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Compression
Moderator: Micka
Compression
As previosly mentioned somewhere i have an 88 3.5 V8 that has been strocked to a 4.2 (dunno if it makes anydifference in compression) and got the following readings160, 160, 140,155, 160, 140, 160 and 160. Are the 140's anything to be worried abouts
do a wet compression test.
that means put a tiny bit of oil in each of those cylinders and do the test again.
if the compression comes good it means you need new compression rings or worse cylinder/piston work may need to be done.
if it doesnt come good then valve work needs to be done, wether that be valve straighting, valve seat cleaning/replacing/redoing, valve adjusting etcetc.
that means put a tiny bit of oil in each of those cylinders and do the test again.
if the compression comes good it means you need new compression rings or worse cylinder/piston work may need to be done.
if it doesnt come good then valve work needs to be done, wether that be valve straighting, valve seat cleaning/replacing/redoing, valve adjusting etcetc.
Yea wet tests suck big time, i've never had any luck getting accurate results and don't even bother and with those figures i'd just drop it in anyway, remember your looking for consistancy in the figures between cylinders not the actual figure itself, unless you've calibrated your gauge recently?
Wanted: Car trailer or beaver tail truck, let me know what you got
Depends on budget and your mechanical knowledge, it shows signs that the rings or valves have started to wear but the motor may still be good for another 100thou?? if you strip it you may aswell fit new rings and bearings regardless and the heads may need looking at, valve recession?? cracks, worn guides??? and is the camshaft worn, lifters ok, put a new oil pump in while your at it, timing chain too! gaskets and well you get the idea! Obviously your driving this thing, Does it smoke? Lack of power? well it is only 4.2 but you know what i mean 
does it knock, rattle?? if not just drive it 
does it knock, rattle?? if not just drive it Wanted: Car trailer or beaver tail truck, let me know what you got
yea thats true fellas, but its more to just give an idea of whats going on in that cylinder.
ive heard a 10% variable is acceptible, more and can start to cause problems down the track, but everybody has a different idea.
i totally agree with cloughy, just drive it till you really need to do a rebuild, which would cost roughly the same as doing it now would. unless something went broke of course
ive heard a 10% variable is acceptible, more and can start to cause problems down the track, but everybody has a different idea.
i totally agree with cloughy, just drive it till you really need to do a rebuild, which would cost roughly the same as doing it now would. unless something went broke of course
Both my 3.5's have 270 +280k's on them, one has had a top end rebuild (cam/lifters/t/chain) the other rattles a bit when cold otherwise both run fairly well. Unless it really lacks power and is undriveable just keep driving it fix other stuff around it (always something needing fixing on an old rangie) untill it really shits you then rebuild it.
Saddle up tonto, its the not so loanrangie! . 98 TDI DISCO lightly modded with more to come.
G`day
if it was a 3.5 i`d agree and say just drive it .
If the comp tests weren`t done with a warm engine the figs will be low .
Though i agree the number isn`t as important as the eveness .
I`d suggest you price 4.2 stroker pistons and then decide if you should do something with it now .
Ring Vitesse , Ritters or Deacons and ask them .
When strokers break they are generally not viable to fix .
Only suggestions .
Peter
if it was a 3.5 i`d agree and say just drive it .
If the comp tests weren`t done with a warm engine the figs will be low .
Though i agree the number isn`t as important as the eveness .
I`d suggest you price 4.2 stroker pistons and then decide if you should do something with it now .
Ring Vitesse , Ritters or Deacons and ask them .
When strokers break they are generally not viable to fix .
Only suggestions .
Peter
What i mean is price the pistons , they use Omega pistons which are a special and top dollar item , better than a grand for a set , i think you`ll find .
Rings a couple of years ago were hard to get but that may have changed , price them .
Ask the blokes that know them , get their opinion on putting rings and bearings in , get a price off them .
When they break they sometimes drag a liner down onto the crank and break the top off a piston which pretty much destroys it and it`s not viable ( value for dollars ) to fix .
The rings not only give it compression but also transfer heat , the worst killer of rover engines is heat or overheating more so , 3.5s can take more of it and more often than any of the other Rover V8s and although your 4.2 is a 3.5 , it has been bored .020/.030 depending on who what when why .
All i`m really saying , trying to inform you of is that the bits in it are not run of the mill , they are all dollar bits in comparision with a STD Rover V8 .
A bit of a freshen up now may pay in the long run but will cost dollars by the sound of it as cloughy says and from the figures you`ve given , it`ll need rings and if it need rings it needs bearings the valves need a tidy , may only need a lap if the cam is crook .
The lower ones i`d guess from your figs are either valves or cam or both .
Another test you could do if your able requires a compressor and a fitting for the sparkplug hole .
All you do is fill each cylinder with air and listen at the sump inlet and exhaust for air leaking .
When it`s fitted crack the air open and closed for each cylinder so`s it doesn`t rotate , open the air and listen , 120psi cut out compressor should pump up to cut out and hold it on a good cylinder .
But it may tell you no more than you already know .
Rings a couple of years ago were hard to get but that may have changed , price them .
Ask the blokes that know them , get their opinion on putting rings and bearings in , get a price off them .
When they break they sometimes drag a liner down onto the crank and break the top off a piston which pretty much destroys it and it`s not viable ( value for dollars ) to fix .
The rings not only give it compression but also transfer heat , the worst killer of rover engines is heat or overheating more so , 3.5s can take more of it and more often than any of the other Rover V8s and although your 4.2 is a 3.5 , it has been bored .020/.030 depending on who what when why .
All i`m really saying , trying to inform you of is that the bits in it are not run of the mill , they are all dollar bits in comparision with a STD Rover V8 .
A bit of a freshen up now may pay in the long run but will cost dollars by the sound of it as cloughy says and from the figures you`ve given , it`ll need rings and if it need rings it needs bearings the valves need a tidy , may only need a lap if the cam is crook .
The lower ones i`d guess from your figs are either valves or cam or both .
Another test you could do if your able requires a compressor and a fitting for the sparkplug hole .
All you do is fill each cylinder with air and listen at the sump inlet and exhaust for air leaking .
When it`s fitted crack the air open and closed for each cylinder so`s it doesn`t rotate , open the air and listen , 120psi cut out compressor should pump up to cut out and hold it on a good cylinder .
But it may tell you no more than you already know .
G`day
yep i know what you mean , trouble is if you pull them apart where do you stop .
What i`ve found is an engine that seals pretty well , generally will come up nothing or 5 to 10 psi with a wet test , you can get a 20 psi variation in the same engine if done cold or warm and still won`t be the same as what`s happening when it`s actually running so a comp test is only giving an idea .
One reason i suggested you talk to the blokes that know them is because all pistons expand when they get hot and the expansion rate of the pistons used in yours may be different and so the 160/190 in the higher ones may be ok or not .
The lower ones i`d suggest you pull one rocker cover off and check how far the rocker arms are moving the valves compaired with the ones with higher comp .
If the lobes on the cam are worn enough on the lower comp cyl that will be the cause if not it will probably be the seal of the valves .
If it`s only the valves or cam ?
If you have to pay someone to work on it it`s probably not viable but if you can do it yourself , you don`t have to use new , you can get your cam /lifters reground and possibly just use a new timing chain and the heads possibly don`t need alot doing but there would be the cost of new gaskets .
Anyhow i`m sure you`ll work it out what ever you decide .
peter .
yep i know what you mean , trouble is if you pull them apart where do you stop .
What i`ve found is an engine that seals pretty well , generally will come up nothing or 5 to 10 psi with a wet test , you can get a 20 psi variation in the same engine if done cold or warm and still won`t be the same as what`s happening when it`s actually running so a comp test is only giving an idea .
One reason i suggested you talk to the blokes that know them is because all pistons expand when they get hot and the expansion rate of the pistons used in yours may be different and so the 160/190 in the higher ones may be ok or not .
The lower ones i`d suggest you pull one rocker cover off and check how far the rocker arms are moving the valves compaired with the ones with higher comp .
If the lobes on the cam are worn enough on the lower comp cyl that will be the cause if not it will probably be the seal of the valves .
If it`s only the valves or cam ?
If you have to pay someone to work on it it`s probably not viable but if you can do it yourself , you don`t have to use new , you can get your cam /lifters reground and possibly just use a new timing chain and the heads possibly don`t need alot doing but there would be the cost of new gaskets .
Anyhow i`m sure you`ll work it out what ever you decide .
peter .
A worm camshaft won't be the cause of low compression but may explain the lack of power.
Has it ever been tuned right? and have you compared how the motor goes to a 3.5 to get an idea if you are really lacking power? the 4.2's are still pretty gutless even when well tuned and fresh! but a bit better than a 3.5.
Has it ever been tuned right? and have you compared how the motor goes to a 3.5 to get an idea if you are really lacking power? the 4.2's are still pretty gutless even when well tuned and fresh! but a bit better than a 3.5.
Wanted: Car trailer or beaver tail truck, let me know what you got
G`day cloughy , fair enough you could be right , it may not be the cam .
My reasoning is that if the inlet lobe on the lower ones are rounded and not opening enough it will lessen the pressure reading on the gauge and i may be wrong but i only ever suggest .
It`s the reason i use words like maybe , probably and possibly etc , coz i`m not there looking at so don`t know .
Peter
My reasoning is that if the inlet lobe on the lower ones are rounded and not opening enough it will lessen the pressure reading on the gauge and i may be wrong but i only ever suggest .
It`s the reason i use words like maybe , probably and possibly etc , coz i`m not there looking at so don`t know .
Peter
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