Topic: CB400F 1977 engine rebuild - piston ring groove clearances

[berbecs]

Dear members,
Thank you for accepting my request to join your community!
I recently bought a Honda CB400F 1977 , I am currently disassembling the engine.

My Haynes manual is somewhere in transit in Europe, and I couldn't find the answer for this here or on Google, so:

I would like to measure the piston ring groove clearance, but I don't know the acceptable gap size. Could you please tell me this measurement, and also that should I measure this with the old piston rings in, or the new one?
 I guess taking the measurements with the new ring(s) in would make more sense..

Many thanks!
Akos

[Tim2005]

Hi, these are the specs from the Honda service manual... centre column is standard values, right column is he service limits

[berbecs]

Thank you very much!

Just a quick question, the top left box says Cylinder I.D. - I assume this is the cylinder inner diameter, right? In my case it is 51mm, does this change anything?

Something else..
I've just measured a piston that I've just taken out from the engine with a normal vernier caliper. Roughly in the middle, where the gudgeon pin is the diameter is cca. 51mm, but at the top just above the first ring it is cca. 50,6mm.

I guess I need new pistons too, right? This seems quite a difference 

Thanks,
Akos

[Tim2005]

Yes, the 400F is 51mm, the 350F is 47mm, I'm not sure why the manual shows that. Looking at the other section, it just says measure the piston at its skirt, maybe someone else on here has a new piston on hand to measure & compare skirt vs top diameter.

I would upload the whole manual for you but it's 40mb and the max size on here is 4

[Tracksnblades1]

Dear members,
Thank you for accepting my request to join your community!
I recently bought a Honda CB400F 1977 , I am currently disassembling the engine.

My Haynes manual is somewhere in transit in Europe, and I couldn't find the answer for this here or on Google, so:

I would like to measure the piston ring groove clearance, but I don't know the acceptable gap size. Could you please tell me this measurement, and also that should I measure this with the old piston rings in, or the new one?
 I guess taking the measurements with the new ring(s) in would make more sense..

Many thanks!
Akos

BB,

I found a older post you may find interesting. Sometimes a new bore can be “worn out” before you get started.

http://forums.sohc4.net/index.php?topic=179579.75

It’s good to read from a mew member.

[HondaMan]

Thank you very much!

Just a quick question, the top left box says Cylinder I.D. - I assume this is the cylinder inner diameter, right? In my case it is 51mm, does this change anything?

Something else..
I've just measured a piston that I've just taken out from the engine with a normal vernier caliper. Roughly in the middle, where the gudgeon pin is the diameter is cca. 51mm, but at the top just above the first ring it is cca. 50,6mm.

I guess I need new pistons too, right? This seems quite a difference 

Thanks,
Akos

The pistons must be measured at the widest portion of the BOTTOM of the skirt, which is the front-to-back measurement. However, vernier claipers will not be accurate: this measurement requires ID bore calipers for the cylinders and micrometers for the pistons.

"Worn out" in these engines almost never refers to the pistons. It is the rings that wear out and cause compression issues.

You also cannot just buy new pistons and drop them in and expect improvement: that will not happen - you will end up with poor compression, oil leaks, and probably a smoking engine. You can replace the piston rings with new ones and get some more miles from the engine, but a top-end rebuild requires special attention. The piston-to-cylinder clearance in this engine is VERY TIGHT, and most machine shops these days seem like they refuse to do it properly. This engine, in particular, requires 0.0004" (that is NOT a typo: it is less than 1/2 of a thousandth of an inch) of piston-to-cylinder clearance between the widest part of the piston skirt and the bores. If the clearance exceeds 0.0012" (that is less than 1.5 thousandths of an inch) then it is considered to be worn out.

So, choose the parts carefully: for the cheapest path forward to obtain a temporary improvement, find some piston rings for your existing pistons from original Honda parts, RKS or MC brands (Honda used both of these, try eBay for some) in the Standard size. Hone the cylinders lightly with a new brake hone so they will seat.

[berbecs]

Tracksnblades1  Thanks for the link, I'm reading the topic!

HondaMan Thank you for all the info!

I am aware that a vernier caliper is not the right tool, I measured it out of curiosity and I was just surprised that how smaller the top part of the piston was.
I stripped the engine because of the noisy cam chain, the chain has bitten into the tensioner mechanism very much.

The engine was working before I stripped it, starts first time, no smoke, no problem except the cam chain, although I cannot refer to the power since I have nothing to compare. I rode it cca 10 kms, it pulled very nice, but maybe I'm in love

I thought once I'm there, I will change everything that needs to be changed. But it is quite hard for me to tell what needs to be changed
I do not want to  desperately throw everything into the bin, quite the opposite. But I don't want to "save" €30 now so I have to spend €1000 next year if you know what I mean...

The pistons are all marked "Honda", the rings look ok to me, here's a picture. I'm still thinking how to clean them, they fit in my ultrasonic cleaner one-by-one, but this carbon buildup is much more stubborn I guess

Thanks for all the help!! I am browsing the forum for a couple of months now, and I'm slowly getting the hang of it...

[berbecs]

At this point, I am quite confused how to determine whether the parts need to be changed or not.

I have not split the cases yet, but

- The crankshaft has cca. 0,7mm side play
- The main shaft has a cca.1,0mm side play
- The con rods have cca. 1,0mm side play
- The starter reduction gear has cca. 3-5mm side play

Are these movements considered normal?

[bryanj]

The only thing worth changing is primary chain, same as 500/550 and identical to a Kawasaki one at about half the price.
End float on crank dont matter unlike a car, others are probably ok

[Tracksnblades1]

Berbecs,
I’m not sure of your background so don’t misconstrue any inferences.
The piston’s shape cold is by purpose. The heavy pin boss area of the piston and higher operating temperatures of the top will  expand more than lower cooler running skirt. At operating temperature the piston shape should expand to the designed shape (round). Google “cam ground piston”. The cam ground piston technology has been in practice as long as I can remember. And I’m old.😜

My reference to a “new bore” that can be “worn out” is from a machine shop experience with a new bore job on an air cooled 2cycle engine. The mostly aluminum cylinder with the molded cast iron sleeve had an expansion rate similar to the aluminum piston’s expansion rate. So the manufacturer’s specification piston to cylinder for their cast piston was very tight. Since the cylinder expanded (got bigger around) as the piston did at operating temperatures and higher loads.

My automotive machine shop,working primarily with water cooled engines,  set it up with .004 instead of .0005-.0008. Effectively making it a “worn out”  “new bore” before it ever ran. Since the cylinder had been radically ported, the exhaust port caught the ring the first time the throttle was pinned.. The second go around I took the jug to a shop that specialized in air cooled, paid more, had it set up on the low side of the specs. Gave it a long break in and rode it. 😁

Measure twice, measure again, when in doubt, ask someone.
A very least replace the primary chain as already recommended.
I personally don’t have much experience with a bore caliper. I prefer the dial bore gauge over any other type, excepting small diameter bores. Once you find a competent machine shop, I would be primarily interested in the actual clearance.
Micrometers that read down to the ten thousands may not perfectly agree with yours..

[berbecs]

My background, well, not repairing inline fours on daily basis for sure. let's say I have seen a workshop from the inside before

The only thing worth changing is primary chain, same as 500/550 and identical to a Kawasaki one at about half the price.
End float on crank dont matter unlike a car, others are probably ok

I have split the cases, and indeed, the primary chain needs to be changed. Thanks for the tip!

But I am still anxious about the play of con rods, it seems like they are loose. Again, I have nothing to compare with, but they look way less tight than anything else in the engine.

On the other hand the crankshaft bearings look OK to me, for example compared to ones that I've seen here in previous threads.
There is no visible scoring or pitting on the shaft whatsoever.

[bryanj]

Side play is of no consequence only up and down play

[Tim2005]

Crank, shells and piston all look pretty good. Has the motor done many miles?

[berbecs]

Crank, shells and piston all look pretty good. Has the motor done many miles?

65000 km according to the speedometer. I bought it from a dealer, so who knows, but I guess they usually turn back the meter, not forward

I will ask a machine shop to measure the components, and we'll see what's then next move

[bryanj]

Wouldnt bother with the cost of measuring they look perfect and very few people have the kit (that includes machine shops) to measure to Hondas accuracy

[berbecs]

Wouldnt bother with the cost of measuring they look perfect and very few people have the kit (that includes machine shops) to measure to Hondas accuracy

All right, thanks!

And what about the pistons? Should those be measured, or I just clean them, put new rings on and good to go?
The ring end gap is 0,4mm (service limit is 0,7), the piston ring groove clearance is just above 0.05mm (Services limit is 0,15mm) I don't have a feeler gauge below 0,05mm at the moment)

I have taken off the rings from only one piston to see what's going on there, so I guess I should swap those to new ones, or swap all of them to new ones.
And the criss-cross pattern from the inside of the cylinders are gone so I guess a honing would be necessary

[bryanj]

Honing removes too much metal, use a "glaze buster" which looks like a flap wheel, get one that is a tight fit in the bore.
The rings tend to wear on the top side in the groove so that there is a lip at the bore, if so change all of them if not no reason not to put back on the rings you took offas long as they go the same way up they came off. There should be markings near the gap and they face upwards, if not there rings are worn. Also only glaze bust if fitting new rings not if reusing old ones.

[berbecs]

Honing removes too much metal, use a "glaze buster" which looks like a flap wheel, get one that is a tight fit in the bore.
The rings tend to wear on the top side in the groove so that there is a lip at the bore, if so change all of them if not no reason not to put back on the rings you took offas long as they go the same way up they came off. There should be markings near the gap and they face upwards, if not there rings are worn. Also only glaze bust if fitting new rings not if reusing old ones.

There is an "R" on the rings, but I was foolish enough to mark which one was at the top and at the bottom, not to mention which way up the oil rings were if that counts...

About the lip at the bore, there is some carbon at the top 5mm, beside that it looks OK to me.

I found these two different type of tools when searching for "glaze buster", are these any good?

[Tracksnblades1]

what is the piston to cylinder wall clearance?
What is the cylinder taper?
A Dial bore gauge, Micrometer, and a old piston
Will remove all doubt.

Top picture is an economical stiff hone.
It can produce average results with an ampere probe
connected to the drive.

Second picture is a glaze buster.
Probably doesn't come with stones above 120/240.

Both will make the bore bigger than Honda sold it with.

[berbecs]

what is the piston to cylinder wall clearance?
What is the cylinder taper?
A Dial bore gauge, Micrometer, and a old piston
Will remove all doubt.

Top picture is an economical stiff hone.
It can produce average results with an ampere probe
connected to the drive.

Second picture is a glaze buster.
Probably doesn't come with stones above 120/240.

Both will make the bore bigger than Honda sold it with.

I will take the parts to a machine shop to figure out the measurements. 

Regarding the tools, what would be the method/tool to "brake the glaze" but isn't too harsh on the walls?
The second tool, with fine pads above 240?
I think it is more likely that a machine shop is able to do it than me buying some €20 tool from ebay, I'm just a bit concerned because 6 out of 10 shops hung up on me when I said " it is a 1977 Honda..."

[Tracksnblades1]

Without knowing the actual taper and bore clearance and your intended goals that's a tough call.

If your planning a re-ring Only or using your old rings and pistons in their original holes.
Then I would check to see if the carbon ledge you wrote of is indeed just a carbon ledge or the original measureable bore. It may need removed if new rings are installed.

There is a crosshatch tool sometimes called a "dingle berry hone". Primarily used to achieve the desired crosshatch pattern for the chosen ring type. Different ring types require different stone grits. Chrome rings need some what of an aggressive stone. But I've seen engine builders use very fine stones with good results too.

HondaMan has extensively wrote about these tiny bore's requirements. I'm the type that's going to bore it. Have someone with a "Sunnen" power hone, hone it, set up for the minimum. Because I'm not going to sell it and it's only going to get bigger. 

Google dingle berry hone.
I can't seem to send a picture on this old ipad

[Tracksnblades1]

Picture is from ASE certification practice.

 Imagine your new sharp unworn ring’s sides and squarer face
having to compensate with this shape.  When cylinder pressure
and rpms are high…😳

[bryanj]

All those pictured are "homes" and will remove too much, look at ebay item 184430930041 you  need this with a 60mm diameter. Called a flap wheel sander in UK no ides of name in US

[HondaMan]

Those rod-crank bearing pictures look normal for the 400F/350F. I'd leave those as they are. Almost all the 350F/400F engines I see have the slightly oversized rod bearings that wear just in the middle of each shell half, seems to have been Honda's choice. This is one reason they run better and longer with a little bit heavier oil, too, like 15w40 or 20w40 (or 20w50 in hot weather). Low-detergent oil is vital (I always 'preach' about that in these engines...).

If you hva the tools to remove the valves from the head, clean them to nice and smooth again on a wire wheel. If you can obtain some fine valve-grinding compound, you can hand-lap them in again for a perfect seal. The valve guides on the 350F/400F don't wear out very fast, so they are likely just fine.

[berbecs]

Many thanks for you all!
I managed to find an enthusiastic guy locally, who rebuilds engines so he's helping me know. We measured the pistons and cylinders so far, more coming.
The piston-cylinder difference is 0,07mm, the ovality of the cylinder is somewhere 0,03mm if I remember correctly.
I'll write all the measurements down out of curiosity and for the record. He'll help cleaning the head in an ultrasonic cleaner, and the valves too, but it seems like it's goind to be fine