First I want to say that I'm a complete amateur at using micrometers, etal. Today I removed the pistons from my 1926 Model T.
I checked the rod journals with a mic and found that they are all round, except #2 seems to have maybe a slight taper of less than 0.001" front to back. I checked the clearance and found that #1 is 0.003" and the others are 0.002". Each rod has plenty of shims. Each pack is more than 0.030" thick.
I then attempted to measure the cylinders and the pistons. Here I ran in some trouble with the measurements. Most likely my inexperience. I used a telescoping snap gauge to measure the cylinder and then mic'ed the gauge. I measured two diameters, 90º from each other. I think I need more practice doing this, in that I got measurements that were nearly 0.010" different each time.
Then I tried to measure the pistons. The first question is where should you make this measurement on the piston. I found the most consistent place to measure with a mic is just above the wrist pin. If I compare this piston measurement with the cylinder I get clearances around 0.010". Not that these are correct, because I'm unsure of the cylinder numbers. Just as a side check I tried to slide a 0.011" feeler (a 0.005" and a 0.006") into the cylinder with the piston and it went in, where as a 0.013" (a 0.006" and a 0.007")would not. Probably not too good of idea in that the feeler is flat and the cylinder round. I have aluminum pistons.
I'm not going to machine the engine cylinders or do anything with the pistons, but I'd like to know what the measurements are, even if it is a general idea. Any tips on where to measure and how? Thanks Mike
Mike, you measure the pistons at the skirts, which is generally around the area or below the piston pin, but perpendicular to it. Here is a pic I posted doing this on my two cylinder project.
You also measure the bore the same way---perpendicular to the rod journals. Although you can't see the points of the bore gauge very well, they run from the valve area across the bore (left to right---NOT front to back).
.010" is quite a bit of clearance, but it's not like it won't run. I actually had .010" on the above engine. It may mean the you don't get proper cylinder wall to ring sealing, and will have a chance for more wear on the walls. Its not the end of the world by any means, if you need to do it on the cheap, put a set of rings in, ball hone the bores and away you go. At least you know what you have and in the future will have to bore the cylinders at rebuild time. With that much wear, I would personally go to .030" over size at rebuild time to regain nice straight bores.
Good luck with your project and have fun!
Just to add, when I mentioned going .030" oversize on rebuild, I was assuming you had/have standard bores in the engine. But I see you wrote you have aluminum pistons, so I do not know if your bores are already over sized. Depending what you have, it may or may not be possible to bore more. The worst case scenario is you can either get another block, or sleeve yours back to original size, but Model T engines are pretty robust and will take a good amount of overbore. If you are at .030" now, you could go to .060" easily. After that it gets a little tricky and depends what shape the bores are truly in and availability of pistons and rings.
The engine ran fine prior to diss assembly. I don't think that I really have 0.010" clearance. I believe my measurement skills are not very good. I was using a telescoping gauge to measure the cylinder. This is what I'm using to measure the cylinder and piston.
That looks like a nice device you have to measure the bore, maybe something like that would be easier to use. Mike
Mike - Perhaps this "goes without saying", but just so you know, aluminum pistons are never perfectly round,....they are "cam-ground". That is because of the uneven heat expansion of aluminum pistons. And of course, ideally, aluminum pistons should be fitted to perfect cylinders (no taper and no out-of-round) with whatever clearance the piston manufacturer recommends. And I think you can go to as much as .080 oversize before sleeving would be necessary,....FWIW,.....harold
P.S. Chad - I would measure the cylinder bores at two separate levels, just barely below top ring level at TDC and again at top ring level at least half way down to BDC, and at each level, measure parallel to wrist pin and again at 90 deg. to wrist pin. That will tell you about any out-of-round as well as any taper.
Again, best to bore in order to have perfect cylinders, however, if not boring, my personal belief is that you can get away with a couple thousandths of taper, as taper will just cause a bit of ring flex, but more than about two thousandths out-of-round is risking brand new rings never seating (sealing) quite properly. Most new pistons will include limits as far as how many thou' away from perfect cyl. allowable,....again,....just my opinion,.....FWIW,.....harold
Mike - Simultaneous typing,...ha ha,......
Again, just my personal opinion, but your telescoping guages and outside micrometer will measure just as well as an inside micrometer if done carefully and with some "double-checking". In fact, as clumsy as I am, I find the inside micrometer more difficult to use than the T-guage and outside micrometer!
Guess I should also say that ideally, that very nice bore gauge that Chad shows is easiest of all to use, but not everybody has such a fine instrument as that!
Harold, you are spot on as far as measuring the bore in different spots. Obviously a picture only shows one spot, and I did forget to mention that.
The snap gauge will work, but repeatability and quickness can be difficult. I have a set of those too, but I don't often use them for big stuff like this---in fact I rarely use them for bores in general as a bore gauge is so much easier to use.
FYI, the bore gauge I have (I have two that cover from 1.5" to 6"), they are both "cheap" knockoffs from E-bay. I was skeptical of their accuracy, but using my Starret mics for comparison, they are spot on. I hate supporting the cheap tool industry, but when they can make them darn near as good for the home hobbyist, what's a guy to do but save his pennies and buy more stuff elsewhere. I think the key is knowing what to buy cheap, and sometimes just taking a chance, and don't be afraid to return it.
Seems that the real trick is to get the telescope in the middle and flat. Maybe I should take a half dozen measurements and average them out. I'll probably call a machinist friend and ask for some help doing this. Hopefully I'll learn a little on the way. In the mean time Practice, Practice Practice. Thanks Mike
You need to feel the telescoping gage. Set it and then insert in the cylinder and then rotate it. If it clears then add a thousandth or two and try again in the same spot. When reach a point where you feel a slight drag then you have your measurement.
I think it's all academic anyway as if you have aluminum pistons the cylinders have likely been rebored and are fine. Someone else will have to tell you how to measure the aluminum pistons as they are not round until the engine reaches operating temperature.
Mike you might want to look at a YouTube video on using snap gauges there are several.
Mike, is there any markings on the top of the pistons? 20 or 30 or .020 or .030 or something like that.
Chad: What is your recommendation on cylinder bore gages? Are the $60 ones adequate for model T work? Mike: I'm sorry to intrude, but I'm facing the same measuring challenge. Respectfully, jb
Snap Gauges will only give you an accurate reading by accident.
Use a post, or inside Mic.
Telescoping bore gages also do not have the accuracy of a inside Mic.
My pistons are aluminum 0.030" over size. A machinist friend of came over and helped me with the snap gauge and pretty much told me the same thing about the accuracy. He showed me how to use the gauge and I got better at it. Then he brought out a dial indicator bore gauge (pretty sure that is what he called it) and measured the bores and the used his mic on the pistons. The results were much better. I have 0.005" on #1 cylinder and 0.006" on the others. So, the pistons will get a good cleaning and once the block is ready for them I'll put them back in. Thanks, Mike