Smoker324

Hi all,

Search didn't really help much -- but I am assessing an 86 block for a DIY rebuild. I know I could simply bring the block to the machinist -- and I will before the actual rebuild -- but I would like to learn how to use measurement tools.

So -- I have a theoretical question related to out of round measurement of cylinder bores. Forgive me if the question is dumb -- but I can't figure it out.

When measuring the bore -- how do you actually determine the out of round spec? Is it as simple as taking a measurement in line with the crank and then 90 off of that? Then compare the 2 numbers?

Or do you take many readings and average the readings? The reason I ask is that I have taken 4 measurements at each level (top, below ridge, middle and bottom) 1 in line with crank, 1 at 45deg to crank, 1 at 90deg and and 135deg to crank. Some of the readings show readings over the .020mm limits -- others are within the .010mm to .020mm spec. The average of all the measurements are within spec on 2 cylinders and slightly over spec with the other 2. (I am not sure if the average tells you anything or not -- not a math guy)

I guess the question is this -- how out of round to too out of round? Can 1 point that is out of spec render the whole cylinder out of spec? Has anyone redone a motor with out of round slightly over spec?

Thanks for any insight!

-Dan C.
__________________
86 944 NA 2.5

Chris White

Taper is more important than out of round. Real out of round is uncommon but taper is not. Measure the bore width at the bottom of piston ring travel, middle and at the top (of ring travel – not bore). Repeat at 90 degrees.
Much eaiser with a bore gauge – you can get the measurements quickly and accurately that way.
Taper will wear out a new set of rings quickly – the rings have to expand and contract each cycle – that will wear out the rings and create sealing problems at higher revs.
If you have a ridge at the top of the cylinder then you have some degree of taper.
What are you measuring with?

jlturpin

My bore gauge has .0005", or 1/2 ten thousandths of an inch measurement scale, and .01 mm is just under .0004".

In a bore that is just under 4", you are going to pull your hair out trying to get consistent numbers with a spring loaded Telescoping gauge. I tried this method a few times, and could not get measurements from the same location to measure the same .001 twice.

Purchase a nice bore gauge, and spare yourself the headache and take out any 2nd guessing.

Smoker324

Hey Chris, JL, thanks for the replies

What is the taper spec anyway? I can see why that would make more of an impact than out of round. I can say that the measurements at the bottom of the bore (at level 4 in the figure below -- well lower than where the piston travels are all about .005-6mm over standard tol grp 1 bore all the way around with not out of round exceeding .001-2mm.) I haven't yet organized all of my other measurements -- but I plan on spending a few hours this weekend on them.

Anyhow, I took my measurements with a bore gauge with a Mahr-Federal .001mm digital dial indicator. For example lets say at level 2 of cylinder 1:

A2= .036mm over standard tol grp 1 bore;
B2= .018mm over standard tol grp 1 bore;
C2= .020mm over standard tol grp 1 bore; and,
D2= .012mm over standard tol grp 1 bore.

Does the fact that A and D exceed out of round spec mean hone/bore/sleeve? OR can I average the 4 measurements out?

I am new to metrology and I admit to getting varying measurements, and the numbers above are the mean of about 20 measurements. They are probably off due to my inexperience. I expect to get better with some practice.

Smoker324

Hey Chris,

I just noticed your avatar -- you are in Marietta -- right around the corner of my Inlaws in Liverpool. I am out there in 2 weeks -- I might take a ride and meet you if you are around.

-Dan C.

Chris White

Sure - give me a call and stop by!
As for the previous post - no, you can’t take the average since what we are really looking for is the worst case delta.
For a street car it might work out OK - but whats the fun in that?
several choices - a good shop that does alusil could hone the cylinders back true and then you could run stock pistons with new rings. It will be a little out of spec for piston wall clearance but it will work OK. You could bore and hone it to the next oversize or you could sleeve it. The best answer depends on what the final use of the engine will be.

Smoker324

Chris,

The final use for the engine will be street use in stock form. This is a project for me to create a good summer and weekend car. So no monster HP goals for this engine -- but who knows what the future holds. This car is a project for me to get away from the desk and wrench for fun.

I am interested in this hone and use new rings -- would they be over-sized rings or stock? I redid measurements last night and they were slightly closer to spec -- but still out about .003.

I will try and stop by in a few weeks. I'll PM you ahead of time.

-Dan C.

jlturpin

Lindsey sell a custom ring pack for slightly oversize. Swaintech can put a .003-.004 skirt buildup on the pistons and you can easily clean up the bores and have good clearance specs, and correct ring gaps.