Compression Results / Head Torque Question

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Thread: Compression Results / Head Torque Question

  1. #1

    Compression Results / Head Torque Question

    Compression results...what would you do?

    7MGTE (cold...very cold engine that hasn't been started in months)

    "Wet" measurement was using appx 1oz of Royal Purple oil in cylinder.

    cyl1 - 150 dry / 185 wet

    cyl2 - 120 dry / 145 wet

    cyl3 - 35 dry / 60 wet

    cyl4 - 130 dry / 150 wet

    cyl5 - 120 dry / 150 wet

    cyl6 - 150 drt / 180 wet

    Cylinder 3 has an obvious issue. I'm new to this platform and have heard the 7mgte has under-rated torque specs for the head? So my idea is to re-toque the head bolt and measure again.

    Am I missing anything here? Should the car be warmed up before re-torquing or would a cold engine be better for this?

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  3. #2
    Unlimited Time Attack A70 Twin2Turbo's Avatar
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    You're not gonna get 120psi increase from torquing the headbolts.

    Time to tear it down and see whats going on.
    -Nick

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  4. #3
    Moderator SideWinderGX's Avatar
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    Doubtful that a retorque would help your low compression numbers, it wouldn't hurt to try. Depends on when the issue arose, milky colored oil, etc etc.

    Could be a bad headgasket, might be the rings in cylinder 3. Either way you'll need to do a teardown.

    7MGTE head bolt torque specs is something like 58 ft-lbs, aftermarket ARP studs (better option) are around 80 or 90.

    You can retorque cold, do one at a time. Crack one loose, then tighten down, go to the next one. Pattern is here: http://www.cygnusx1.net/Supra/Librar...aspx?S=EM&P=55

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  6. #4
    Quote Originally Posted by Twin2Turbo View Post
    You're not gonna get 120psi increase from torquing the headbolts.

    Time to tear it down and see whats going on.
    In theory if the head bolts are backed out considerably near cyl 3 I'd assume it would greatly affect the results?

    I do agree that 120psi is not likely come back from a re-torque. However, I have only 20 miles into this project/car and the idea of ripping an engine apart before re-torquing head bolts is that of potential time saving and I also get a secondary measurement for my compression test.

  7. #5
    YotaMD.com author
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    I'd open up the valve covers and torque the head bolts all to ~75 ft-lbs. Take very careful note of which ones rotate (if any do) and how much they rotate. If you get a few in the middle that are obviously looser, perhaps you could get lucky and end up with slightly improved compression. I think that's a long shot, but it's worth trying.

    I'd be mentally prepared for a BHG repair. If you can't/don't want to do a full rebuild, you can just aim for an OEM head gasket job with simply a tested and milled head and manually cleaned block.

  8. #6
    Quote Originally Posted by suprarx7nut View Post
    I'd open up the valve covers and torque the head bolts all to ~75 ft-lbs. Take very careful note of which ones rotate (if any do) and how much they rotate. If you get a few in the middle that are obviously looser, perhaps you could get lucky and end up with slightly improved compression. I think that's a long shot, but it's worth trying.

    I'd be mentally prepared for a BHG repair. If you can't/don't want to do a full rebuild, you can just aim for an OEM head gasket job with simply a tested and milled head and manually cleaned block.
    Thanks for the input! Checked out your BHG link in the signature, perfect write up if I end up needing this route (thumbs up)

  9. #7
    7M Fanatic sixpack's Avatar
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    The Toyota head torque specifications were 58 ft/lbs, but were incorrect from day one, and why that engine has such a bad rep.
    Too bad you didn't come here when you first bought the car, and read up on it.
    That was the time for a re-torque, to the correct level, like 75 ft/lbs as suprax7nut said.
    Now you have to pull the head.
    The only other thing it may be is a valve that is burnt or too tight, but I have never seen a 7M valve tighten up to the point of it killing compression.
    If burnt, you still have to pull the head......

  10. #8

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    If you're very lucky it could be sticky valve(s) on that cylinder. Check the valve lash on that cylinder but don't assume they need adjusting if there's way too much feeler gauge clearance. Using too thick shim could also cause the same problem not letting the valve(s) close completely. You could pressurize that cylinder (at top dead center) with compressed air using your compression tester hose and listen for escaping air from the intake side or the exhaust side but you would probably want to take off the manifolds for the best diagnosis. Worst scenario would be rings, head gasket, burnt valve(s) not sealing. The dry spread is 30 psi on the other cylinders which is pushing it anyway. I think the Toyota manual says 10 psi? I'm using ARP studs with nuts and the ARP grease torqued down to 90 foot-pounds. Sometimes when you re-torque an old oil and coolant soaked composite gasket, it squishes messing up the metal seals. Especially if there is warpage in the head.


    Note: I edited this post a few hours after the posting to make a correction.
    Last edited by Bruder; 01-17-2019 at 12:14 PM.

  11. #9
    7M Fanatic sixpack's Avatar
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    This isn't a chevy.
    Haven't ever seen, or heard about a sticking valve in a 7M, and I have been building them since '93.
    And never, ever reuse a composite head gasket on a 7M....
    Are you by chance a first time 7M owner, but an ASE certified GM mechanic ?

  12. #10

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    You could do a leak down test to see if it is valves. You could also run it and see if you get combustion gasses in the coolant with a kit from Napa or Autozone but it sounds like a tear down .

  13. #11
    YotaMD.com author
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    Quote Originally Posted by sixpack View Post
    This isn't a chevy.
    Haven't ever seen, or heard about a sticking valve in a 7M, and I have been building them since '93.
    And never, ever reuse a composite head gasket on a 7M....
    Are you by chance a first time 7M owner, but an ASE certified GM mechanic ?
    FWIW, I had a stuck valve on my 7M about 8 years ago. Bought the motor used from a forum member. I bought it, gave it a once over, had the head machined flat and pressure tested. As I went to assemble, the cam cap threads were pulling out. I thought it was weird, but I had helicoils handy and figured "I know how to fix this, no worries." Exhaust studs also pulled out. I soldiered on and got it all together. A few months down the road I lost all compression on one cylinder (#4 I think). I pulled it apart and the valve was still sitting there stuck open a few mm. That head remains in my garage now as a decorative piece, cut in half.

  14. #12
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    Quote Originally Posted by f00g00 View Post
    You could do a leak down test to see if it is valves. You could also run it and see if you get combustion gasses in the coolant with a kit from Napa or Autozone but it sounds like a tear down .
    This is my advice as well. A compression test will help determine there is a problem, but a leak down test will help you identify the root cause. When performing the test, listen for air escaping the exhaust manifold or the intake manifold; that will tell you it's valves. If you hear air coming out of the oil fill inlet or dipstick, that would be rings. And visible bubbles from the radiator inlet says there is a blown head gasket. There are plenty of write ups for how to do this and what to look for on the interwebs. Good luck!
    Last edited by jonnymkiii; 01-23-2019 at 07:57 AM.

  15. #13
    7M Fanatic sixpack's Avatar
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    Quote Originally Posted by suprarx7nut View Post
    FWIW, I had a stuck valve on my 7M about 8 years ago.
    Weird !
    Did you ever determine why it was stuck ?
    Happening in just a few months of a rebuild kind of points at something not being put together correctly.

    Anyway, all of the above scenarios for leak down testing all end up with the same outcome, the head has to come off at minimum.
    And if it has over 100k on the engine, you should pop the pan off, freshen the bearings & rings at the same time, not to mention seals.

    To do otherwise is what they call 'building on soft & shifting sand', the outcome will be the same, in a few years you will be doing it all again, and spending more money than just doing it now.

  16. #14
    YotaMD.com author
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    Quote Originally Posted by sixpack View Post
    Weird !
    Did you ever determine why it was stuck ?
    Happening in just a few months of a rebuild kind of points at something not being put together correctly.

    Anyway, all of the above scenarios for leak down testing all end up with the same outcome, the head has to come off at minimum.
    And if it has over 100k on the engine, you should pop the pan off, freshen the bearings & rings at the same time, not to mention seals.

    To do otherwise is what they call 'building on soft & shifting sand', the outcome will be the same, in a few years you will be doing it all again, and spending more money than just doing it now.
    I had to go back and read my build thread on that car to see what happened (love the forums for exactly this reason). I stated in my thread on SM I had to helicoil 12 threaded holes in the head. O_O Eek. Broke, college me just wanted that thing to run so bad, I guess. Shortly after that incident I got my first "real" job out of school and finally had some play money to do things right. #3 intake valve was the culprit. I could move it freely by hand, but then it stuck open again during a compression test that same day. Apparently I had a spare head at the time and I just gave up on the soft one with the stuck valve. That must actually be the other spare head I still have in my garage. The cross sectioned one is a different 7M head that had a crack on a valve seat area.

    So I still have the stuck valve head on hand and could investigate it sometime down the road, but I don't have any answers now on the cause.

  17. #15
    YotaMD.com author
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    Sixpack, check out the pics here. I got curious and grabbed the head out from under my bench. Sure enough, #3 valve on intake. Looks like the valve was sloshing around on the seat. I'm thinking worn guide, retainer fell off or wasn't installed properly. Ever seen that before?
    Attached Images

  18. #16
    7M Fanatic sixpack's Avatar
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    Almost looks like the seat came out of the head.
    Seen that on other engines, and have seen guides come apart, letting the valve wobble around more than normal (early Nissan heads).
    It would be interesting to see what it looks like once the keepers are taken off, and the valve pulled out.

  19. #17
    YotaMD.com author
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    Quote Originally Posted by sixpack View Post
    Almost looks like the seat came out of the head.
    Seen that on other engines, and have seen guides come apart, letting the valve wobble around more than normal (early Nissan heads).
    It would be interesting to see what it looks like once the keepers are taken off, and the valve pulled out.
    It does look like the seat is just... gone. Maybe it fell out during machining and they didn't notice when they put the valves back in? Very bizarre.

  20. #18
    7M Fanatic sixpack's Avatar
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    It may have been damaged before they assembled the head, and just didn't notice a crack.
    Thank god I have all the stuff to do my own heads !

    I recommend to everyone to at least buy a valve spring compressor to disassemble their own stuff, to clean & inspect.
    Then inspect the valves & seats after you have them machined/ground before installing the stuff yourself.

    Cutting seats, and grinding valves hardly cost anything, it's the labor to take things apart, and re-assemble that costs.
    And it is usually done by wana-be peons, or people who despise imports, so just how good of a job do you think you are going to get ?

    I don't expect everyone to have the money to buy their own head equipment, but if you plan on staying with the engine (even a JZ) then it's really a small investment to be able to do the work yourself.

    It costs about $300 for a good set of Neway valve seat cutters to do a minimum of a 3 angle job, or less if you find used pieces on eBay.
    I have nearly a thousand in my set, but I have everything to do 5-angle on both 4-valve heads, and 2-valve Datsun heads, my chosen play things.
    And I picked up a good Sioux valve grinder off eBay for $300, but had to buy a modern spindle to use collets for our little 6mm valve stems, and it was about $600 or more.
    Last edited by sixpack; 01-23-2019 at 01:45 PM.

  21. #19
    SupraForums Member seoul4korea's Avatar
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    Leak down test..... Then get ready to fire the parts cannon and start a build / rebuild thread.

  22. #20
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    Quote Originally Posted by sixpack View Post
    Almost looks like the seat came out of the head.
    Seen that on other engines, and have seen guides come apart, letting the valve wobble around more than normal (early Nissan heads).
    It would be interesting to see what it looks like once the keepers are taken off, and the valve pulled out.
    I had a seat come loose with the car running, which the piston then compressed into the head. Luckily the cylinder walls were not damaged, but I did have to rebuild the motor.

  23. #21
    Car is out of storage now and ready for more work:

    Here's what I found today - 11 of the 14 head bolts were torqued below 60 ft/lbs. Five of those were below 50 ft/lbs.

    So I'm going to torque them all down to spec in correct order and thinking of running another leak down test again.

  24. #22
    SupraForums Member seoul4korea's Avatar
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    Run the test again mate.

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