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unit of measure for bore gauge

Practical Engine Building

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Is a 0.0005in bore gauge correct for measuring main bearing clearances? as well as the piston bore clearances?

is a 0.0001in bore gauge better?

The number usually indicates the smallest division shown on the scale. The "tenths" (0.0001" or a reading of one ten-thousands of an inch) would be a more desirable for a bore gauge. But you can still have acceptable results with the lower resolution tool -- you can often read to about twice the resolution (ie, you could tell a 0.00025 difference).

As David said, the finer measurement will give more accuracy, but it also comes down to the ability to use the tool correctly - holding correctly so hand heat isn't transferred, the force used, it being square to the bore, etc. You live in the Caribbean, and you may even see a measureable difference between early morning and late afternoon if the tool and/or block are kept in a cool area or are exposed to the heat of the day.

I expect there are guides available on YT, possibly by the tool manufacturers.

ok thanks Gord and David, that helps me a lot.

according to my factory manual my bearing clearances are correct between 0.021-0.048mm (0.0008-0.0019).

If I increase my clearances by 0.0005in, should I take the maximum factory clearance measurement 0.048mm (0.0019in) as a base and add 0.0005in? or if for example I find a measurement of 0.033mm (0.0012in) I have to increase this same measurement by 0.0005in?

if you want to increase your bearing clearances by 0.0005", then the new range is 0.0013 - 0.0024. So your measurement of 0.0012 is very close to the new minimum, which might be acceptable to you especially if it's a new bearing.

If I understand you, you currently have a clearance right in the middle of the factory recommendations - before worrying about changing that, I would want to be sure it's a good idea and is actually required.

Do you have any specific reason for increasing it?

thank you David I understand better how to proceed to increase the bearing clearances.

Gord, it will be a forged motor with a 180% increase over the original power (160hp stock => target 450hp approximately) I have chosen an increase of 0.0005in over the indications given in the course "Engine Bearing Clearances ".

What is the rule to follow to know the amount of clearance to add compared to its power objective? can your for example decide to add 0.0010in if my power target is 900hp?

what is the limit of this rule? I guess increasing the bearing clearance too much can be bad.

It's going to depend on a lot of factors, well above my pay grade, but some bearing suppliers have over and under-sized shells for the more common engines. For example, King bearings, who also have some documentation on the affect clearance, viscosity and rpm has on load and heat handling.

This may be of interest - http://www.substech.com/dokuwiki/doku.php?id=effect_of_oil_viscosity_on_hydrodynamic_friction_of_engine_bearings

thanks Gord

this is a very interesting article, I understand better now I thought there was a particular rule to follow to choose are the main bearing Clearances

in fact it is much more complex than that.

So if I understand correctly, we must take into account the configuration, the desired power level but also the recommendations of the main bearing manufacturer?

Rather than re-inventing the mousetrap, I'd see what others with a similar engine and torque/power output are using. If it isn't a totally unique engine, there should be forums specifically for it, or the vehicles it's fitted into, with the information you need.

OK thanks a lot

I start the research