Engine compression

[WhiteRed]

Hello, anyone knows what is the compression on an engine L20B of a Datsun 620 1976?

 

My engine had an engine rebuilt recently and the compression varies from 100 to 125 psi, but in the manual says that the compression must be 170 psi to be normal and 120 psi minimum. Are this numbers right? Or the engine rebuild was a fraud?

[datzenmike]

It varies with wear. 170 and up would be very good. 120 and lower would indicate that a rebuild is soon coming even if it doesn't burn oil. 

 

Compression must be contained by the valves, so if the seat/valve faces are worn/pitted this will also allow some compression to escape. 

 

Depends on the quality of the rebuild. The cheapest/easiest is a simple hone and ring job where the condition of the cylinder is deemed 'good enough' and it is honed to aid in the break in of the new rings. If the cylinder is already too worn, has a taper or is now oval in shape new rings won't give good results and a waste of time rebuilding. In this case the cylinders should be over sized and new matching over size pistons and rings installed.

 

From your description, I would say it was not rebuilt. Have a look at the gaskets, oil pan and head. The exposed edges should look new if recently replaced.  

 

 

[WhiteRed]

Thank you, I’ll have this in mind and I’m going to carefully check the engine.

[Stinky]

Did you prop your throttle blades open?  If not, your mileage may vary (you aren't gonna get a good reading).

 

What is your elevation?

 

How many miles are on it?  

 

You say that it varies between 100-125....do you mean for the different cylinder readings?  That is not good.  It should vary between the cylinders by less than 10%.

 

How does it run on the road?  Does it burn oil?

[datzenmike]

Efficiency depends on good compression. 120-130 is at the bottom end. If the compression is within 10-15% it will run fairly well without shaking. At the low end of the compression limit means the oil control rings are also not doing their best job and oil consumption goes up.

 

If you were told it was rebuilt it probably wasn't or was very poorly done as in a quick hone and ring job on a block that should have been over bored and new pistons put in. There are other reasons for low readings even if newly rebuilt. Badly worn valve seats will leak compression. A leak down test is very good for determining if rings or valves.