Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: roverboy on October 29, 2024, 08:42:44 AM
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hi im new sohc and would like avice on bore to piston clearance
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Might be worth a PM to Trigger or his other half Nurse Julie as Trigger is/ was the to go to professional rebore man.
On 500s it is now Bryanj.
Trigger did my 400, Bryan my 500 both did excellent work.
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Welcome to the forum. Do you mean piston to bore wear limits or clearance for a re bore?
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Honda manual for 750 notes :- 0.01~0.04 mm as minimum clearance.
That's at the skirt and 90 degree to gudgeon pin location for piston diameter.
This is with Honda pistons, obviously, and of that material type. If you used piston with a different material spec, then their manufacturer would advise you of clearance for that material.
Car engineering shops often look at these figures for old Honda with disbelief and usually make statement of "it'll seize" at that gap :)
You'll need to insist on your spec if they are not well versed in these engine and materials along with their tight tolerance.
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hi thanks to all your replies the reason i,m asking is that i had a rebore and the piston to bore clearance is set to 0,0004 1th and i am worried it will be to tight as the book says 0,0004 to 0.00016 would like your comments please
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What book are you refering to - is it the original Honda workshop manual?
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Are you talking metric or imperial, ie inch or mm? Piston to bore clearance after rebore, hone to spec and X hatch should be 1 thou inch, which = 0.0254 mm. If you are using genuine Honda pistons, the clearance can be down to 0.8 thou inch. Any greater clearance than that, the engine will smoke badly. Not sure which book you're looking at but those figures don't sound familiar for a piston to bore clearance. .
Edit.... All the above is assuming it's not some fancy race engine with very high tech pistons and nicosil liners etc.
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It is written as stated by riverboy in original Honda workshop manual Julie, that "thou" spec.
I don't understand it either ;D and looks like some spare noughts are literally spread within that.
I was just sitting puzzling as to what that converts to, and doubting the outcome.
Isn't that saying 4 millionths in that 0.00004 inch figure ?
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Now I've got a spare nought ;D
And I said liberally, not literally (addressed to my fugg'n spellchecker) and substituted while I wasn't looking ;D
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It is written as stated by riverboy in original Honda workshop manual Julie, that "thou" spec.
I don't understand it either ;D and looks like some spare noughts are literally spread within that.
I was just sitting puzzling as to what that converts to, and doubting the outcome.
Isn't that saying 4 millionths in that 0.00004 inch figure ?
Maths isn't my strong point Nigel, especially when so many zeros are involved. But, if those figures are correct is that saying its been rebored to 4 thou clearance as if that's the case, the oil loss will be on par with the Torrey Canyon.
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I learnt engineering in metric, so have to convert into thou to understand it if necessary.
This may be right :- 1st 0 after decimal place is tenths, 2nd 0 is hundredths, 3rd 0 is thousandth and so 0.001 should be one whole thou ;D
Honda manual says minimum of 0.0004 inch ... which should be 4 tenths of 1 thou ... effectively just under a 1/2 thou, that's if I understand it correctly ;D
It's weird as it's a decimal division of finest English inches .... all seems a bit cock eyed to me.
If it's definitely bored in that spec, with correct materials, then it should be fine to run it.
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It's 4 ten thousandth's of an inch
Not helped when they changed the definition of a million. It's not generally 10 to the power of six as it was, the yanks call 10 to the power of five.
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hi roverboy here thanks for all you replies will have to study the information from you all thanks again ps were the engine when new painted or untreated as would like to keep the bike original again thanks for your help
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I'm unsure of your knowledge here roverboy, but don't want to teach anyone's granny to succeed eggs ;D
These are notably tight in that tolerance and characteristics of their design, an important part of why they sound so smooth when all to spec and assembled correctly. There's no real risk in running them at the stated tolerance, but many opinions out in the great wide web as to what should or shouldn't be done.
As comparison, and linked to your title, the rover V8 (if that's in your interest) has, for the same dimension of skirt to bore clearance listed as 0.0003~0.0017 inch specification. Honda 0.0004~0.0016 for comparison.
This engine quite similar in metallurgical design intent, cast aluminium block and piston with ferrous bore liner to arrive at similar clearance, even with larger diameter piston.
Your measurements need to be noted with all the components at same temperature .... barrels, pistons, measuring equipment etc which usually means individually measured pistons and bore matched to that, letting the barrels cool after machining to the same as pistons for comparison validity. This is sandardl inspection procedure though and will validate the existing specifications you have ready to assemble.
Engine oil condition being another significant component in how they run. Whichever oil used, condition of oil is paramount in these to avoid compromise.
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many thank for you help by the way the rover name is my other love rover 90 p4 kind regards
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A nice car and not often seen around much today.
My interest is from the new type :) P6 onwards, a very interesting marque generally.
I seem to recall from watching an engineering workshop, clearance issues with bore on 90 type rover engine, they were guided by the owner as to the tolerance they were to use. They felt it was very large though, I'll have to go and watch again.
These Honda use (Japanese engine in general, I think its fair to say) raised silicone content aluminium alloy casting to better match expansion of piston to cylinder block over extended heat range. Effectively "travelling" in expansion as a pair and able to maintain their desired clearance in practical operation. This as opposed to coping with significant piston expansion by leaving a larger space, then to arrive at correct clearance only while hot.
The Rover V8, similar but through slightly different route, with the skirt and crown designed with quite a large gap to avoid the skirt being distorted by crown expansion, has the gudgeon pin low in the skirt with fairly high crown but with the skirt accepting more side thrust from rod geometry. This needed a tight clearance on skirt to not clatter around when cold.
Different methods, but similar tolerance and outcome. Both are typically quiet either hot or cold. Most current engines use that Honda specification now though, in type that is.