MTFCA Forum

I'm currently having the '22 Coupe engine bored out to .020 over and I will be installing new aluminum pistons. What clearence should I allow for heat expansion of the pistons so it doesn't seize.

I go .005-.006. Any more might be noisy @ start up. A little more wouldnt bother me in a used bore. Rather hear a little slap when cold than seizing when too hot. The area above the top ring needs extra clearance also. I give .020-.030 per side. Thats .040-.060 smaller than the skirt diameter. I have had no noise issues. The new China pistons may not need this much clearance. They are a super quality piston. The Eggy's/Jahns expand a lot more making the extra clearance mandatory.

I bored my 15 .020 over last year. I have about 1000 miles on it and I set them up at .0045-.005” clearance . I just kind of took it easy for the first few miles and it came out great. China pistons. I certainly would not run what is printed on the box. Too tight.

greenacres36 wrote: ↑

Tue Jun 07, 2022 5:23 pm

I bored my 15 .020 over last year. I have about 1000 miles on it and I set them up at .0045-.005” clearance . I just kind of took it easy for the first few miles and it came out great. China pistons. I certainly would not run what is printed on the box. Too tight.

As I recall the box says .002. That must be for each side .004". That would be a "normal" motor clearance for this bore size. Motors manual.
Having no cylinder water jacket, more clearance is still wise.

The new pistons are listed at modern specs of 2 to 3 thou and by engine machining specs, piston clearance is not measured by running clearance so the manufactures clearance is too tight for the T's for the reasons listed on above comments. So I bore to .0035" for a starter T and .004" for a crank start T, had no problems with that. Just as important is that the wrist pins will push through with a light finger push, many do need a touch of honing to re-size.

Just a question. China pistons! Are these ceramic? Last sets I have used were made in Taiwan. They are of excellent quality and balance. Is Taiwan considered Chinese in the USA. I know China would like it to be Chinese.

Allan from down under.

Allan wrote: ↑

Tue Jun 07, 2022 10:09 pm

Just a question. China pistons! Are these ceramic? Last sets I have used were made in Taiwan. They are of excellent quality and balance. Is Taiwan considered Chinese in the USA. I know China would like it to be Chinese.

Allan from down under.

The piston box I get is marked "made in China." As much as the Chinese would like to have Taiwan completely under their thumb, I am thinking they would be 2 different sources. These Chinese pistons have a beautiful machine finish(almost jeweled), perfect balance & dimensions. Wrist pins can be a bit snug & typically need some honing , as mentioned.

Some interesting thoughts above. Perry, you may realize this and just did not state this properly, but your machinist will likely be boring each of the cylinders to 3.765"±, and it will be HONED to the finish size, -not bored. Next, as mentioned above, the piston manufacturer has taken into account the clearance they have engineered their piston for and have (under)sized the piston accordingly. I typically use the rule of thumb of 0.001" per inch of bore to have the finished size of the bore, -however, most pistons that are available to us generally have a few tenths variance between them in a set. With one basic exception, I mic all of my piston first after I bore the block, then I hone each cylinder to match the piston size plus clearance that will go into that hole. In your instance, I would hone to a target of 0.0037" over the actual size of the respective piston with the exception of #4 hole in which I would hone to 0.004± since that cylinder typically runs a tad hotter due to less water circulation. The exception I mentioned above is I also sonic check each cylinder wall, and if I have one that is a tad thinner than the others due to water/rust corrosion, then I may hone another ¼-thousandth in that hole just to be safe.

One other thing to consider is honing machines differ in quality. Mine will repeat within about 2 tenths, and I use good quality diamond stones. Using a torque plate also keeps the walls straighter during the honing process which allows tighter piston-to-wall clearances to be used without scuffing. Talk with your machinist about what he suggests. If he is unfamiliar with Model-T engine machine work, maybe it would be prudent to find an alternate machinist who has proven experience with a T engine?

Brent gave an excellent explanation above of how cylinder blocks should be bored and honed. I'll add a little bit of info to what he shared about the use of a torque plate.

Whenever you bolt a head on a block and torque the fasteners the cylinder bore changes shape depending on the block, by as much as .003 to .004 thousandths. In addition to causing piston and cylinder wall scuffing it causes blow-by because the rings can't seal to the warped area of the cylinder which is generally in the area of each head bolt.

If the cylinders are bored and honed with a plate and a gasket to simulate the the torque load it turns out to be perfectly round; and it returns to the same shape again when the head and gasket are torqued again at final assembly. You may ask why worry about a few thousandths? Well, without the blow-by and extra friction you may pick up a couple of extra horsepower and more torque which means about a 10% increase in power, which when you only have 20 hp, 2HP is a significant increase without and engine changes. It will also extend engine like.

The 2" cast iron plate and spacers not shown in the photo was machined in house. Plates also usually have tubular spacers that go under each head bolt to simulate the thickness of the actual head. It would be a good club or region project to have some made up by a plate maker so that members could rent them when getting their block bored to help with the cost and to cover the shipping.

I think I've read earlier posts that have debated this subject, but when someone says that they allow "0.004 to 0.005 piston clearance measured at the cylinder skirt, does that mean that's the final honed DIAMETER is 4 to 5 thousandth greater than the measured aluminum piston diameter?

NealW wrote: ↑

Wed Jun 08, 2022 3:23 pm

I think I've read earlier posts that have debated this subject, but when someone says that they allow "0.004 to 0.005 piston clearance measured at the cylinder skirt, does that mean that's the final honed DIAMETER is 4 to 5 thousandth greater than the measured aluminum piston diameter?

That is correct. The final honed ready to assemble clearance.