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BT7 oil pressure relief valve

Drone Dog

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the bentley manual shows the spring free length as 2.687.

does that include the cap or is that just the spring length?

Thanks
 
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Drone Dog

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i thought the same thing.

i was reading another site today and they were talking about the spring length. Since i happened to have an extra new spring from Moss (besides the one in the car), i went out and measured it. the number of coils is the same and the width is the same as specified. but the length of the Moss spring is 2.432 inches. about .255 inches shorter than the book. i decided to pull the sring in the engine back out and it also measured the same length.

Does the shorter spring fully seat? not something you may notice when the engine is cold but it could affect a hot engine.

Now i guess the Moss spring could be made out of a thicker wire than the original. But in the posts i was reading, one person had added a spacer to their spring to compensate and one had tried stretching the spring so it stayed the proper free ength after being compressed.

i tried stretching one of my springs and i got it to about 2.71" but then as soon as i compressed it again just with my fingers. it went back to about 2.54".

wondered if anyone had noticed this on this site and if they had done anything like the above?
 
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I don't remember if I pulled them out of a Healey's engine, or an MGB, but there are factory shims that were available. Once I discovered them, I'd put one (1) in every engine I'd build thereafter__somewhere between 112-120 A, B, & C Series BMC engines. You'd get about another 10 psi of oil pressure with them.

The "new" 1500 Midget engine didn't respond quite the same way__as I watched a spin-on oil filter start BULGING as soon as I was raising the RPMs with my hand on the throttle! Unfortunately, I didn't reach the key before a seam split, but the mess was minimal and I didn't have it spraying the engine compartment.

Funny, I only built one (1) ever 100 engine; back in the late 70s, most of the ones we'd see had cracked cylinder heads, making them ripe for sbc-swaps.
 
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Drone Dog

Drone Dog

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did you put the shims in because the springs were short in the first place or to adjust for spring compression after use?

the trick i guess is getting the right thickness of shims. not too much the oil pressure shoots way up when cold but enough it will close completely.

i just looked on the Moss site and the spring shows as 2-9/16. i believe that is 1/8 short right out of the box. but i think when i got my second one, i compared it to the first and they were both the same length before i ever put the second one in. can't say that for dead sure but i think so. that means mine came about a 1/4" short.
 
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I can picture in my mind what a shim looks like, but don't remember the dimension, probably somewhere in the range of .080" & .120"; .100" would make the math easy...
 

Healey Nut

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A washer in the base of the cap works just as good
 
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Drone Dog

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just thinking how much is enough?

my thought would be that you put the shims in with the engine cold. put in as much as possible provided you do not run the cold PSI up above 60 lbs. once it starts running over 60 psi, you stop or pull a shim back out.

then check your engine psi when it gets hot. if it is the same, you do not need a shim to seal the valve. if it is higher, you were not getting the valve to seat properly with just the spring. it seems the only way to know if your valve is seating properly.

shims seem to be the way over stretching the spring. i stretched one of mine numerous times and after a few times of compressing it by hand, it went back to about 2.52" So i never got it stretched enough to have it compress back to 2.687.

questions that comes to mind:

once installed, did the factory spring compress down to the 2.432" length? should the spring be expected to come back to the original factory specified free length after use? or was that designed in?

is this just a product of newer standard springs that come close in size? and thus we should expect to adjust for them with shims?

Moss shows their spring as 2.562" on their site. i sent them a question and asked them about the size difference.
 
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Some years ago, somebody was marketing a spring cap with a bolt in the end of it allowing a range of adjustment, easy enough to replicate, but...

You'd only have to rig it up with a linear screw-actuator with a stepper motor and a PID controller to have whatever oil pressure__within practical limits__that anyone could desire.

What? I only elaborated on what you started ;)
 
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Seems like everybody--myself included--is obsessed with getting maximum oil pressure (there's a guy on the 'other' forum who brags about having, like, 50psi at idle and 80 at speed). Something else to consider is flow; really high pressure could be an indication of problems as well:

https://www.boosttown.com/engine/oil_pressure_vs_flow.php

And, has everyone checked their pressure vs. a calibrated gauge? My BJ8 used to show zero at idle on a hot day; of course it bothered me but I drove it for many tens of thousands of miles before I thought to get the gauge checked and overhauled--I then magically gained 15-20psi at idle (speed reading didn't change). After another, better overhaul the pressure readings didn't change.
 
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Drone Dog

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i think everyone would like to have good oil pressure. certainly not a tell all on engine health. but i think it was ingrained in to most of us that remember the 60's.

For me i don't have a lot of history with this car and i would just like to know everything is working properly. After reading posts about shims and the pressure relief valve, i got to wondering... how do i know mine is seating properly? then when i saw the length... i wondered even more.

Sorry not trying to make it complicated but if something as simple as a shim is needed, then that is an easy one. i had never read anything about it on this site but figured someone here had checked it out and had some background.

Now after reading Randy has done it on 120 engines and gets about 10 lbs higher reading....that's a lot. heck now i want to try it, but also don't want to create a new issue by going too far.

yes i did check my gauge... but i have checked all of them. just like to know they are working properly.
 

Healey Nut

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More pressure doesnt relate to more flow , as you increase pressure all you do is increase velocity of the oil . Volume is dictated by the pump . Personally my cars run at around 55/60psi cold and around 10psi at idle when hot thats good enough for me .
I will take high volume first before higher pressure .
100psi with a trickle of flow isnt going to keep the bearings cool 50 psi with higher flow volume will .
 
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Healey nut, i understand your thoughts. i am certainly not trying to get my car to show 80 psi. i want the high to be the 60 it currently reads at cold idle. i want the valve to release pressure as it was designed.

but if something as simple as putting a washer behind the spring could change your hot idle from 10 psi to say 20... wouldn't you do it?

i have not seen mine go below 20 even on the hottest days and at the cars hottest temps at idle. but i would still like to know if it would be 30psi with a simple shim in there... again as long as the valve still works as designed and keeps the upper end at 60psi. if i try a shim and it stays at 20psi, then fine. i will pull it back out.

but i would like to know.... maybe i am saying i got too much time on my hands.... haha.

i know it is hot and sticky here so i may as well experiment because i am not taking many rides. that is for sure. been a tough summer. this is the least i have ever had my cars out.
 
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More pressure doesnt relate to more flow , as you increase pressure all you do is increase velocity of the oil . Volume is dictated by the pump . Personally my cars run at around 55/60psi cold and around 10psi at idle when hot thats good enough for me .
I will take high volume first before higher pressure .
100psi with a trickle of flow isnt going to keep the bearings cool 50 psi with higher flow volume will .

Yep. But, since a flowmeter in the oiling system isn't practicable we're stuck with using pressure as a proxy. It surprised me that, although I'd installed a DWR 'high capacity' pump the pressure didn't change noticeably after my latest overhaul. I assumed with higher capacity I'd get more pressure, but with a good gauge it seems 15-20PSI at hot idle and 45 at speed is the limit. I do think multi-vis oils give a little less pressure when hot, despite the claims on the labels.

An interesting (to me) side story is that BMC allegedly switched from a rotor-type pump on the earlier cars to a sintered iron vane-type for BJ8s. I believe the DWR HC pump is the rotor type, but the one they recommend for over 6K RPM operation is the vane type. The earliest versions of the rotor-type supposedly put too much strain on the drive and gear, sometimes resulting in stripped gears, and the vane-type was cheaper anyway. The vane-type presumably offers less resistance to the drive train at higher speeds.
 
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Since we are not talking about doing anything that would modify the pathway of the oil or change internal resistance I believe that higher pressure would naturally result in higher flow.

Because my oil pressure was getting low (10 psi idle/45-50 at 3K, both hot) earlier this year I replaced my worn oil pump with one from DW and put in new tri-metal rod bearings and center bottom main bearing with engine in situ. I also fit and lapped in a new OPRV and spring. Oil pressure increased 10-15 psi both hot and cold across the rev range.

I am happier and so, I think, is the engine.
 

Healey Nut

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The pump dictates the flow , the pump is designed to pump a certain amount of oil . You can only push a certain amount of oil through a certain size pipe with a certain size pump .
The pressure is ditacted by several things .The main ones being
1- the condition of the pump ,gear wear etc .
2- the condition of the crank bearings etc . If the bearings are worn the clearances are larger allowing the oil to flow faster through them which lowers the oil pressure due to less restriction in the system
3 - condition of the relief valve .
The clearances in the bearings and oil piping are just restrictions to flow the smaller the restrictions the more pressure you can back up in the system .The size of the restrictions dictate the controlled bleed off of the pressure in the system .
Turn on your garden hose at home with no nozzle on the end faucet wide open , lots of flow ,now put your thumb over the end ,reduced flow increased velocity but the pressure behind your thumb goes up .
I remember an old timer engine guy and me discussing oil pressure and his remark was “ get it hot and pull the valve cover off with the engine running at a steady idle ,check to see you have a good wet rocker shaft with oil steadily reaching all points of the rocker shaft .If its all wet then your good to go .”
His reasoning being if the pump cant pump the oil to the top of the engine then either .......see 1,2,3.
 
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Perhaps more accurately: The greater the oil pressure the closer one gets to achieving maximal flow, and once there the OPRV kicks in (or out).
 

Patrick67BJ8

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Some years ago, somebody was marketing a spring cap with a bolt in the end of it allowing a range of adjustment, easy enough to replicate, but...

You'd only have to rig it up with a linear screw-actuator with a stepper motor and a PID controller to have whatever oil pressure__within practical limits__that anyone could desire.

What? I only elaborated on what you started ;)
From my memory when I owned a MGB in 1971.
i recall a book called the Complete Official MGB. It was black with white lettering. There was a piece in it about increasing the oil pressure by adjusting the Oil Pressure Relief Valve. I’m not sure, but there was possibly a modification of the valve detailed in this book.
 

Healey Nut

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Some years ago, somebody was marketing a spring cap with a bolt in the end of it allowing a range of adjustment, easy enough to replicate, but...

You'd only have to rig it up with a linear screw-actuator with a stepper motor and a PID controller to have whatever oil pressure__within practical limits__that anyone could desire.

What? I only elaborated on what you started ;)

Awesome idea Randy but lets throw in Bluetooth capability so it can all be livestreamed direct to the drivers phone with a audio feed to a Bluetooth headset and even a heads up display onto the windshield....okay maybe thats just a tad much:joyous:
 
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Drone Dog

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got a reply from Moss today on their springs:

Here is a chart showing the variations that we found;

No of Coils
Free Length
Type
Series
Model
Total
Active
Inch
MM
100-6

BN4, BN6
13
NA
2.562
65.09
3000
MK I
BN7, BT7
13
NA
2.687
68.26
3000
MK II
BN7, BT7
3000
MK III
BJ7, BJ8

The New Old Stock pieces (OE samples) were at 14.8 coils, 12.8 active coils, 2.687 inches.

It turns out it doesn’t matter, all of them are suitable for use.






Here is an excerpt from an internal document; Based on a detailed comparison of the new, unused OE springs and our springs, there is no appreciable difference between them.
We have checked the spring rate and the free height cold, and we have headed them to 200+ degrees and repeated the tests.
Our springs behave like the new, unused OE springs.
Ours are actually a little stiffer than the OE springs.
 
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