18RG compression test results.

[maurice c]

HI guys,
I have searched for this one but found no definite answers. I was looking at an RG in Sdyney a couple of days back that gave me 120 125 125 120 on a compression test, I was expecting about 145 a cylinder. How tired is this motor? Its a 250 head. Thanks.

[Major Clod]

My freshly rebuilt (ie not even run in yet) 88253 head RG with standard RGU pistons (8.7:1) pulled 160psi per cylinder at operating temp the other night, if that gives you any help.

[Steve M]

9.7:1 0.5mm OS pistons with 88253 and 88270 head using 88251 cams got 180-188psi.
same pistons using 88270 head and 30/70-70/30 reground cams got 190-198psi.

[Steve M]

The limit is around 142psi anything below that needs rebuild straight away.
179 is stock for 9.7:1 pistons.

[maurice c]

thanks fellows

[The Witzl]

yep i get about 180psi for my rebuilt 9.7:1 pistons with 88253 head.

[allencr]

Wet results?

[maurice c]

thats dry only.

[tricky]

Cold? Expect results to be low for cold tests. 5psi between cylinders isn't bad.

[bmxer54]

I've just done a head gasket on my 18rg.
before I did the gasket compression read 125 90 170 170.
now after doing the gasket I'm getting 185 140 220 140 doing a wet test. would this suggest a may have a cracked head? I'm just baffled by the difference

[Javal]

I've just done a head gasket on my 18rg.
before I did the gasket compression read 125 90 170 170.
now after doing the gasket I'm getting 185 140 220 140 doing a wet test. would this suggest a may have a cracked head? I'm just baffled by the difference

Man you're really good at digging up ancient threads. No posts for over 8 years!

On topic though - You really need to do back-to-back dry and wet tests to interpret those numbers with any degree of accuracy. Also the amount of oil added needs to be as close to equal as possible and the time between adding the oil and taking the reading minimal and consistent. Any cylinder that gets more oil in it than the others will raise the compression reading on that cylinder during the test. Due to the inconsistency in your readings, i'd take the time to have another crack at it and get some fresh readings. Gotta be a bit scientific with your data collection before drawing a conclusion based upon it - same number of compression strokes, consistent RPM (fully charged battery, all plugs out), throttle open the same amount, readings taken at the same temperature etc.

If you still get some pretty crazy readings I think a leak-down test would be in order.

[73GTV]

Any way you look at it, it is rebuild time, You need to inspect that head and cylinders, a blow down test will only delay the news that you refuse to hear in the first place. (Eight years, that is cool, at least the information was still present for review, which reflects the strength of this forum.)

[Javal]

Any way you look at it, it is rebuild time, You need to inspect that head and cylinders, a blow down test will only delay the news that you refuse to hear in the first place. (Eight years, that is cool, at least the information was still present for review, which reflects the strength of this forum.)

Well, yes and no. Given how erratic the readings taken are, i'd first want to know that the readings are true. There's absolutely no harm in collecting a broader set of data, especially given how quick it is to do - you could reinforce or negate your last readings within half an hour. Even if the engine needs to be rebuilt, i'd want to know why it needs to be pulled down, and that's the beauty of the leak down test. Comp test only gives you dynamic compression, leak down tells you where your lost compression is going. For example, if back to back dry and wet comp tests were done, all cylinders came up 30psi evenly on the wet test and then a leakdown test was done with losses through valves, you've just displayed that all you need to have rectified is a valve seating issue and probably saved yourself a few dollars on a top + bottom rebuild.

Hence why I suggest doing the tests again, accurately and uniformly to have a broader dataset on which to draw a conclusion as well as doing the leak down test. Not only can you draw a more accurate conclusion, you can have some confidence in it.

[73GTV]

I know you are correct, however as a tear-down happy mechanic, I want to see what the damage is or the components has failed or are worn-out. Whether it is a cracked head below the valve seat, worn valves or excessive wear to the cylinders. I like the challenge of the parts measured to specification so my first response is to disassemble, inspect and replace components.

So let us yank that baby out, strap it to an engine stand and let the tear-down begin.

[Steve M]

I agree with Javal. Get more data. I've heard many people complain about the inconsistency of their compression tests. It seems that the tool is sometimes the problem and sometimes the technique.

Are you using a push-in compression tester or a screw-in? My screw in is developing a loose hose, which will one-day affect the reading. Push-in testers must be pushed in straight and firmly to work properly.
Are there any leaks in the tester connectors/hoses that could throw your figures out depending on how the tester is held or twisted?