340 Main Bearing Clearance

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1969GTS340

1969GTS340

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Hey FABO - I just finished checking main bearing clearences using measuring tools, not platigage. .009, .007, .008, .007, .008

This is a race application motor and I'm curious what the engine gurus on here think of these numbers?

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I'll try picking up an inside mic tomorrow. I just re-measured at a 90* my previous measurements were taken ~60* to 70*. Much better results but I'm hoping with use of an inside mic I'll get down to where I need to be. .002, .004, .004 .005, .004, .004

Thank you for the help.
 
1969GTS340 said:
I'll try picking up an inside mic tomorrow. I just re-measured at a 90* my previous measurements were taken ~60* to 70*. Much better results but I'm hoping with use of an inside mic I'll get down to where I need to be. .002, .004, .004 .005, .004, .004

Thank you for the help.
Click to expand...


You can get a dang nice bore gauge for a hundred bucks on Amazon. That’s the best way to measure what you are doing.
 
I shoot for .0025-.003"
Snap gauge and caliper are not the right tools. Bore gauge and Mic then subtract the difference.
 
Rat Bastid said:
You can get a dang nice bore gauge for a hundred bucks on Amazon. That’s the best way to measure what you are doing.
Click to expand...
Thank you.

I couldn't believe what I was reading for min there! Lol I was saying to myself.. like why not just wear a blindfold too while you're at it!

.002-.0024 is about nice.
 
You might want to consider using Plastigage for a second confirmation. Plastigage is less dependent on tools and the operator experience and will give consistent results within about half a thou if used properly. Crank in place, cut with a knife and lay a 1/2" long strip in line with the crank line on the crank bearing , light film of WD40 on the cap side bearings, torque caps, don't rotate anything, remove caps and measure in place. Should be in the range of the green, 0-3 thou.
 
At your first measurement you would have to run wheel bearing grease to get a pressure reading on you oil pressure gauge. lol I would shoot for .002 to .0025
 
Thanks again guys. I'm convinced on the need for a bore gauge now. I was hopeful that what I was using would provide as good, if not better
results than Plastigage. Regardless, I'll order a bore gauge today and may double check with plastigage as suggested as well.
 
Question on choosing a Bore Gauge - I see shaft lengths varying from 6" to 10" . What length are you guys using
that avoids colliding with adjacent bearings/caps ?
 
1969GTS340 said:
Question on choosing a Bore Gauge - I see shaft lengths varying from 6" to 10" . What length are you guys using
that avoids colliding with adjacent bearings/caps ?
Click to expand...

You can do mains with a 6 inch shaft if you go from both ends. And I think if you have the 10 inch you still need to go through the rear to get 5, and maybe number 4 main. Also, if you look you can find a bore gauge that will go under 1 inch and up to a 6 inch bore. Might as well get a gauge you can use for wrist pins and lifter bores while your at it.
 
1969GTS340 said:
Question on choosing a Bore Gauge - I see shaft lengths varying from 6" to 10" . What length are you guys using
that avoids colliding with adjacent bearings/caps ?
Click to expand...
I bought the Jegs bore gauge, 100 bucks back then. I do one cap at a time and sharpie the readings on the cap or pan rail.
I've used it to align caps as well...
Last week I just measured a Suzuki 4 cyl for a friend who is re ringing it. std bore, no 2 was 2 thou smaller bore than the rest..bore taper thrust side was .0015-.002 for all.
Oor. 0018
 
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I too measure mine one at a time. I’ll mic the journal then zero the bore gauge to the mic. Then check the corresponding main. It may not be the correct way but it’s worked for years for me. On a 340 sized main I like .0025-.0030”.

Monte Smith (RIP) always said if you build it loose you know it. If you build it tight everyone will know it.
 
1969GTS340 said:
Thanks again guys. I'm convinced on the need for a bore gauge now. I was hopeful that what I was using would provide as good, if not better
results than Plastigage. Regardless, I'll order a bore gauge today and may double check with plastigage as suggested as well.
Click to expand...
I am going to go against the grain a little and say you do not need a dial bore gauge or inside mic unless you plan to do a lot of measuring in the future.
The snap gauges and micrometer shown in the first post can get you within .0002” or better when used properly. That method was used for years. Just make sure the snap gauge is level in all directions, and the spring loaded ends are pointing up and down if the block is level upside or upright. Snug the lock when satisfied you are at the widest point vertically, then measure the snap gauge with a micrometer. Do it a few times and you will see how repeatable and accurate it is.
I am not against buying tools that work well, But no need to spend a bunch on a one time use tool if you already the snap gauges and a 2”-3” micrometer.

No matter which method you use, be sure to measure vertically. If you rotate the tool away from vertical on a new bearing, it will show wider readings, by design. The specs are for vertical clearance.
 
Bobzilla said:
I am going to go against the grain a little and say you do not need a dial bore gauge or inside mic unless you plan to do a lot of measuring in the future.
The snap gauges and micrometer shown in the first post can get you within .0002” or better when used properly. That method was used for years. Just make sure the snap gauge is level in all directions, and the spring loaded ends are pointing up and down if the block is level upside or upright. Snug the lock when satisfied you are at the widest point vertically, then measure the snap gauge with a micrometer. Do it a few times and you will see how repeatable and accurate it is.
I am not against buying tools that work well, But no need to spend a bunch on a one time use tool if you already the snap gauges and a 2”-3” micrometer.

No matter which method you use, be sure to measure vertically. If you rotate the tool away from vertical on a new bearing, it will show wider readings, by design. The specs are for vertical clearance.
Click to expand...
You can light a fire rubbing wood together as well.
Some ways are just easier than others... I'll just use a match or a lighter... or dial bore gauge.lol
 
Bobzilla said:
I am going to go against the grain a little and say you do not need a dial bore gauge or inside mic unless you plan to do a lot of measuring in the future.
The snap gauges and micrometer shown in the first post can get you within .0002” or better when used properly. That method was used for years. Just make sure the snap gauge is level in all directions, and the spring loaded ends are pointing up and down if the block is level upside or upright. Snug the lock when satisfied you are at the widest point vertically, then measure the snap gauge with a micrometer. Do it a few times and you will see how repeatable and accurate it is.
I am not against buying tools that work well, But no need to spend a bunch on a one time use tool if you already the snap gauges and a 2”-3” micrometer.

No matter which method you use, be sure to measure vertically. If you rotate the tool away from vertical on a new bearing, it will show wider readings, by design. The specs are for vertical clearance.
Click to expand...


I appreciate your feedback on this. I did meet up with an old machinist buddy of mine who. along with FABO members, schooled me on the proper method for using the snap gauges. I re-measured ( .002, .004, .004 .005, .004, .004 ) yielding better numbers but no cigar. The issue this time was partially due me to not tightening the snap gauges enough, but at risk of marking the bearings with this method, I chose not to go any tighter. With that experience, I purchased a Fowler dial bore gauge that should be delivered tomorrow. At $108 shipped, I'll have peace of mind and if it gets dusty enough from laying on the shelf after this motor is finished, I may offer it up for sale on here and recoup some of the cost.
 
If you get a variance of .006 from one attempt to the next, your technique or tools are faulty. (.007-.008, then .002-004)
A dial bore gauge is a great, useful tool, and i have one, but in your spot i would CERTAINLY try plastigauge first. Follow instructions above. Tens of thousands of engines have been put together successfully using plastiguage.
 
1969GTS340 said:
I appreciate your feedback on this. I did meet up with an old machinist buddy of mine who. along with FABO members, schooled me on the proper method for using the snap gauges. I re-measured ( .002, .004, .004 .005, .004, .004 ) yielding better numbers but no cigar. The issue this time was partially due me to not tightening the snap gauges enough, but at risk of marking the bearings with this method, I chose not to go any tighter. With that experience, I purchased a Fowler dial bore gauge that should be delivered tomorrow. At $108 shipped, I'll have peace of mind and if it gets dusty enough from laying on the shelf after this motor is finished, I may offer it up for sale on here and recoup some of the cost.
Click to expand...

The dial bore gauge will leave marks on the bearing much more noticeable than the snap gauges. Just be careful and don’t move it around more than necessary.

Setting up the dial bore gauge may feel like a puzzle the first time. If you can gently mount the frame of your micrometer in a vise with soft jaws to support it you will have an easier time setting up. If you hold the frame of the micrometer in your hand long enough during setup or journal measurement, the micrometer will expand a little and your measurements will be off a tiny bit.
 
A final update. The Fowler dial bore gauge arrived yesterday. Much easier and consistant for me to use. Glad I spent the 100 bucks. All the mains came in between .0025 and 0030. The crank is installed now and now on to file fitting the rings. I appeciate everyones suggestions and instructions.
 
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