for bad picture).



Also the primary chain tensioner looks exactly like you would expect after 40+ years.



And I need new primary chains, wear limit is 70mm...



So I cracked open the engine cases, next step is to inspect and measure the gearbox and order some plastigauge.
Any tips on what I should definitely look at?

[NalydStrea]

No one is answering, and that's fine, because this is the #1001 engine rebuild thread.

But I was measuring my connection rod bearing clearance with plasti gauge, and they were all 0.051mm (0.002").



So do I need to replace the bearings? Standard clearance is 0.02-0.046mm (0.0008-0.0018") and wear limit is 0.08mm (0.0032").
The problem is, there are no markings on the conrods...
I have the markings 'BBBBB' on my case and 'AAAAA' and '4444' on my crankshaft, but that's it.
No markings on the conrods.





Does this mean I have aftermarket conrods?

[Jerry Rxman Griffin aka MuthaF'er]

You're at your limit on the crankshaft at .051. Don't know what bearings are in there currently but there are 4 sizes available. If you are not already using the thickest ones you can decrease that clearance but only slightly.

I'd almost bet those are RC rods which are modified stock rods from looking at the dots. The nuts look larger which RC did. They would also be shot peened for extra strength. Are the small ends bushed?! That would be the tell tale.

The 4444 refers to your rod journal crank sizing. Stock rods would have lettering and numbers on each side. One for size and the other for weight.

Some people prefer to run the crank looser with more clearance but that usually sacrifices some oil pressure.

Can you see any colors corresponding to bearing sizes on the edge of the bearings? ie yellow, green, brown, black
 

[NalydStrea]

You're at your limit on the crankshaft at .051. Don't know what bearings are in there currently but there are 4 sizes available. If you are not already using the thickest ones you can decrease that clearance but only slightly.

I'd almost bet those are RC rods which are modified stock rods from looking at the dots. The nuts look larger which RC did. They would also be shot peened for extra strength. Are the small ends bushed?! That would be the tell tale.

The 4444 refers to your rod journal crank sizing. Stock rods would have lettering and numbers on each side. One for size and the other for weight.

Some people prefer to run the crank looser with more clearance but that usually sacrifices some oil pressure.

Can you see any colors corresponding to bearing sizes on the edge of the bearings? ie yellow, green, brown, black
 

No color coding on the conrod bearings, and also no bushings on the small end...
That's what makes this so difficult to me, because I have no idea what bearings are in those conrods...

The main crank bearings are all green on the edge.