Engine overhaul options/decision
 

Hello

I'm new to the barn and just started a A project.

I dismantled the for at least 25 Years sleeping engine. Head pulled.jpg Two oil rings where broken and one compression ring has gone during the removing. After cleaning (the cylinder surfaces felt smooth again) I mesured the cylinder an piston diamters. Engine bore mesurements.jpg

With my simple understanding I'm not sure which oversized pistons are in. Should be the 0.06 in, right?


What treatment would be recommended in this case? Could a new honing and new ring sets work out or is a borint to the next oversized piston diameter a must? Engine bore original and play.jpg


Btw is the piston play of 0.002 in to 0.007 in correct?


Thanks a lot for any reply

Re: Engine overhaul options/decision
 

You probably want to take it to your machine shop. They can accurately gauge the taper in the cylinder, and tell you what size pistons to get. I would not reuse the old pistons.

Re: Engine overhaul options/decision
 

If I read your measurements correctly it looks like you have from 0.017 to 0.024 inch clearance between the pistons and the cylinders. I think the clearance is supposed to be 0.002 to 0.004 inch if my memory serves me. In any case, new pistons are needed.

How is the rest of the engine? Bearings?, Piston pins?, Camshaft? etc.? I would probably take it somewhere to get it rebuilt.

Re: Engine overhaul options/decision
 

Using a SAE, and not a metric micrometer, right?

Re: Engine overhaul options/decision
 

If it were my engine.....hone it and new rings and replace the valve springs. Half the springs were compressed for 25 years and they will not recover..

Re: Engine overhaul options/decision
 

I agree,

There is about .004-.005 taper in the bores which is not bad for these old engines.

If the bearings look good, hone the cylinders and lap the valves if that is all they will need or grind the valves and seats if necessary. This engine will run just fine for many more miles. be sure to change the timing gear if it is a fiber gear and look at the cam.

If you want the engine to be absolutely like new, then re-bore the cylinders, new pistons, valves cam and bearings etc. but for how most of use our Model A's. I think it will work just fine to work with what you have.

My opinion,

Chris W.

Re: Engine overhaul options/decision
 

By the way,

Your current engine has been over-bored by 0.060 inches which is a very common size.

Chris W.

Re: Engine overhaul options/decision
 

Minimum piston-to-wall clearance should be .003" with modern piston and the way we drive. For stock engines I bore to .003" then hone another .0005". On performance engines I allow more clearance.

Re: Engine overhaul options/decision
 

If you have any machine work done I would highly recommend using a shop with experience on model A engines that is recommended by other A owners. I would also recommend requiring the shop to magna-flux and test for cracks before doing anything.
I have a nightmare story with my engine that is still unresolved over a year later.

Re: Engine overhaul options/decision
 

buy a fresh engine and install....... yes you will have to ship it.

Re: Engine overhaul options/decision
 

Thanks for all the feed back.


I'll definitely contact a machine shop. Unfortunately there are not many arround the corner with Model A engine experience.


@nkaminar, Yes the clearence of up to 0.024 inch is way too much according to the red book p 1-128. In general the bearings, piston pins and camshaft look great. The old oil was homogeneous and had absolutely no chips or cooling liquid in it. So far I found two more irregularities. The main oil line from pump shaft to chamber reservoir looks not good Main oil line.jpg. It seems as parts from the iron cast has vanished. The piston 3 babbitt bearing has lost a part of the surface Piston 3 rod bearing surface.jpg. I'm not sure how concerning the first one is? Definitely need to solve the rod bearing.


@Gene F, I used a metric micrometer an calculated the inches


@jerrytocci, good point, I'll keep that in mind



@CWPASADENA, do you think there is a chance to go on with the excessive piston cylinder clearence? If I conclude right then the compression will not build up as good and the oil consumption can increase.

Re: Engine overhaul options/decision
 

If you know no machinists with model A knowledge, find a machinist that knows his job and is careful and takes pride in his work.