Topic: Engine Tear Down

[ncstatecamp]

I am doing an engine tear down on a cb750k2 to fix an issue with low compression in cylinder 2. While I have the top end off fixing what ever the issue is with cylinder 2, other then gaskets what else is recommended to be replaced or checked?

Things considered/heard

Clean polish everything possible
Cam chain, tensioners
Lap valves
Rings
Replace Head bolts, or any and all bolts possible (especially those damn phillips)
Bearings (uh not sure which help? )
Cylinder Studs
Hone Cylinders
To check the bore to see if its still in round

My initial compression test was all 130ish for the 3 good cylinders but with a few drops of oil 150-155ish and held to that the next day without any oil (it hasnt been running in who knows how long  so im assuming it was kinda dry in there.)

Being new to this forum (been lurking for a bit) any and all advice or help would be appreciated.

Many Thanks ahead of time!

[Rob69]

I am doing an engine tear down on a cb750k2 to fix an issue with low compression in cylinder 2. While I have the top end off fixing what ever the issue is with cylinder 2, other then gaskets what else is recommended to be replaced or checked?

Things considered/heard

Clean polish everything possible
Cam chain, tensioners
Lap valves
Rings
Replace Head bolts, or any and all bolts possible (especially those damn phillips)
Bearings (uh not sure which help? )
Cylinder Studs
Hone Cylinders
To check the bore to see if its still in round

My initial compression test was all 130ish for the 3 good cylinders but with a few drops of oil 150-155ish and held to that the next day without any oil (it hasnt been running in who knows how long  so im assuming it was kinda dry in there.)

Being new to this forum (been lurking for a bit) any and all advice or help would be appreciated.

Many Thanks ahead of time!

You pretty much have everything covered. If you don't have a service manual, get one  and make sure you have all the necessary tools, and leave no stone unturned:

http://wwwclassiccyclesorg.mediafire.com/?dbk51cbcx8wfbj9

The compression readings should be about 170 psi with no more than a 10% difference between any of the cylinders.

[ncstatecamp]

So I should do all cylinder rings regardless, and go all the way down to the valves? I have printed off about 500 pages of manuals so im set there.

Ive heard both ways on replacing the studs and wasnt sure if they were worth the risk/reward.

Also which bearings should I pay attention to the most?

[cgswss]

Your main goal is to remove the jugs and hone them, install new rings, then put her back together.  Of course you will need a new head gasket so that will be replaced.  While apart, I would do a couple of things.  If you have a big enough piece of glass (I use one of my storm windows but not everyone has those) lay the cylinder head on it and see if you can slip a piece of paper between the head and the glass  The head should be dead flat and not allow you to do this.  do the same thing on the jugs (upside down).  set the head upside down and try to get it as level as possible, then pour gas into each chamber.  If the gas leaks out, we know you need to also do a valve job.

While everything is apart, I would clean the carbs.  no need to remove them from the rack but take the float bowls off and use the little tube on a can of carb spray to check each passageway to make sure everything is clear.

When you start putting everything back together, get some assembly lube from the auto parts store and lube each stud and bolt when you are assembling the motor.  This will make your torque setting more accurate.

I would wait to put in a new filter.  After its back together ride the bike for about 500 mile then change the oil and the filter.

[cgswss]

I wouldn't do the valves unless you see them leak gas when you fill the chambers (head upside down)  IF you have to o the valves, then I would replace the valve guides and seals at the same time.  If I had to do a valve job, I would do a 3 angle valve job.  You may be able to rent the cutters from an auto parts store.

[dave500]

if youve got the head off and dont at least de carbon the valves and replace the seals your only doing a half job,,if your de carboning the valves and dont re lap them your doing another half a job

[cgswss]

Doing a proper valve job requires some special tools I'm sure you don't have.  Your "wet" compression numbers would seem to say you may not need to.  However "to do it right" you should go ahead and do it.  Just make sure you can get a valve spring compressor and the valve seat cutters.  While you can make a usable valve spring compressor from a really big c clamp, its a whole lot easier with the proper tool.

Also you might consider a "big bore" kit as long as you have to do the rings anyway. there are some out there that are almost cheaper then getting OEM rings.  working with fresh holes and new pistons will make sure your "fixing the problem" you started out to do.

http://www.ebay.com/itm/69-77-HONDA-CB750K-836cc-BIGBORE-PISTON-KIT-W-GASKET-/130712839062?pt=Motorcycles_Parts_Accessories&hash=item1e6f17a396&vxp=mtr#ht_1189wt_1085

[wrenchmuch]

When checking compression remember that different gauges will yield different readings due to things like the kind of tube used to connect the gauge to the cylinder ( does it swell under pressure , its length ) . To get a meaningful comparison to the figures in the manual you need the gauge Honda used to arrive at those figures . Using that gauge your pressure could be closer to ideal . the gauge you have is good for checking balance across cylinders . I get the idea from your posts that this engine hasn't run in a while . Your rings might just be stuck , not worn . You might try a 50/50 mix of acetone / AFT fluid poured into the cylinders to loosen up the rings . Let it sit a week then crank it to blow out the fluid . Be ready with a rag across the plug holes . Change the oil and try and start it . If it runs I would drive it for a couple of weeks then reassess compression .

[ncstatecamp]

Your main goal is to remove the jugs and hone them, install new rings, then put her back together.  Of course you will need a new head gasket so that will be replaced.  While apart, I would do a couple of things.  If you have a big enough piece of glass (I use one of my storm windows but not everyone has those) lay the cylinder head on it and see if you can slip a piece of paper between the head and the glass  The head should be dead flat and not allow you to do this.  do the same thing on the jugs (upside down).  set the head upside down and try to get it as level as possible, then pour gas into each chamber.  If the gas leaks out, we know you need to also do a valve job.

While everything is apart, I would clean the carbs.  no need to remove them from the rack but take the float bowls off and use the little tube on a can of carb spray to check each passageway to make sure everything is clear.

When you start putting everything back together, get some assembly lube from the auto parts store and lube each stud and bolt when you are assembling the motor.  This will make your torque setting more accurate.

I would wait to put in a new filter.  After its back together ride the bike for about 500 mile then change the oil and the filter.

This is a project bike I picked up. The engine hasn't been run in quite some time (to my knowledge) and the old filter was pretty wrecked so that has to go. Already have the carbs off and have completely polished and replaced all the brass. The glass pane idea is a good one which I will try to do. Quick question, im sure ill be able to tell once I've dived in, but uh.... what are jugs  noobie question I know.... I will also defiantly remember to pick up some assembly lube.

cgswss: what is a three angle valve job?

wrenchmuch: Ah that mightve worked. I will remember this for future projects but the motor is already off the frame (stripped to get powder coated) and I've kinda always wanted to see the inside of one of these first hand!

Thanks all for the advice!

[lucky]

So I should do all cylinder rings regardless, and go all the way down to the valves? I have printed off about 500 pages of manuals so im set there.

Ive heard both ways on replacing the studs and wasnt sure if they were worth the risk/reward.

Also which bearings should I pay attention to the most?

You have it all apart. check ALL the bearings.
Measure the cylinder bores. Have a machinist do it- Give the machinist the specs from the book.
You do not need to replace any of the studs.
If you want high strength cylinder head studs that is an optional upgrade.
A stock engine does not need them.

[ncstatecamp]

You have it all apart. check ALL the bearings.
Measure the cylinder bores. Have a machinist do it- Give the machinist the specs from the book.
You do not need to replace any of the studs.
If you want high strength cylinder head studs that is an optional upgrade.
A stock engine does not need them.

Thats what i was thinking about the studs but what about the studs if the cylinders need over boring?

[cgswss]

A normal valve seat is 45*.  with a 3 angle valve job you still cut a valve seat of 45* but you also undercut the seat with a 60* cutter an over cut it with with a 30* cutter.  This allows you to very closely control the with of the valve seat and the under cut and over cut lets the gases start flowing sooner and should also improve the flow.  When we build racing engines wet bot cut the seats and machined the valves.  on a race engine we would go with very narrow seat to get the quickest possible flow.  Of course we were tearing down the engines every week so we didn't have to worry about how long the seat would last.  on street engines we would use a wider seat but still about 1/2 as wide as the "stock" seat.  A 3 angle valve job improves your performance everywhere in the power band.  the only negative is the more narrow you make the valve seat the shorter the valve job will last.  A good "street" 3 angle valve will give you 25,000 miles.

[dave500]

youll most likely be ok with just lapping the valves not actually cutting the seats.,the simple plan A engine apart and back together can get out of hand if you want to big bore it etc.

[ncstatecamp]

youll most likely be ok with just lapping the valves not actually cutting the seats.,the simple plan A engine apart and back together can get out of hand if you want to big bore it etc.

Precisely what I'm trying not to do! Hopefully it's at most lapping valves and cleaning everything and the cam assembly if need be at most.