I am putting together my 383 stroker in a 1997 350 2 bolt main block. The block was not line honed.

I am using plastigage to check clearances. I know it is not precise, but I don't have the proper measuring tools.

With a set of standard bearings I was getting to my best estimate .0018" on the mains. Too tight says the machinist. The crushed plastigage was smaller than the .0015" scale, but not even at .002" so that's how I guess .0018", yes its a guess....

So I got a set of King MB557HPSTDX for +.001" more clearance and put them in and checked clearance.

Well #1-4 main bearings are still showing the same clearance! It's like I didn't even change the bearings... The #5 rear cap is now around .003" so it is good.

Whats up with the #1-4 clearances not changing even using a +.001 set of bearings? The bearings are stamped STDX so they should be correct.

When I did first mock up with a set of STD bearings to check rod interference with the block I used regular motor oil for lube and the crank turned effortlessly. The weight of the counterweights caused the crank to turn on its own. I don't think it is a main bore alignment issue.

 

https://blog.k1technologies.com/how-...ing-clearances

 

Double check to make sure there is nothing behind the bearing shell, between the shell and the block/cap. Clean and dry.

 

You won't see a difference unless you accurately mic the journals, and the clamped bearings. Your "visual eyeball" tolerance was between .0015 and .002, or 5 thousandths with the plastigage. Your new bearings are a thousandth looser than the other set, so your new "visual eyeball" tolerance will still be between .0015 and .002 until you can accurately measure. Your eyeballs and plastigage aren't accurate enough to see a thousandth of an inch, and it could be anywhere between that .0015 and 2 thou range.

 

I wish I had a micrometer and then I would be able to check 100%

 

They are clean.

 

I know it would be impossible to tell the difference between .0015 and .002 by eye. But I thought if the plastigage read .0015 with STD bearings and then I checked again with STDX .001 more bearings, it should be narrower than the .002 scale.

I want to clarify that I checked the crushed plastigage against the scale on the crank mains. I just used the shells for pictures as I had put the crank back in the box for storage....

This first image is checking the clearance with a STD bearing against the .0015 scale. It is narrower indicating greater than .0015 clearance.



This image shows checking clearance with the STD bearing against the .002 scale. It is slightly wider indicating not yet at .002 clearance.




This image shows checking clearance with the STDX bearing against the .0015 scale. As you can see it is almost identical to the STD bearing.



This image shows checking the STDX clearance against the .002 scale.

 

I also read that as .0018"

 

Its not. Invest in a micrometer. The money spent now is cheaper than a ruined motor. Plastigage is the cruel mistress of engine building. It may seem convincing, but it can lead you astray.

 

I agree. Plastigage IS NOT a substitute for proper and accurate measuring tools.
What I said was, from the picture he posted, that it reads .0018"
The fact that he got the same PG reading with STD and +.001" bearings shows that PG is not a measuring INSTRUMENT.

 

Misunderstood you. You are correct.

 

Indeed a strange occurrence to get the same measurements on the scale with STD and +.001" bearings. One would think that it should somewhat reflect properly on the scale.

 

Plastigauge is better than nothing, but if your plan is to build engines, you need to invest in the proper tools. Incorrect bearing clearances will prove to be far more costly than a good set of micrometers and snap gauges. If this engine is a one-off project, maybe you can find a source from which to rent or borrow the tools.

 

Every time I put together a engine I say it will be the last one. 6 engines later I should have bought the proper tools. If the new +.001" bearings show no difference with the plastigage I will load up the block and crank and go by the machine shop and have them measure it out.

 

Well, hindsight is always 20-20.

Buying good-quality hand or power tools is, IMO, always a good investment. Even if you only use a new tool only once or twice, you'd very likely still be able to recoup it's cost if you sold it, and considering today's market on just about everything that is manufactured, you'd probably make money.

Personally, I've never even one time regretted spending $$$ on top-quality tools, but I can remember a couple of times I regretted buying a cheap one.