Author Topic: CB250RS Rebuild Progress  (Read 6210 times)

Offline K2-K6

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Re: CB250RS Rebuild Progress
« Reply #105 on: October 28, 2018, 07:08:02 PM »
I like the Triton leaning against the wall nonchalantly in an "Ogri " style.


Offline Allington

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Re: CB250RS Rebuild Progress
« Reply #106 on: October 28, 2018, 10:48:57 PM »
Done the Tritton thing again so many times in my head over the years not making my mind up just which way to go but I have space here and I’m pretty sure it will happen at some time very soon.
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Offline Laverda120

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Re: CB250RS Rebuild Progress
« Reply #107 on: November 08, 2018, 05:22:40 PM »
Engine now rebuilt and back in the frame. I completed 60 very cold miles today starting the running in process all over again! All appears to be fine so fingers crossed the genuine Honda piston won't melt at 480 miles as the pattern one did  :(!

Offline Nurse Julie

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Re: CB250RS Rebuild Progress
« Reply #108 on: November 08, 2018, 06:06:59 PM »
Fingers crossed it will all be ok, great job.
Im a Nurse, trust me, this won't hurt.....much !!!

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Offline Seabeowner

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Re: CB250RS Rebuild Progress
« Reply #109 on: November 08, 2018, 06:07:59 PM »
Great, 10/10 for persistence and a lot of hard work.
Phil
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Offline Bryanj

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Re: CB250RS Rebuild Progress
« Reply #110 on: November 08, 2018, 07:01:32 PM »
Well done again, i really hope its all ok now. I liked those when they were new, fantastic handling and a decent turn of speed for a small bike---but then my first bike was a 250 Royal Oilfield all 7hp of it!

Offline Laverda120

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Re: CB250RS Rebuild Progress
« Reply #111 on: November 08, 2018, 07:13:25 PM »
Thanks all and thanks for all the advice during the strip down. The weather for tomorrow is dry so maybe I'll get some more (4000rpm =40mph 😴) miles in! I'll take it up to 4,500 cruising when I clock up 100 miles. I'll keep the revs to 5000rpm until I reach 500miles and then increase by 1000rpm for every 100 miles thereafter. I'm just conscious of another potential meltdown if I don't go easy during the running in period.

Offline Laverda120

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Re: CB250RS Rebuild Progress
« Reply #112 on: November 20, 2018, 09:34:58 AM »
Here we go again....... I need some expert advice from someone! 🙄. I very, very carefully rebuilt the engine with all new gaskets, new exhaust valves, checked the valve guides (all good) and fitted a genuine Honda +50 piston and ring set  with a rebore. After 260 miles the engine has used 700ml of oil, this cannot be right 🤔. The engine doesn't smoke on tickover but I asked the o/h to follow me on her bike last weekend and she told me it was smoking blue smoke all the time after pulling away. Sure enough after swapping bikes I could see it smoking. It does it all the time so it doesn't appear to be valve guides.
Am I just being paranoid, should I give it another few hundred miles to settle down or is this an indication it has been rebored incorrectly? I'm 100% positive I don't have a broken ring. The engineer who did the rebore took it out to 0.8 as he said the clearance given in the Haynes manual is too tight for a large single cylinder air cooled engine?
I ran the bike in for the first 100 miles at 4500rpm and have now increased it to 6000rpm when accelerating through the gears but cruise at 5500rpm. The Honda manual states 'Do not exceed 80% of the rev range for the first 1000 miles'. Having the original rebored (DSS pattern) piston sieze up after 480 miles despite carefully running the engine in I have been even more careful this time but still making sure I am not labouring the engine.
Is it normal for one of these engines to use this amount of oil and smoke like a chimney during running in, I know all the old CB250RSA Road tests advise keeping a careful eye on the oil levels but considering this engine only holds 1.5litres of oil the difference between max and min is only 400ml, that's about 100 miles at its present oil consumption! 🙄
To say I'm cheesed off with this bike at the moment is a true understatement, it's rapidly getting to the stage where it will be thrown into the back of the garage to rot😡.

Offline Trigger

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Re: CB250RS Rebuild Progress
« Reply #113 on: November 20, 2018, 09:46:17 AM »
Not really understanding the piston to bore clearance that you have had done. Are you saying that you have fitted a 0.50 mm and the engineer has bored it to 0.80mm ?

Offline hairygit

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Re: CB250RS Rebuild Progress
« Reply #114 on: November 20, 2018, 09:52:37 AM »
I hate to say this, but what Honda specify for clearance is right. It may well seem tight to anyone used to working on lesser bikes (Brit stuff, Harley's etc) but Honda alloys used in their pistons are specifically formulated for low expansion rates. The clearance it has been machined to may actually be on the service wear limit, or worse, exceed it. I'm willing to be proved wrong, but I'm sure Trig would agree with me on this. 700ml of oil is an awful lot to burn so quickly. How much pressure is in the cases because of blow by? How does the oil look/smell.

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Offline K2-K6

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Re: CB250RS Rebuild Progress
« Reply #115 on: November 20, 2018, 11:50:33 AM »
I didn't understand the tolerancing numbers either,  can you give more exact figures that it's been bored to?

It's certainly not a good feeling with something wrong again with so much invested in it.  Hopefully it can be be determined what is wrong.

I don't think it'll be a risk to run it if you keep a very close eye on the oil level,  it's unlikely to seize with oil going through at that rate, unless it runs dry.

It does look like it's going past the rings if constantly smoking,  but as hairy pointed out, what is happening with the breathers? Are they verified as clear?

If compression is ok? It would suggest the combustion is not pressuring the cases. If the oil rings are letting oil pass upwards,  then it could be tolerancing that has stopped them sealing.

Can you get a compression reading from it when warm?

Offline Laverda120

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Re: CB250RS Rebuild Progress
« Reply #116 on: November 20, 2018, 03:48:54 PM »
The clearance I'm referring to is the piston to bore clearance. I sent a copy of the Haynes Manual clearance details to the engineer. This gives a clearance between 0.01—0.04mm but, the engineer said that was incorrect and the clearance for a large Japanese air cooled engine should be 0.08mm, for British engines he bores much larger to provide the required clearance. As Hairy says, Honda clearances are tight and I remember both K2—K6, Trigger and Tomb mentioning this in an earlier post. I think I have therefore answered the original question, it's been over bored. I don't have a compression tester but I know it's pretty good, I can feel it when kick starting it (no electric foot on the RSA).
I don't want to run it anymore and risk a seizure. Luckily I have a spare barrel I bought for £10 on ebay following the original seizure and this is on standard size. I'll remove the head and barrel on the engine and check it. If it is bored oversize I'll reuse the new piston and get the barrel bored to suit the 0.5 oversize. Although the rings have only covered 200 miles I'm not sure if I should renew these or reuse with the rebored spare barrel?
Thanks for the advice so far.
Cheers
Dave

Offline Trigger

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Re: CB250RS Rebuild Progress
« Reply #117 on: November 20, 2018, 04:29:28 PM »
I would do the clearance to 0.02mm on a oem Honda piston and 0.03mm on a aftermarket one. 0.08mm clearance is brit bike clearance, shot the engineer  ;)

Offline Oddjob

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Re: CB250RS Rebuild Progress
« Reply #118 on: November 20, 2018, 04:30:14 PM »
0.08mm is just under the service limit of 0.10mm but I don't think the engineer should have ignored the specs set out by Honda, assuming of course that Haynes got those figures correct.

I'd personally have the spare barrel rebored to suit the 0.50 oversized piston and this time use the Honda specs, considering it's his fault I'd imagine the engineer will do this FOC. Still leaves you with the gasket costs etc but I'd reuse the pistons rings to save some cost.
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Offline Laverda120

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Re: CB250RS Rebuild Progress
« Reply #119 on: November 20, 2018, 05:02:17 PM »
Thanks both. I'll take the barrel off, measure it and give the engineer a call if it is overbored. I'll keep you advised.