Compression Issue

A

armchair67

Senior Member
I tried searching a bit, but did not find quite the answer to my problem. Two nights ago, my daily driven Model A started making an odd noise, seemingly coming from the engine at somewhat low rpm under load. I have continued to drive the car back and forth to work the past couple of days, still running pretty good but just a little bit less smooth running. Tonight, I pulled the recently installed plugs and found them to be pretty sooty. I did a compression test-the front two cylinders showed 59 and 59 psi respectively. The rear two showed 22-25, which is obviously not a good thing. I have ordered up a touring engine since I plan to put a whole bunch of miles on this car but in), the meantime( the next month and a half to two months, I have a choice-pull the head and see what the story is, or try to milk this thing along and hope it stays together. I put about 1,000 miles a month on this car-any thoughts?
 
The gasket probably blew between 3 and 4. I'd replace the gasket rather than be stuck on the side of the road. Do you really drive in all that road salt?:confused:
I don't even like to drive my antique cars in the rain.
 
ya what Tom said
blown between 3&4
do not drive it you are gonna get AF in the oil and muck up your bearings

easy enough to change out the head gasket yourself.
Then choose a proper head torquing procedure from whatever source you trust the most (there are so many on here that your head will spin, and most are not worth doing on a lawn mower) You can PM me for the way we have done it for 45 yrs without a failure. I am not gonna subject myself to the hate mail I usually get for publicly stating how I think best to do it.
Use the Felpro copper 7013, never the silicone gasket, never the composite, and choose 60 ft-lbs of torque since Henry short sheeted us one stud per cylinder. Oh, and have the head milled a couple thou to be sure it is FLAT

I am sure you have noticed a huge drop in power......
Blown between 3&4 is the most common affliction there is on a Model A.....because few are torquing correctly
 
Thank you-I will pull the head tomorrow night after work and check that gasket. Will PM for the torque advise-I am a newbie to Model A's so I appreciate any and all good advice!

I do drive it in the salt, but I pull it into my shop once a week for a very good wash down. My plan is to drive it daily for one solid year, and after that, whenever the weather is good. It's been holding up well regarding the snow and subzero temps, but the guilt of putting it through the salt is a bit much. Chicago loves to salt in the winter, needed or not!
 
Thanks. I am lucky to have a well known local Model A engine builder and can bring him the head to shave or pick up a replacement depending on how backed up he is (4-6 week wait for complete engines right now)
 
armchair67 said:
Thanks. I am lucky to have a well known local Model A engine builder and can bring him the head to shave or pick up a replacement depending on how backed up he is (4-6 week wait for complete engines right now)
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Armchair, before automatically assuming that the head needs a shave, it's best to see if it's needed using a machinist straight-edge and feeler gauges to check it for flatness. While you're at it check the block gasket surface for the same thing.

If the head and block are not out of flat then surfacing is not needed and your gasket failed due to some other reason/reasons.
 
IMO checking with a feeler gauge only works if the head is sitting on a known machined true machinists table, if that is what you have in mind.
 
it is possible to have a twist in the head and not see it using a straight edge and a feeler. A very slight skim, of less than .002 will tell if the head is flat. I have seen two common reasons for the gasket to blow between 3 and 4: one is that the distributor cable nut gets loosened to pull the distributor, and re torquing it does not do the job. The other is a warp in that area, as that is the area that gets the hottest. I had a head that looked straight with an edge and feeler gauge, but a skim showed it was slightly twisted, causing a gap when torqued down.
 
pat in Santa Cruz said:
it is possible to have a twist in the head and not see it using a straight edge and a feeler. A very slight skim, of less than .002 will tell if the head is flat. I have seen two common reasons for the gasket to blow between 3 and 4: one is that the distributor cable nut gets loosened to pull the distributor, and re torquing it does not do the job. The other is a warp in that area, as that is the area that gets the hottest. I had a head that looked straight with an edge and feeler gauge, but a skim showed it was slightly twisted, causing a gap when torqued down.
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You have to know how to do it properly. I can check them with feeler gauges and a straight-edge to less than a .001 of an inch whether they are twisted or straight in any direction.
 
1. I'm with Pat in Santa Cruz all the way on this one. No straightedge for me. I have seen both methods used and the table is superior IMO

2. I have seen too many composite gaskets crumble apart with time, and cause a water leak. I think they can't stand up to the heat. Miles is changing the head gasket on a late model BMW in the shop right now and the fire rings are OK, but the composite has crumbled to nothing, which is what caused the water leak, just as we predicted
 
tbirdtbird said:
1. I'm with Pat in Santa Cruz all the way on this one. No straightedge for me. I have seen both methods used and the table is superior IMO
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A surface plate that is accurate is accurate indeed. In good shops that see a lot of work, the accuracy of a surface plate should be checked with a laser about every 6 months and correct if needed.

However, if you use a surface plate, you will only be able to check flatness on the perimeter of the head unless you have other tools and know the proper method to use them. On the other hand, with a straightedge you can check in a straight line from any two points on the plane in any direction. Plus, accuracy with a good machinist straight-edge and simple feeler gauges can be done by someone who knows how to perform a check properly to within .001 of an inch. How much more accuracy does one need on a Model A? Check with a few automotive machine shops and I doubt you'll find very many that go to the trouble to use a surface plate for checking heads.

However, if one is inclined to use a surface plate then set the head up on machinists jacks, or similar, on the plate so that the surfaced side of the head is facing up. Support the head at three points. Draw a small circle with a Sharpie or similar on the surface of the head above each support point. Use a height gauge and a dial test indicator, or a surface gauge and a dial test indicator, and raise or lower the jacks until a dial test indicator measurement in each circle is the same, indicating that the head is parallel to the surface plate at each of the three points. Now sweep the head surface with the dial test indicator and it will indicate the amount of bow or crown in the head.
 
Thanks very much guys, I thought for sure it was something much more major! Turns out there was a breach in between number 3 and number 4 cylinder in the head gasket. Replaced it and swapped the steel fan for an aluminum while it was apart, running like a top again. Was down for two days, making it for a total of 3 that I was unable to drive it since purchasing it in August (the third was when I had three foot snow drifts in my alley last month). Appreciate the advice-I am learning, coming from many years wrench on vws which by the way don't use head gaskets.
 
Good to hear it was an easy fix. Be sure to recheck the torque several times until it holds at 55lbs.

Corvair also didn't have a head gasket, but used copper sealing rings. Someone gave me a Corvair when I was in the Army because it had lost compression. I expected to find the aluminum head cut away by the hot gases, but the cast iron cylinder was burned away and the head remained in good shape.
 
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