Compression Pressure
Forum rules
By using this site, you agree to our rules. Please see: Terms of Use
By using this site, you agree to our rules. Please see: Terms of Use
-
- Minor Friendly
- Posts: 17
- Joined: Wed Mar 30, 2022 8:27 pm
- MMOC Member: No
Compression Pressure
Hi, can anyone tell me what the compression pressures should be on 1098cc engine please?
My engine is showing 125. 137, 90, 130 psi dry and 168, 170, 130, 175 psi wet. So looks like it needs piston rings.
My mechanic mate has asked me to try and find out what the dry compression pressure is for a fully warmed up rebuilt bedded in engine.
Thanks
Jon
My engine is showing 125. 137, 90, 130 psi dry and 168, 170, 130, 175 psi wet. So looks like it needs piston rings.
My mechanic mate has asked me to try and find out what the dry compression pressure is for a fully warmed up rebuilt bedded in engine.
Thanks
Jon
Re: Compression Pressure
140+ is fine, to my knowledge. More importantly, they should be within a few psi of eachother. Your readings for cylinder 3 look particularly bad. It seems likely that there's something going on with the valves on that cylinder. Have you set the clearances?
-
- Minor Friendly
- Posts: 17
- Joined: Wed Mar 30, 2022 8:27 pm
- MMOC Member: No
Re: Compression Pressure
Hi, yes we did the valve clearances.
Re: Compression Pressure
Someone more knowledgeable than me will probably be along soon but I think a leak down test would be a good idea.
That 90 psi reading seems too low to just be ring related and it's a long way out from the others, hence why I suspect valve trouble on that cylinder.
That 90 psi reading seems too low to just be ring related and it's a long way out from the others, hence why I suspect valve trouble on that cylinder.
Last edited by James k on Tue Aug 02, 2022 10:10 pm, edited 1 time in total.
-
- Minor Friendly
- Posts: 17
- Joined: Wed Mar 30, 2022 8:27 pm
- MMOC Member: No
Re: Compression Pressure
Thank you
-
- Minor Maniac
- Posts: 10768
- Joined: Thu Jul 19, 2012 12:05 pm
- Location: Hampshire
- MMOC Member: Yes
Re: Compression Pressure
I would suggest that the problem on No:3 cylinder is not down to a valve. If it was the valve the pressure would remain relatively the same when in fact the wet reading has gone from 90 psi to 130 psi.
I would be looking to remove the head and examine the valves and valve seats - if nothing obvious is found then look at replacing the pistons rings after de-glazing the bores.
I would be looking to remove the head and examine the valves and valve seats - if nothing obvious is found then look at replacing the pistons rings after de-glazing the bores.
-
- Minor Legend
- Posts: 1655
- Joined: Thu May 12, 2016 6:33 am
- MMOC Member: No
Re: Compression Pressure
My mechanic mate has asked me to try and find out what the dry compression pressure is for a fully warmed up rebuilt bedded in engine.
Does this engine fit that description? Without further details, I might suspect valve guides were changed?
As per James, a leak down test at 150 psi should identify a leaky valve with the manifolds removed. Otherwise, the engine has not been successfully run in. But ‘rebuil’ does not mean overhauled/reconditioned.
Head needs to come off anyway, if the valve clearances have been properly adjusted.
Does this engine fit that description? Without further details, I might suspect valve guides were changed?
As per James, a leak down test at 150 psi should identify a leaky valve with the manifolds removed. Otherwise, the engine has not been successfully run in. But ‘rebuil’ does not mean overhauled/reconditioned.
Head needs to come off anyway, if the valve clearances have been properly adjusted.
-
- Minor Friendly
- Posts: 17
- Joined: Wed Mar 30, 2022 8:27 pm
- MMOC Member: No
Re: Compression Pressure
Sorry I should have better explained.
The engine is unknown. It's in a 61 minor I recently bought.
The reason for the compression test is that it is blowing oil mist out the breather pipes and oil filler cap (when removed).
My mechanic mate wants a comparison figure so that we know what the compression figures should be.
The engine is unknown. It's in a 61 minor I recently bought.
The reason for the compression test is that it is blowing oil mist out the breather pipes and oil filler cap (when removed).
My mechanic mate wants a comparison figure so that we know what the compression figures should be.
-
- Minor Maniac
- Posts: 10768
- Joined: Thu Jul 19, 2012 12:05 pm
- Location: Hampshire
- MMOC Member: Yes
Re: Compression Pressure
In addition to my comments above - I would be thinking along the lines 'how much oil if any does the engine use' before thinking too far ahead.
The 'A' Series is good a throwing oil out of any aperture when opened especially when the rocker cover oil filler cap is removed with the engine running.
Do you have the oil separator canister fitted to the front cam follower cover?
The 'A' Series is good a throwing oil out of any aperture when opened especially when the rocker cover oil filler cap is removed with the engine running.
Do you have the oil separator canister fitted to the front cam follower cover?
-
- Minor Friendly
- Posts: 17
- Joined: Wed Mar 30, 2022 8:27 pm
- MMOC Member: No
Re: Compression Pressure
Unfortunately it doesn't have an oil canister fitted and I don't know how much oil it uses as it's not on the road yet.
This video shows the problem, ignore the leaking radiator it now has a new one.
https://youtu.be/zqG9TItQCI8
This video shows the problem, ignore the leaking radiator it now has a new one.
https://youtu.be/zqG9TItQCI8
-
- Minor Friendly
- Posts: 40
- Joined: Wed Dec 01, 2021 10:19 am
- MMOC Member: No
Re: Compression Pressure
To answer your originasl question Bob, A good engine should have at least 150, dry that is.. My engine just had new rings and a valve job, and is showing 160 to 170.
Last edited by islecastle on Wed Aug 03, 2022 11:15 am, edited 1 time in total.
1973 Ford Cortina Mk3
1967 Morris Minor 1098 Convertible
1978 MGB GT
2000 Mercedese Benz SLK 230K
1967 Morris Minor 1098 Convertible
1978 MGB GT
2000 Mercedese Benz SLK 230K
-
- Minor Maniac
- Posts: 10768
- Joined: Thu Jul 19, 2012 12:05 pm
- Location: Hampshire
- MMOC Member: Yes
Re: Compression Pressure
Having watched the video - rings it is - but saying that I would not do anything until the car has been run for about 500 miles so as to get a better understanding of the car and its if any problems.
-
- Minor Legend
- Posts: 1655
- Joined: Thu May 12, 2016 6:33 am
- MMOC Member: No
Re: Compression Pressure
Now we are getting some decent information regarding the problem, I agree, it looks like the engine may be in need of an overhaul.
Checking the plugs after a good long run will provide more info on the engine condition - if pot number three is drinking oil, it should be evident, for instance.
Checking the plugs after a good long run will provide more info on the engine condition - if pot number three is drinking oil, it should be evident, for instance.
-
- Minor Friendly
- Posts: 17
- Joined: Wed Mar 30, 2022 8:27 pm
- MMOC Member: No
Re: Compression Pressure
Thank you everyone for the info and advice, it's very much appreciated. I need to do some thinking about how to proceed.
-
- Minor Legend
- Posts: 1376
- Joined: Tue Mar 26, 2013 8:20 pm
- Location: Chelmsford, essex
- MMOC Member: No
Re: Compression Pressure
If it's been standing for a long time it may be a stuck ring, soaking it in something like redex for a few days may help.
-
- Minor Friendly
- Posts: 17
- Joined: Wed Mar 30, 2022 8:27 pm
- MMOC Member: No
Re: Compression Pressure
Well this might just be the cause, amazingly it hasn't damaged the bore but I think i'll get it re-bored anyway.
-
- Minor Addict
- Posts: 759
- Joined: Sun Feb 01, 2009 6:24 am
- Location: New South Wales, Australia
- MMOC Member: Yes
Re: Compression Pressure
I tend to agree, but one of our experts will be along soon to give a firm diagnosis.Well this might just be the cause
-
- Minor Legend
- Posts: 1376
- Joined: Tue Mar 26, 2013 8:20 pm
- Location: Chelmsford, essex
- MMOC Member: No
Re: Compression Pressure
I'm amazed it had 90psi !
-
- Minor Legend
- Posts: 1655
- Joined: Thu May 12, 2016 6:33 am
- MMOC Member: No
Re: Compression Pressure
amazingly it hasn't damaged the bore but I think i'll get it re-bored anyway.
No point, at all, of reboring if the bores are perfect. However, somehow I think this one may be in need of a rebore.
Checking the wear is the first thing to do..
Your mechanic mate should at least know how to check for wear in the bore diameter.
No point, at all, of reboring if the bores are perfect. However, somehow I think this one may be in need of a rebore.
Checking the wear is the first thing to do..
Your mechanic mate should at least know how to check for wear in the bore diameter.