Is the Haynes Manual correct?

[Laverda Dave]

I dropped the CB250RSA barrel and head off a the engineering shop today. It looks likely a rebore will be required although he is going to try a gentle hone first as there is a lot of melted piston in the top of the bore. If this doesn't work it will be a 0.50 oversize piston (I had it bored 0.25 over previously).
The engineer thinks the Haynes Manual maybe incorrect regarding the cylinder to piston clearance. The manual states a clearance of 0.01 —0.04mm. He thinks this is very tight for a 74mm piston? He doesn't want to risk a potential seizure.
Does anyone know if the Haynes Manual measurements are correct or is there a Honda measurement or even a 'rule of thumb' for Japanese bikes (Brit bike clearances are much bigger).
Dave

[Nurse Julie]

Sorry Dave, don't know the answer to that but, can you clear up something for me please. I know its a single with twin exhausts but how many spark plugs does it have?

[AshimotoK0]

Sorry ..I can't help with the clearance ...one for Trigger/K2-K2 or Bryan I reckon. I would guess the same clearance as a CB750 big bore or a CB500T perhaps.

Here is the article I copied you for other people to see the engine construction if they wish.

https://www.dropbox.com/s/318cabahyog0q9g/CB250RS_Strip_CMM_2011.pdf?dl=0

[Trigger]

I dropped the CB250RSA barrel and head off a the engineering shop today. It looks likely a rebore will be required although he is going to try a gentle hone first as there is a lot of melted piston in the top of the bore. If this doesn't work it will be a 0.50 oversize piston (I had it bored 0.25 over previously).
The engineer thinks the Haynes Manual maybe incorrect regarding the cylinder to piston clearance. The manual states a clearance of 0.01 —0.04mm. He thinks this is very tight for a 74mm piston? He doesn't want to risk a potential seizure.
Does anyone know if the Haynes Manual measurements are correct or is there a Honda measurement or even a 'rule of thumb' for Japanese bikes (Brit bike clearances are much bigger).
Dave

There is a big difference between 0,01 and 0.04mm, is the piston you are using a OEM one or copy ?

[Gixxer-18]

This is from the FSM:

[Laverda Dave]

Hi Trigger. I haven't bought the new piston yet as waiting to hear what size I need, hopefully it will be 0.50 over. I have seen a IMD oversize piston on flea Bay. I think this is the same make as the 0.25 piston I purchased from DS. I haven't heard any feedback on this make of piston good or bad?

Julie, it has one spark plug (that was new at the rebuild, now slightly mangled due to the blow-up......).

Ash, thanks for the article reminder, I'll print this at work tomorrow when nobody is looking! You may have guessed I've been delayed sending you the parts manual, it's currently back in use! I'll send it when the re-rebuild is complete!

[Laverda Dave]

Hi Gixxer. That looks interesting. I can't quite read it, it goes a bit fuzzy when I zoom in. Does the book give a cylinder to piston clearance?
Thanks
Dave

[Trigger]

Hi Trigger. I haven't bought the new piston yet as waiting to hear what size I need, hopefully it will be 0.50 over. I have seen a IMD oversize piston on flea Bay. I think this is the same make as the 0.25 piston I purchased from DS. I haven't heard any feedback on this make of piston good or bad?

Julie, it has one spark plug (that was new at the rebuild, now slightly mangled due to the blow-up......).

Ash, thanks for the article reminder, I'll print this at work tomorrow when nobody is looking! You may have guessed I've been delayed sending you the parts manual, it's currently back in use! I'll send it when the re-rebuild is complete!

Be careful of over sizes on copy pistons. I have had a set of barrels in that were already at 0.50 and worn. Bought a set of 0.75 from DSS and they were only 0.55 to 0.61. Also had copy pistons stamped as 1.00 mm, that only measured up at 0.69.

On a OEM 74mm piston i would do the clearance at 0.8 thou and a copy at 1 thou 

[K2-K6]

Gixxer's manual is stating a service limit of 0.1mm, so bordering on knackered at that. Given that,  the Haynes manual looks ok for original Honda parts.

Difficult as you don't have a material spec to understand if it matches original piston.

The problem you've experienced doesn't look like a clearance problem,  it's related to combustion heat being too high and melting the piston crown rather than bore friction.

[Laverda Dave]

Thanks for the replies. The IMD piston is not OEM but it is Japan made so maybe the quality is there?? I'll see what size I need, buy one and measure it going on Triggers advice. I'll ask the supplier if he has a material spec although I don't think I'll get very far with that request! If the measurement is well out I'll return it and ask for another. If that fails and I have to use the only piston I can get I'll go to 0.8 clearance as its a pattern part.

[Tomb]

Looks like you are quoting the figures wrong looking at the photo, what they say is 0.1mm (0.004")

Which is a tenth of a millimetre, or four thousands of an inch.

What you're telling your engineer is not the same.

The manual states a clearance of 0.01 —0.04mm.

Can I suggest giving him the manual to look at. 4 thou is quite big clearance and is the upper wear limit, not what it would have been originally machined to.

[Laverda Dave]

Hi Tomb. Yes, I agree with what you are saying the clearance is very, very tight if I were to use the Haynes Manual figures (attached), the Haynes must be incorrect! Even the IMD piston supplier has said a clearance of 0.01mm would be virtually impossible to achieve and if it were the piston could sieze when it gets hot! I think I will go with Graham's advice of 1 thou for a pattern piston.
Dave

[Tomb]

1 thou is 0.025mm, and the manual quotes 0.01mm to 0.04mm, so 0.025 is right in the middle of that upper/lower limits, so the same thing. I bored my old cast iron barrelled Triumph with 2 thou (0.04mm ish) which is a big clearance, so 1 thou is good - and basically what the manual is saying.