Engine rebuild - bore glazing

[Mr-Gear]

Hi All,

I have a rebuilt CB400F engine where the bores have been machined oversize to 51.25mm paired with 0.25mm oversize pistons/rings (pattern from DSS).

During assembly, ring gaps and piston OD was measured within spec.

Break-in procedure followed as per the Honda shop manual guidance using mineral engine oil, local riding working through the gears at various RPM ranges.

Since the rebuild and subsequent break in, the engine appears to be performing well and compression values are ~150psi evenly across all cylinders after what is now 2000 miles. There have been a few occasions where the engine has been held at at a constant speed on the motorway as I like to travel on the bike (I'd estimate 6-7K RPM for an hour or so).

Out of curiosity, I popped an inspection camera into the bores and saw that cylinder 1 has polished areas (perpendicular to crankshaft) at the bottom portion of the cylinder wall. (see attached picture). This feature appears across all 4 cylinders in the same places to the same extent.

Being magnified by the inspection camera could just be making it look worse than it is. I have not disassembled the engine to investigate further at this stage, as it is all nicely finished with Cerakote and I wouldn't want to risk damaging the finish unnecessarily.

Question:
Is this normal or expected?
If not what could be some suspected causes?
Is this detrimental to the engine?

Thank you.

[Bryanj]

The "honing" looks a bit rough to me and the smooth bit would be the rings making it smoother

[McCabe-Thiele (Ted)]

Curiosity killed the Cat.

Fitting an oil pressure gauge makes you worry about oil pressure, as long as the oil light goes out when you start the engine that all Mr Honda deemed necessary.

Putting an Inspection Camera down you bores is verging on the paranoid.
Was there a solid reason you even looked, oil consumption or something?

It's probably oil marking from the oil thrown up  by the crankshaft journals.
The pistons are travelling at a fair speed when going down on the firing stroke, then changing direction at BDC on the return for exhaust, repeat for induction & compression. Pistons distort under such loads at high rpm, the fact that it's on all four sounds good to me. As Bryan has said it could be just the bores bedding in, check it again at 10,000 miles, it might be the same  all over.

My mobile is great at making things look bent or out of shape, especially on close ups.

[TrickyMicky]

If the shiny portion(s) are towards the front and rear of the cylinders, then this is quite normal.  On the firing/downward stroke, as the crank turns the con-rod is not in a vertical plane, and it gives the piston a 'slanting' tendency thereby exerting more pressure at these points. If ever you get the chance to look at a large  marine diesel, or a steam locomotive engine you will se that the connecting rod from the base of the piston operates a crosshead, which keeps the piston square in the bore. I would put your camera away, and if it's not using oil, stop worrying and enjoy riding.

[Mikep328]

I'm on a Norton Commando website regularly where quite a few people have installed oil pressure gauges and now spend a lot of time worrying/commenting about oil pressure.  There is NO history of Commandos failing due to lack of oil pressure.  But with a gauge it can now be observed and, therefore, worried about!

If the engine is running well, not using oil, no smoke, etc, no need to worry about things just because you can!!! 

[Mr-Gear]

Thank you all, the responses are appreciated. I am a little on edge because I had a failure (detailed in a previous thread of mine) during a trip in Europe where the bike didn't make it home.

Hopefully this post reaches others who may be wondering the same.

[McCabe-Thiele (Ted)]

Thank you all, the responses are appreciated. I am a little on edge because I had a failure (detailed in a previous thread of mine) during a trip in Europe where the bike didn't make it home.

Hopefully this post reaches others who may be wondering the same.

That explains your exploration down the bores.

My anecdotal view for what it's worth, looking back to when these air cooled bikes were new, they were in the days when you bought a bike to go to work, nip out to see your mate etc. All relatively short journey work on bendy A or B roads.
Aside from the M1 there were relatively few motorways outside London or opportinities to ride at a constant rpm for long distances & time. (Water cooled engines are in my view a different animal.)

Back in the 1960's my longest journey on my Honda 250 Dream was Derby to Chester each Sunday night, returning home on a Friday evening after work (75 miles each way). No motorway back then, or dual carriageways of any length. It was a journey that took 2.5 -3.00 hours, up and down the gears all the time. When engines are hot, running on mineral oil, on Motorways  its likely to break down the oil relatively quickly.

To ride an old relatively small engine capacity capacity bike great distances is in my opinion not good for the engine. My Yamaha XJ900 was the only air cooled bike I rode on Motorways, I did not find it an enjoyable experience.

I'm sure there are members here who have ridden into Europe on a bike like yours, I think they will not have been riding for hours a day at a constant speed, just my opinion.

Back in the 1960 I had an 850 Mini, I travelled home at weekends on the M1 from Bradford to Derby, no speed limits, being young and stupid I would drive flat out whenever I could. After 50 or so miles I would stop at the services for petrol. I found that the gearbox would not move out of 4th, when it did I could not easily select a gear. Lots of clicking as the engine cooled down whilst I had a Cuppa. All would be well again with the gearbox until I reached my destination. It was a common issue on the old A Series engined Minis.