Datsun Roadster Forum-311s.org

FYI you put the studs in after you set the head on the block, then hand thread the studs install the nuts and torque the head. Also use great care taking the carbs apart as it is easy to damage the body and throttle shaft.

Dave

UPDATE: It's been a while but I managed to squeak out some time aside from the wife and two kids to get things back together. I picked up the head from Rebello with new valves/seats/cam grind, picked up a set of ceramic coated headers from Mike Young and with Dave Premo's help and parts was able to rebuild the old pair of solex's I had purchased and fired everything up. This video shows first start (and probably pushing out some old gas) before I balanced the two carbs a bit better. I still don't have the exhaust hooked up so its quite loud with just the headers.

First Start

After starting the car and balancing it a bit better I found quite an alarming problem. The crankcase is building a huge amount of pressure where it's pushing oil out of the return tube near the dipstick and if I open the valve cover oil filler cap there is a noticeable pshhh and pressure release. Right after discovering this I pulled the plugs and did a compression test. 1 - 2 - 3 - 4 is 125, 115, 175, 185 which is all over the map. The bottom end on this car before dropping the valve was pretty good and it didn't really burn oil. The only thing I can think of is when I was torquing the head on the torque click type torque wrench I was using did not seem to be working right. I switched to the old 'bent rod' type torque wrench and finished the rest of the head bolts. I'm thinking of re-torquing the head bolts to see if the other torque wrench may have been part of the problem. What is the procedure for this and with head bolts is 65 psi wet still the recommended range? (break slightly loose and retorque?) Any help is appreciated...I really don't have the bandwidth to the pull the motor and start over.

Oooh, not good. Did you contact a valve with the piston when you dropped the seat? Broke a ring? Broke a piston? Obviously it should not be building pressure like that, and with the compression numbers you're showing it has to be pushing past the rings and into the crankcase. Maybe past a bad seating head gasket but that seems less likely to me. At least I would think. Nothing to lose by retorque, so I would just break them free in proper order and start over. To break free I BELIEVE you start one at a time on each end and work your way to the middle, alternating ends on each one, the reverse to torque, but it has been awhile and I don't know where my manual is at the moment... others need to chime in. Bummer man...

Thanks for the quick reply Greg. The seat did hit the piston and left a small ding but I basically stuffed rags around it and sanded down the sharp part of the ding so as not to create a 'hot spot'. It didn't look too bad and the piston looked intact. That is also the cylinder that has 175 compression so I would be surprised if that is the issue. I'll wait on some other folks (paging Will!) to chime in here so I get the correct retorque technique.

On my recent head rebuild and install I started from center loosened one bolt 1/4 turn then retorqued to 65. Moved on to next bolt and on and on. No problems. I hope your head gasket wasn't breached with oil/coolant and that the retorque works out for you. With that much positive pressure in the crankcase it sounds like a breach.

OK so the compression on the rear two pistons is fine - but the front two have issues! Which is weird because they don't both come up at the same time so if your valves hit, etc it would be 1-3 or 2-4 that had problems.

After rebuilding an engine, I had LOTS of blow by until the rings seated. Then it went away.

But you didn't rebuild your bottom end. Hmm.

1) Could it be that the throttle plates aren't open all the way on the front carb when you do the compression test?
2) Can you easily do a leakdown teat? If so you'll be able to hear if it's the valves vs the piston-rings.
3) I doubt it's the head gasket leaking that much pressure. If it IS the headgasket you'll definitely be able to hear it if you do a leak-down test.

HTH
James

Dave Premo mentioned the throttle body idea so I need to do another compression check with the bodies open. Is there a chance the rings became 'unseated' somehow? I doubted that the head gasket could leak that much - I was just grasping at straws. I did pour some oil on top of all the cylinders as I was rotating the motor back and forth but doubt that would cause those cylinder rings to unseat themselves. My only other thought is somehow the new valve stems are not sealing. I'm at a loss until I can check more. Will said he has a leakdown tester and could help in a few weeks. That may tell us more.

Valve stem seals would only cause it to burn oil with the oil leaking past the stem. I've never heard of rings "unseating". You have super high pressure in the crankcase... some is normal, but high levels are not. Is the crank case vent plugged? How fast does this happen? If it builds up over time, then maybe it is not venting in the first place via the valve cover tube and is building up such that it has to push it out the dip stick etc. I'll consult with my engine guy. Left him a message...

If you have access to a compressor and a compression tester that uses a screw in hose and a quick coupler for the gauge, you can use a pair of tweezers and remove the shrader valve (same as the valve in your tires) that is in the bottom of the compression tester hose, screw that hose into the spark plug hole as you would with the compression tester, rotate the engine until the valves fully close on the cylinder in question, and attach your air line to the hose with 10-20 or so lbs of pressure and listen to where the air is going. You don't need to know how much it is leaking per se, just if it is running out like water via the crank case or valve cover (i.e. getting past the rings like crazy). After the rebuild, your valves should not leak at all or very very little, so I doubt you will hear much out the tailpipe or intake manifold (exhaust and intake valves)... again, we are not looking so much for percentage of loss, just looking for a gross leak.

What does the oil and coolant look like? I'm trying to imagine any other way that pressure could be put into the crankcase via the head gasket, and I'm just not seeing it unless it is somehow getting past the gasket seal on an oil passage. I suppose that you did "unseat" the piston rings by oiling them up and maybe they glazed. The bottom ring is the oil sweeper, so excessive oil on top could potentially cause some problems (?). If that were the case I guess I would run the shit out of it and get it hot, but that may not be possible if she's blowing oil out all over the place!
One thought is to blast in a lot of brake or carb cleaner into the cylinders, turn the engine multiple times by hand or starter with no spark, change the oil and see if that has cleaned the surfaces (ring and cylinder walls). I would use brake cleaner as it cleans and strips oil off EVERYTHING, but others may chime in and say that is a no no...
If it works, then change oil again just to be sure all the brake cleaner is out (I would).

Where/how does your valve cover vent?
Dan

That is the most important question... Dan cuts through! I dribble on for 3 messages:)

CameronSF wrote: I found quite an alarming problem. The crankcase is building a huge amount of pressure where it's pushing oil out of the return tube near the dipstick and if I open the valve cover oil filler cap there is a noticeable pshhh and pressure release.

This part puzzled me. If the valve cover is properly vented, opening the cap shouldn't make a difference. Return tube? not sure what that is either, unless there's a puke can. I always have air/fumes coming out the vent hose from the valve cover, when it fell off the airfilter there was quite a bit of oil leaking from it and you could smell it while driving.

Compression numbers are odd, agree it's time for a retest and a leakdown test. Correct me if I'm wrong, but he could substitute studs one at a time for bolts and then bring them all up to the higher torque for the studs. Been a while since I had a U20 head off thankfully, so I forgot if all the bolts are the same length. so is there any possibility any of the headbolts could be too long/wrong location?

Talked to my engine guy... he said that the smaller exhaust seats are likely causing an internal inversion wave through the lag coupler input shafts, which as we all know is tied into the phase inhibitor for the oiling system.

Not really, you likely have broken rings or more hopefully a plugged or improperly routed venting system. My idea about unseated rings via oil on top is a no.

Any news?