Compression/Ring replacement

Alex 67 · 01-05-2002, 08:14 AM

What is the normal compression for a 67 I have three cylinders at 120 and one at 80. Is the hight # to high and is the lower # to low or far off from the others to constitute a ring job. I assume that if I do the ring job I should do all the rings. What does this usually cost. How many hrs. The low compression is coming from the number 4 cylinder. I am smelling some gas and is this due to the 4 cylinder not being able to burn the fuel. The car runs OK but runs a little ruff. Will I cause any more damage to the car if I run it. Could the ring free itself on a long drive the car had been sitting for some time b-4 I purchased. A friend said it is possible.

Ads912 · Ads912's Garage

Before doing a compression test you should adjust all of your valves. A tight valve will give you a low compression reading.
When you do the compression test, hold the throttle all the way open. Maybe you already did this.??
The limits of this engine are 170 psi to 140 psi and the cylinders should not vary by more than 25 psi.
As far as a ring job goes, yes, you would want to replace all of your rings. They come as a set, for under $100, if memory serves.
The major cost comes from the labor in removing the engine and dismantling the top-end. If you are going this far, you should have the heads checked by a machine shop.
The gas smell could be from a carb that needs adjusting or a bad spark plug or faulty ignition wire.
If a ring is stuck, it could free itself.
Good luck with these tests. Hopefully you won't have to pull the engine.

Green 912 · 01-06-2002, 05:15 AM

Before you plan to disassemble and do a ring job you should perform a leak down test. This test will ascertain where the leakage is originating. You might have a bad or stuck ring set as it is not an uncommon thing to have happen from long periods of unprepped storage. You might also just have some carbon stuck on the intake or exhaust valve.

Alex 67 · 01-06-2002, 07:38 PM

I found a 1968 chiltons repair manual for the 912 and it says that compression for the 1600 engine to be 114-128 with a minimum of 85 could this be wrong?

Alex 67 · 01-07-2002, 11:56 AM

I performed a leak down test and it shows the problem coming from the 4 cylinder. I also took a look at the valves and they are free of any deposits. I am going to readjust the carbs and check the plugs again to see if this helps. Any more suggestions?

Green 912 · 01-07-2002, 06:12 PM

Where is the problem? A compression test is not a leak down test. The leak down test should be done as it will give needed information to where the compression is being lost. Rings, head leak, intake or exhaust valves. How were you able to see the valve seating surfaces? Carbs will not affect the readings from a leak down test nor will the spark plug as the plug hole is used during the test.

Roland Kunz · 01-08-2002, 06:41 AM

Hello

Make the leak down while you turn the engine over and you can see what happens while the piston goes up to TDC.

On the long run it is better to check the piston rings for sure before your cylinderwalls are hurt or the pistionringlad will wear away.

If you are there don´t make half jobs as you pay to have them done mostly more then twice.

Grüsse

pokemon · 01-08-2002, 09:14 AM

A compression test should be done cold and hot.
Normally you should find that the compression cold is lower then with a hot engine.
Due to metal expansion, play between piston-rings-cylinder should decrease and hence the compression should go up.
Remove all plugs to give your starter an easier live. Use a wire to keep the carb butterflies open all the time. Otherwise each time you open them to test each cylinder, a squirt of gas is pumped into the cylinder. Do that four times and you break down the oil film from the cylinders!
Check the HOT readings with the ones in the manual.
I think 912s have low compression mostly due to burned valves. Maybe a valve grind job may do the job.

Patrick