Piston and cylinder clearance / measurement???

M

Michael_Velling

Member
Hello,

maybe i dont understand this correct :(

I have to buy new pistons and the machineshop here in germany have to bore the cylinders correctly, but in germany no one who knows the correct clearance or diameter. Can you help?

Standard Piston Diameter = 3,8745 Inch
Piston 0,0600 Oversize from Macs = 3,9345 Inch
Right?

Cylinder bore Standard = 3,876 Inch ???
Cylinder bore for Oversize = 3,936 Inch ???

How much must the cylinder "bigger" then the piston? I found a source that tells: 0,015 Inch ???

I think that is very tight, or not?

Thanks a lot for information
 
Michael,
I rebuild Model A and B engines. For an engine that is built for standard service, I hone the cylinders to provide a minimum of .0035" clearance. If the engine is to be used as a touring engine and will see duty under prolonged runs at higher speeds, I hone to .004" clearance. I also slightly increase the clearance on #3 and #4 cylinders as, from my experience, I think they are a bit more prone to scoring. 2 tenths of a thousandths is sufficient increase for these two rear cylinders.
Good Day!
Dave in MN

www.durableperformance.net
 
Last edited:
I think you meant .0015", which is too tight.

The piston maker will have its clearance spec.

I like to run them looser. I prefer a bit loose to a bit tight. The standard is .001" per inch of bore. In an 'A' I like at least .004" if not .005"
 
Rodrelic said:
The Model A Ford Restorers org has specifications page shows a difference of .0015 of stock bore/piston.
https://model-a-ford.org/resources/technical-reference/specifications/
Click to expand...

Rodrelic,
From the link you provide, I can't find the reference you refer to that states a clearance of.0015" between piston and bore. I speculate these numbers could be for split skirt pistons. The solid skirt pistons currently available require more clearance than .0015".

I have repaired a number of engines using solid skirt pistons that did not have sufficient clearance. Scored pistons and scratched cylinder walls are the found problem along with all the metal shards in the bearings....expensive.

One of my customers made a statement after I repaired his engine:
"If they had opened up the clearance then, (when the engine was built) I wouldn't be opening my wallet now." True.
Good Day,
Dave
 
Hello,

Thanks to all of you.

Tommorow i go to the machine shop an we will measure the cylinders. Then i order pistons and so on at snyders. After the pistons are in Germany the machine shop have to measure the diameter of the oversize pistons.

And now....

The cylinder bore must be 0,0035 to 0,0040 larger then the new pistons.

Correct?

Gesendet von meinem SM-T815 mit Tapatalk
 
The Model A engine still has the physics of material thermal expansion.
The original pistons have a "split skirt" this means that the piston has a cut that allows the skirt to compress or expand. This type of piston can survive with the 0.0015 in. clearance.

Modern pistons are made without the relief cut and require more clearance.
There are quite a few variables but, typical clearances are 0.001 to .002 per inch of bore size.

The only advice I can offer is to follow the directions for clearance that the pistons come with.

John
 
Michael_Velling said:
Hello,

And now....

The cylinder bore must be 0,0035 to 0,0040 larger then the new pistons.

Correct?

Gesendet von meinem SM-T815 mit Tapatalk
Click to expand...

You got it.

Modern engines have water jackets that are the length of the cylinder. Model A engines only have water jackets for part of the cylinder, necessitating that the pistons in a Model A engine require more clearance than a modern engine.
 
katy said:
You got it.

Modern engines have water jackets that are the length of the cylinder. Model A engines only have water jackets for part of the cylinder, necessitating that the pistons in a Model A engine require more clearance than a modern engine.
Click to expand...

How hot does it get down at the bottom of the cylinder? I have never known it to be any hotter there than what the crankcase area is. What am I missing, and what excessively heats up that area?
 
Johnneilson has explained it correctly. I bore the block to .003" oversize and hone another .0005". (.0035" total) Always have the new pistons in hand before boring the block, to be certain of the clearances. If you are going to run the engine hard, give them .004". Not necessary to allow for more clearance at the rear of the block, however it is a good idea to clean the water jackets to get rid of 90 years worth of rust. You can turn the block upside-down on an engine stand and use a piece of speedometer cable or wire rope to mechanically loosen all the crud and let gravity remove it from the block.
 
Jim Brierley said:
Johnneilson has explained it correctly. I bore the block to .003" oversize and hone another .0005". (.0035" total) Always have the new pistons in hand before boring the block, to be certain of the clearances. If you are going to run the engine hard, give them .004". Not necessary to allow for more clearance at the rear of the block, however it is a good idea to clean the water jackets to get rid of 90 years worth of rust. You can turn the block upside-down on an engine stand and use a piece of speedometer cable or wire rope to mechanically loosen all the crud and let gravity remove it from the block.
Click to expand...

This is the best advice. The pistons I got from Snyders had a smaller suggested oversize. I used their specs, and had to pull the engine and have it rebored because it was too tight, overheated, and would lock up till it cooled. It’s been several years, but I think I ended up with .0045 with sleeved back to standard cylinders.
 
The popular, foreign made pistons sold by most of the suppliers call for .003 clearance. That is too tight for these pistons, they are solid skirt and they mistakenly give the spec that is used for a split skirt piston which is a different animal. They need to be set up looser as has been suggested here.
 
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