Author Topic: CB250RS Rebuild Progress  (Read 6160 times)

Offline hairygit

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Re: CB250RS Rebuild Progress
« Reply #120 on: November 20, 2018, 05:06:13 PM »
Whatever you do, if it is overbored, do not let him get his hands on the other barrel, anyone that ignores recommended tolerances and makes such a balls up cannot be trusted ever again. And if that IS the case, name and shame him please, so nobody else has the same issues!
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Offline K2-K6

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Re: CB250RS Rebuild Progress
« Reply #121 on: November 20, 2018, 07:02:31 PM »
It's difficult to comment on the engineering experience that leads someone to make an arbitrary decision about something like this.  I wouldn't be inclined to bet against the shear range of Honda's R+D efforts though.

The basis of these pistons, and metallurgy that's involved, is one of the principles of all Honda's road bike engines, and the design concept.

Conventionally pistons had been sized to allow clearance for the piston to expand into and fit the bore with a running tolerance once up to temp. In other words,  the piston expanded more than the bore so you had to predict the end result and machine cold to accommodate that process.

I don't know if they pioneered it? But Honda's development with the piston suppliers used high silicon content aluminium cast pistons to control (along with design structure) the expansion, and make it more or less the same rate as the cylinder structure.  Meaning that the cold and hot tolerance is virtually the same. 
It's this aspect that people inexperienced in them fail to account for.......or don't believe is true.

https://en.m.wikipedia.org/wiki/Hypereutectic_piston

The above link gives a broad overview of why this happens in discussing various piston material compositions.

One set of tolerancing criteria just cannot be overlayed onto another metallurgical scenario,  or certainly not without a throughly investigated process.

It has effectively produced a "worn out" bore as far as tye numbers tell us.



Offline Trigger

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Re: CB250RS Rebuild Progress
« Reply #122 on: November 21, 2018, 08:18:19 AM »
I see over bore a lot and a lot of my work is correcting others mistakes  :o I still scratch my head when i see a barrel that has been honed wrong without a good cross hatch  >:(

As i have said before, it doesn't matter if a engineer is George Stephenson, a aerospace engineer, race or rally, if he doesn't know, he doesn't know  ;)

Offline Tomb

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Re: CB250RS Rebuild Progress
« Reply #123 on: November 21, 2018, 09:07:13 AM »
Whatever you do, if it is overbored, do not let him get his hands on the other barrel, anyone that ignores recommended tolerances and makes such a balls up cannot be trusted ever again.

So true!

I remember the conversation
http://www.sohc.co.uk/index.php/topic,16637.msg140753.html#msg140753 and the comments made appear to have been found proven unfortunately.

I have worked on lots of race engines that have been modified, mostly crank case work for bigger liners, race transmissions/clutches, and am amazed at how badly some of it is done by so called experts. All our engines are machined by us now.
As Trig says - cross hatching is essential for bedding in.

Lastly I would check you have the oil rings seated in the piston correctly, if they are 3 piece oil rings they can be fitted wrong easily.

Tom
'73 CB550 with CB500 engine café racer
'62 CB77 Sprinter
'70 CD175
'78 CB550 with sidecar
'80 Z50R
And a load of old Yamaha 1100's

Offline Trigger

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Re: CB250RS Rebuild Progress
« Reply #124 on: November 21, 2018, 03:02:40 PM »
Whatever you do, if it is overbored, do not let him get his hands on the other barrel, anyone that ignores recommended tolerances and makes such a balls up cannot be trusted ever again.

So true!

I remember the conversation
http://www.sohc.co.uk/index.php/topic,16637.msg140753.html#msg140753 and the comments made appear to have been found proven unfortunately.

I have worked on lots of race engines that have been modified, mostly crank case work for bigger liners, race transmissions/clutches, and am amazed at how badly some of it is done by so called experts. All our engines are machined by us now.
As Trig says - cross hatching is essential for bedding in.

Lastly I would check you have the oil rings seated in the piston correctly, if they are 3 piece oil rings they can be fitted wrong easily.


I don't think it would mater if the oil ring is wrong at 0.08mm. I did advise 0.8 thou to 1 thou and, at 0.08mm that is over 3 thou :o That is service limits on a Honda SOHC  ;)

Offline Tomb

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Re: CB250RS Rebuild Progress
« Reply #125 on: November 21, 2018, 04:03:35 PM »
The reason I say check the oil rings is even at the bore size he has now, its still at the top end of wear limits, and an engine running at these clearances wouldn't be expected to use that much oil.

And its always one to check rather assuming its not the fault.
Tom
'73 CB550 with CB500 engine café racer
'62 CB77 Sprinter
'70 CD175
'78 CB550 with sidecar
'80 Z50R
And a load of old Yamaha 1100's

Offline Trigger

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Re: CB250RS Rebuild Progress
« Reply #126 on: November 21, 2018, 06:05:33 PM »
Very true Tomb, never assume anything when it comes to engineering  ;) I did have a engine in sometime ago where the owner had fitted the rings as they were stacked in the packet  :o

Offline Allington

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Re: CB250RS Rebuild Progress
« Reply #127 on: November 21, 2018, 06:41:36 PM »
Very true Tomb, never assume anything when it comes to engineering  ;) I did have a engine in sometime ago where the owner had fitted the rings as they were stacked in the packet  :o

Better than that other bloke who left half the bits at home  ;)
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Offline Nurse Julie

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Re: CB250RS Rebuild Progress
« Reply #128 on: November 21, 2018, 06:46:03 PM »
Very true Tomb, never assume anything when it comes to engineering  ;) I did have a engine in sometime ago where the owner had fitted the rings as they were stacked in the packet  :o

Better than that other bloke who left half the bits at home  ;)
🙄🙄🙄🙄🙄 There's always one.....Steve 😀😀😀
Im a Nurse, trust me, this won't hurt.....much !!!

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Offline Laverda120

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Re: CB250RS Rebuild Progress
« Reply #129 on: November 21, 2018, 06:51:39 PM »
The oil rings are the three piece type. I fitted these in accordance with the Haynes Manual and followed the diagram as in: first thin ring, second the scraper ring making sure the ends did not overlap and thirdly the top ring. The ends of the rings were set at 20mm apart to avoid them being in line. The oil rings were followed by the middle and top rings. These had the 50 stamped on them on one side only and these faced the crown of the piston as per the manual with the gaps spaced equally around the piston to avoid blow by. The three part oil ring had no markings but I did check with a watchmakers eye piece just in case but found nothing. These are all genuine Honda rings.
I assume all the rings are therefore fitted correctly?
I was hoping to venture into the garage today but was dragged out to start Christmas shopping 🎄🙄, do John Lewis do rebores 🤔?

Offline hairygit

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Re: CB250RS Rebuild Progress
« Reply #130 on: November 21, 2018, 06:58:57 PM »
Probably best to see if Trigger will do the bore for you!

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Offline Allington

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Re: CB250RS Rebuild Progress
« Reply #131 on: November 21, 2018, 07:00:18 PM »
Very true Tomb, never assume anything when it comes to engineering  ;) I did have a engine in sometime ago where the owner had fitted the rings as they were stacked in the packet  :o

Better than that other bloke who left half the bits at home  ;)
🙄🙄🙄🙄🙄 There's always one.....Steve 😀😀😀

 :'(
"A little bit of knowledge is a dangerous thing..."

FS1E - RD200 - CB500 - Norton Commando - Z900

Offline Laverda120

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Re: CB250RS Rebuild Progress
« Reply #132 on: February 11, 2019, 06:59:20 PM »
Well, after a lot of faff the bike now runs and is much, much quieter than before!
In the end I bought a s/h barrel from ebay for £10 rather than throw a £100 genuine Honda ±50 piston and rings into the bin. I had the barrel vapour blasted as the paint was in a bit of a state and repainted it using PJI satin black (although once cured in the oven it is more matt than satin). I had the barrel bored to the middle tolerance as specified in the Haynes (and Honda) manual. The engineer cried 'no, it will sieze' but after explaining the Honda metallurgy methodology he relented and did what I asked. He warned me when he has bored previous CB250RSA engines to these clearances they have siezed as they run very hot. He also said the ones he worked on previously were dispatch bikes with pattern pistons so I guess they might well have siezed if subjected to london's cut and thrust traffic from the word go!
Im not counting my chickens yet given my previous two experiences but I ran it for 42 miles, it was amazingly quiet (the previous oversize rebore must have introduced piston slap of another level!). The best thing, it used no oil whatsoever, previously even after 42 miles the oil level would have dropped slightly!
The only slight downside is I noticed a slight weep coming from the end of one rocker shaft, oil weeps from these are a common problem on these engines. It wasn't there before so I'm at a loss why its strarted now? I know I'm not taking the rocker cover off again to remove the rocker shafts and replace the o rings. What should be an easy job isn't on these engines due to the stupid pressed in dowls being impossible to remove without a spark eroder!
If anyone has any suggestions for an alternative potential cure I'm all ears 👂😊.
Thanks for all your previous advice, hopefully it will run as it should from now on 🤞!
Tomorrow I'm also collecting the CB250RSA holy grail.... A genuine 'Hondastyle' nose fairing, as rare as hens teeth to find in undamaged condition.
If anyone has a set of front engine bars they wish to sell please let me know.
« Last Edit: February 11, 2019, 07:02:14 PM by Laverda120 »

Offline Jamesh_sussex

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Re: CB250RS Rebuild Progress
« Reply #133 on: February 11, 2019, 08:11:35 PM »
Good work. Just checking - you using mineral or semi-synthetic oil?

Offline rozabikes

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Re: CB250RS Rebuild Progress
« Reply #134 on: February 11, 2019, 10:26:10 PM »
Sounds promising. Bet you are relieved..... :)
One day I'll have the time to restore it, not just talk and dream....