Compression Test

[jack wingard]

Got my 29 Sport Coupe almost a year ago. Checked the head bolts then, all under spec some where even 35 lbs.

Wanting to do a valve clearance check. But I first did a comp test. #1 & #4 are 74-75 lbs. #2 & #3 are at 52-60 lbs. This seems it might be a head gasket problem and if so what kind of head gasket is the best to use? Thanks in advance.

[Mulletwagon]

May be normal. Bad gasket usually results in much lower compression. If it runs OK, probably good to go for now.

[alexiskai]

Might be valves with those numbers. Like Mulletwagon said, possibly not worth fixing unless it's showing up in other ways.

[30 Closed Cab PU]

Probably more info than you need, trying to be thorough.


Continue to run and re-torque cold every 50-100 miles a few times and then take compression readings again. This may be enough to cure the low readings.


Graphtite head gaskets work well, but if head is pulled again down the road are difficult to clean off the block/head.


Copper Clad gaskets work well, some prefer to spray them with copper coat.


Stay away from the modern silicone impregnated gaskets, are known to go bad. Some have good luck with them, but quite a few reports of issues.


Proper torqueing is important. Make sure you follow the recommended torque sequence. Re-torqueing after a short run hot to prevent coolant leakage into the cylinders - not uncommon for the 1st re-torque to show 35-40 lb. readings before the re-torque . Re-torque again cold, then every 50 -100 miles until torque holds. Then I prefer to re-torque in fall before storage to insure no coolant leakage (I run green antifreeze), and then again in the spring due to sitting for a period. Torqueing the 1st time is to 35, 45, then 55 lbs. in proper sequence. Threads on bolts should be clean but dry, no lube. When re-torqueing I prefer to re-torque each bolt /nut one at a time, by backing the nut off by about 1/16 - 1/8th of a turn and then re-torqueing.


For further diagnostics without removing the head you can do a leak down test. Also can check for exhaust leak into the coolant with a Napa Block Test Kit.


Also can try some Marvel Mystery Oil in the gas (4 oz. per tank) for a few tanks, may help the valve sticking/seating. Some spray a little MMO directly into the Intake manifold with the motor running. Another trick is to use a water misting bottle and spray MMO or Water into the Carb air intake with the motor running - may clean valve seating.


If replacing gasket make sure to have the head magnafluxed for cracks, and ground for flatness. Check the block for cracks, especially around the valves, and thin areas of the cylinder bores. Can check for block flatness with a straight edge for low/high spots and for being level. Use of gland rings on the exhaust manifold are recommended to help prevent teh manifold from drooping over time on cylinder 4. Many prefer the copper clad exhaust gaskets.

[30 Closed Cab PU]

One last note, if the motor was a fresh rebuild, it can take a few hundred miles for the valves to seat properly and the rings to seat properly. When my motor was rebuilt compression readings were all over the places, and had to retorque the head multiple times. Then compression readings stabilized at 72-75 lbs.

[jack wingard]

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Thank You all so much for the info. For now I'll try a Leak down test with a re-torque of the head.

[Sunnybrook Farm]

Once it starts the compression numbers should come up once all the parts get to functioning. An engine that sits for a long time will lose compression and the valves will not work right. Before I tore into it I would make sure it is torqued, put some MMO or something in the spark plug holes then just crank it over and over until it fires. If it ran before and the fuel system is working then it will eventually fire. I have run a battery down almost getting one to start but give it a rest as the starter and cables get hot while cranking.

[V4F]

a friend has the same compression difference . had head surfaced , no change , reset valves , no difference . it runs fine , so he quit trying to find the problem . strange yes , but not a killer

[katy]

Copper Clad head gaskets work well, some prefer to spray them with copper coat.

I prefer to spray them with silver paint, works well.

[jack wingard]

torqued the head and solved the problem

[canadian]

Originally Posted by jack wingard

torqued the head and solved the problem

Jack , With the Compression Numbers you have that Engine will run fine, Dont over think these little engines. Mine has about the same. The way I check my adjustments, carb, timing , points Etc...Is Hand Crank to Start it with a Pile of Onlookers...I done it several times on a Walmart Parking Lot...Confident

[chap52]

Glad you got it figured out... Chap