Measuring piston clearance after rebore

[BobUkPipedream]

Hi All,

Well I am slowly progressing, crank is now good as are the con rods all at 1.5 thou clearance with no binding. They were a pig to balance, I ended up doing twenty measurements and averaging…

So I thought I would check my pistons. These are new cam ground 0.20 oversize. I told the machine shop that bored the block to go with 0.0035 clearance minimum. They protested as the box said 0.002-0.003, I quoted this forum and that really I should run 0.004. They said they would do 0.0035 minimum.

I am not sure they did, but I may be measuring wrong. So I am measuring at 90 degrees to pin as below. Feeler gauges are metric but correspond to very very tight 0.002 at base of piston in first photo and about 0.0035 at mid point in second photo just under wrist pin (90 degrees to wrist pin but at just below height of wrist pin). I am assuming it is too tight, if so I am buying hone and fixing it myself. However what do you all think?

[speedytinc]

.002 is too tight. I like .0045-.005" Even .006", I would rather hear a little piston slap until the motor warms up than risk seizing.
Modern honing machines can be set for an exact amount of removal & stay perfectly parallel.
Doing the job by hand isnt so easy in both regards. An accurate cylinder bore gauge would be handy to check your work as you go.
If possible, take the block back & have em do it right.

Also check wrist pin fit. They are typically too tight. The pin should just fall out on its own weight when tilted on its side.

[Kerry]

Not the bore, you have a bit of variance on the piston skirt, a bit of sand paper will fix that.

[speedytinc]

Not the bore, you have a bit of variance on the piston skirt, a bit of sand paper will fix that.

Thats a possibility. Certainly worth checking with a calibrated micrometer.

In my experience, these Chinese pistons are perfect.(at least the ones I have installed)
I have measured them in all ways & weights.(trust but verify) They are real jewels. Of the rare few repop parts available.
The hobby is fortunate to have this fine product. Sad they cant be American made.
A far cry from the old Egge/Jahns pistons.

I realize I claimed "perfect". Well with the exception of the recommended bore clearance & the wrist pin fit.

[Kerry]

Agreed, todays pistons are good, but measuring the clearance at the same spot of the bore as the photo's show, and the only thing you have moved is the piston, then it can only be a variance in the piston wall.

[BobUkPipedream]

Thanks all for replies.

I have just checked all 4 pistons in all 4 holes and they all measure the same. So in-line with the pin about 11-12 thou at base. 90 degrees to pin at base, less than 2 thou. At the level of pin, but 90 degrees to it is 3.5 thou or so.

I had assumed that the taper of the piston continues to the base? So the advice on 4 or 5 thou clearance, is that at the pin level and ignore the very tight base of skirt? Or should it be minimum of 4 thou at base of skirt?

I have been through the pins polishing them and they all now slide out effortlessly, which is really annoying when you take the piston out the box and forget to hold the pin…

If I do hone, I suppose I am starting from a good straight bore and would only be taking a thou or so.

[speedytinc]

The clearance is @ the maximum diameter of the piston skirt. Too tight.

Dont assume the bores are straight. VERIFY, VERIFY, VERIFY. Your shop already didnt follow your direction.
They should have known that .002 is to tight. Modern piston clearances in this bore size would be .001" per inch. .00375 here. At least they were 40 years ago.
I would want a bore gauge to confirm accuracy.

[Jerry VanOoteghem]

Rather than use a feeler gauge, I would want to measure the bores with a bore gauge, and the pistons with a micrometer.

[Kerry]

Still sounds like a piston problem, no way they are to taper from the oil ring base of the thrust sides of the pistons to the bottom by .0015" I double checked with new pistons I have in stock to be sure, spot on the same top to bottom.

[Dan Hatch]

Check your wrist pins too. Most need a little honing cause they are too tight and will lock up.

[BRENT in 10-uh-C]

As an engine rebuilder myself, I guess I understand both sides of this ....somewhat. My initial thought (-from my experiences) is when a customer starts instructing their machinist to do something that counters what the manufacturer has suggested, the potential for liability always looms. And when the results of something are not as successful as the customer had hoped for, the first to be blamed is the machinist. So, when someone tells me "I read on the internet ...!", I generally start seeing little red flags waving and my stomach starts feeling wheezy. Surely we would all agree that some of the worst advice ever given on a subject is often originated on internet Forums.

I guess my advice differs from others here but the first thing I would do Bob, would be to load your block & pistons and go revisit your machinist to have him measure each piston and cylinder bore with the proper metrology for you. If you were in my shop, I would use the same micrometer (-that I am going to measure the pistons with) to set my bore gauge. That way I can show you accurate bore measurements of each area at 90° intervals and compare clearances to each piston. At that point, you have accurate numbers to assist you with knowing whether the machine work is correct, and which pistons are best suited for which hole. Based on your pictures above, likely your numbers are inaccurate.

[BobUkPipedream]

Hi all,

Well I called another machine shop that had straightened the con rods for me and took the block and pistons to them. They do lots of vintage stuff including stuff for vintage racing and such. Anyway I said I thought I only had two thou based on my crude measurement with feeler gauges. Well they measured everything right there and then with a very fancy new bore gauge and mic and after several minutes said that I have… 2 thou clearance…

So I left the block with them and they will hone it out to a max of 4.5. I settled on that figure based on the fact that they have new accurate gear, understand the need for more clearance and I will have a newly cored rad and I took the core plugs out and pressure washed the water ways in the head. I have also made sure my pins are good, I polished the holes using autosol and the pins slide out under their own weight. Finally my car is a speedster so will not be lugging around loads of weight under load.

I am also getting them to skim the deck as it did have a bit of unevenness and I noticed the threads are pulling a bit on the head. The head bolts look a bit stretched, so when I get it back, I am going to chase threads, soak the holes in brake cleaner, blast them out with compressed air and then use new bolts.

This car is driving me crazy though, everything seems to just turn into a mini nightmare and every time I buy something or get machining done, it goes wrong in some way; the new bent Scat crank was just one small episode in a list of silly problems. I must have really done something bad in a previous life… However I am winning slowly one battle at a time and hopefully by June it will be running.

[Scott_Conger]

If holes for headbolts look "iffy" now, then now is the time to remedy them properly. Since things are not already clean, it must be assumed that the block has not been cleaned because you're saving the old babbit? With a new SCAT crank? How are lifter and valve clearances? How are valve seats? Again, now is the time to remedy these things...

[BobUkPipedream]

The lifters were okay in hole size, but I am putting in new std lifters to get round any dishing. The valves stem holes were a bit worn so oversize valves and recut seats.

The babbit was good overall, and even after bedding to 1.5 thou, I still have quite a few shims. I did fit a new centre cap just because I could and a main cap to get runout back to new. Generally I don’t think the engine did many miles as triple gears were good. I did put new triple gear pins in, but they were only a bit worn. The original bores were only a bit worn too, it felt a bit like sacrilege to change to Alu pistons, but at the time, I felt it would be best for the crank.

I wished I had a proper machine shop so that I could do it all myself, so much easier and satisfying. The most stressful thing for me is relying on other people for those machining jobs.

[Scott_Conger]

You made me smile...the most stressful thing for ME is relying on ANYONE for ANYTHING, so I feel your angst.

[speedytinc]

"everything seems to just turn into a mini nightmare and every time I buy something or get machining done, it goes wrong in some way; the new bent Scat crank was just one small episode in a list of silly problems."

Welcome to the reality of the hobby.
Model T mechanical's are a can of worms.
Isnt that to be expected on a 100 + year old thing that has gone thru dozens of "mechanics/wrench slingers" hands?
I, personally love the challenge with everyone I get into.

[RGould1910]

Amen to avoid having others do work on your car. I'm convinced that shabby workmanship is the reason we do as much as we can ourselves. When I started in this hobby, the only tools I owned were carried in a tool pouch. Gradually I learned how to do most things needed to restore a car. I still farm out some things but am darn careful who I choose to do the work.i could tell you some real horror stories!

The other thing. There is nothing wrong or imprecise using long feeler gauges as you did. I've know respected auto machinists who do the same. The proof is in the results you found.

[michaelb2296]

In order for others to be able to go to a modern machine shop and ask for their engine to be bored out to proper specs, I have made a table that I HOPE brings together everybody's thoughts on what the proper CLEARANCE should be for a model T engine.

Can someone please look at the table and comment on whether or not it is correct? If it is NOT correct, I will update the table. If it is correct, then GREAT. Hopefully it will keep people from having a BIG HEADACHE when they talk to their machinist about their engine needing bored out to the correct size.

I get confused when people start using feeler gauges vs. a machinist talking to me about clearance. Is he talking about outside circle vs. inside circle for clearance? Is he talking about holding one circle to the edge of the outside circle and measuring the distance for clearance? I'm not trying to upset anybody I just want things to be concise for everybody especially the machinist.

Thanks,
Michael

p.s. be helpful! Thanks!

[Jerry VanOoteghem]

In order for others to be able to go to a modern machine shop and ask for their engine to be bored out to proper specs, I have made a table that I HOPE brings together everybody's thoughts on what the proper CLEARANCE should be for a model T engine.

Can someone please look at the table and comment on whether or not it is correct? If it is NOT correct, I will update the table. If it is correct, then GREAT. Hopefully it will keep people from having a BIG HEADACHE when they talk to their machinist about their engine needing bored out to the correct size.

I get confused when people start using feeler gauges vs. a machinist talking to me about clearance. Is he talking about outside circle vs. inside circle for clearance? Is he talking about holding one circle to the edge of the outside circle and measuring the distance for clearance? I'm not trying to upset anybody I just want things to be concise for everybody especially the machinist.

Thanks,
Michael

p.s. be helpful! Thanks!
Model_T_BoreSize.jpg

Not sure what "hand crank" and "engine with starter" has to do with anything. Also, while not an engine rebuilder myself, my understanding is that each piston is measured and bores are sized to fit as needed. Generally stating that .010" oversize pistons warrants a 3.765 bore is kind of "theoretical". Rather than referring to any chart, actual measurements should be taken and bores sized as appropriate.

[michaelb2296]

Its just how people see things Jerry… it doesnt. It’s their opinion or statement in this post or another.

So your recommendation is “happy medium.” I assume most will be in that category. Whats really important here is that people understand “clearance” which i believe is bore size minus max piston skirt size and how to tell that to the machinist who likes tighter tolerances.

Folks, Please dont change your posts…. There are so many people who follow different rules of thumb etc….
.001 per in. Of piston or .00375
.0045-.0065
.004 for this .005 for that

[Kerry]

Michael, your chart dose not really work that way, the pistons are set with the clearance, example, if the block is bored .020" over factory STD size, then that is what it's bored, the pistons come up pre machined with the .002-3 clearance, in the case of wanting more clearance then the extra is added to the boring.

[michaelb2296]

So you’re saying the piston can come from the manufacturer STD as 3.748 (-.002 ) and that the bore should be 3.752 (.004) or 3.753 (.005) or 3.754 (.006) …. Right?

Table above has been updated!!!!! Again, I think it's preference for .004 vs. .005 vs. .006 so.....

Maybe a better question is, "Has anybody had galling at 0.004'' clearance between the piston and the bore?"

[J and M Machine]

So you’re saying the piston can come from the manufacturer STD as 3.748 (-.002 ) and that the bore should be 3.752 (.004) or 3.753 (.005) or 3.754 (.006) …. Right?

Table above has been updated!!!!! Again, I think it's preference for .004 vs. .005 vs. .006 so.....

Maybe a better question is, "Has anybody had galling at 0.004'' clearance between the piston and the bore?"

When we bore and hone cylinders to their respective size using the Yong Chang pistons, readily available from the suppliers.
We set the clearance at .003" and to date we haven't had any issues .

The difference is we thermal clean the blocks and heat transfer is as original engine was designed.
We don't have the issue of scuffed or seized engine due to tight clearances.

We also tell the customers to clean the radiator and people that listen, the cars don't overheat.

For an engine that hasn't been properly cleaned .004' wouldn't be unheard of due to the lack of heat transfer.

At .005 and .006' the pistons will knock and expect reduced engine life as blowby will be more prevalent.

[michaelb2296]

Ok…. Now I’M frustrated! So which way do i go? Is there more to this story? Am i going with too much clearance at .004? I feel a revision of my table coming on….. lol. No offense to anyone else, but I'm going with what the machine shop said. It's a no brainer.... get your engine cleaned, radiator cleaned, and set the tolerance at .003" If you want more slop, go for it.... your choice.

[J and M Machine]

Ok…. Now I’M frustrated! So which way do i go? Is there more to this story? Am i going with too much clearance at .004? I feel a revision of my table coming on….. lol. No offense to anyone else, but I'm going with what the machine shop said. It's a no brainer.... get your engine cleaned, radiator cleaned, and set the tolerance at .003" If you want more slop, go for it.... your choice.

If you don't have the block thermal cleaned as I have shown than .004" is ok.
It's when you say .005-.006" that's pushing it .
also it depends on piston size too as always check each piston and then size each cylinder accordingly.
We have seen slightly different sizes so always make sure they are all same size rather than believing what the box says.
The cylinder concentricity also is important when it is final honed that it be straight along with the "cross hatch"
https://www.jandm-machine.com/cylinder_honing.html

I have included the link to just add more information .

[Jerry VanOoteghem]

So your recommendation is “happy medium.” I assume most will be in that category. Whats really important here is that people understand “clearance” which i believe is bore size minus max piston skirt size and how to tell that to the machinist who likes tighter tolerances.

I'm not recommending anything, except that the bore size be governed solely by the measured, maximum piston diameter and the desired clearance. The desired clearance being at the discretion of the experienced antique engine rebuilder. As for the interpretation of "clearance", it's been debated here in the past. In the end, any machinist or engine rebuilder worth their salt knows what clearance means. If you want to be more specific, you might state "overall clearance". I've worked with precision machinists for nearly 40 years. I can tell you that educating them in something they already know will not go well.

[Norman Kling]

Follow the instructions in the manuals supplied by the club. One of the big problems is that the modern engines have a cooling system with water pumps and thermostats which control the engine temperature. The thermosyphon system in the Model T , especially if all the rust and hard water deposits are not cleaned out is that there will be different temperatures in some areas of the block than in others. And the temperature will change according to outside temperature and how hard the engine is pulling hills etc. Aluminum expands at a greater rate than iron, so when the engine gets hot it gets tighter. If you follow the instructions for a Model T, less problems.

[Kerry]

There was a time and I'm talking in my life time when I first started in learning the trade, that setting clearance was done in reverse, one would bore and hone the block in even increment's, 10-20-30 etc to what was required to clean them up and pistons were supplied semi finished and then made to fit the bore.

[Art Wilson]

Another area that needs to be looked at when assembling an engine is the amount of connecting rod clamp force on the wrist pin. If it is excessive it can cause the wrist pin to warp and bind the pin to the piston. I have seen that happen with large engines with 1inch wrist pins and with Model T wrist pins that have turned blue.

[Kerry]

Art, I've been re-building T engines for ever, the only time I've seen pins turn blue is when a piston has no oil hole for them.

[speedytinc]

Another area that needs to be looked at when assembling an engine is the amount of connecting rod clamp force on the wrist pin. If it is excessive it can cause the wrist pin to warp and bind the pin to the piston. I have seen that happen with large engines with 1inch wrist pins and with Model T wrist pins that have turned blue.

I have seen a wrist pin that seized because of the hole in the rod being nasty rough. (oil holes present in the wrist pin bosses) Tightening the clamp bolt distorted the pin enough to egg & tighten a proper fitting pin into a now tight fit in the piston. Honing the rod hole is sometimes necessary, but a good practice always. After tightening the clamp, check that the pin still moves as freely as before clamping. Lots of mine fields & tricks to know about.

[Kerry]

I know it is a bit of a drift from the original topic but while on the pins the biggest problem is those who fit the rods the wrong way around, now that's a time bomb waiting to happen.