Head torque questions for 948

[ichthos]

While I am waiting for my new Payen head gasket to arrive, I thought I would verify torquing information. 1)My book lists 40 ft/lb for the head, and rocker pedestal nuts as 25 ft/lb - is this correct? 2) When I torque the nuts on the head in the order shown in my book, would it be best to tighten everything to 20 ft/lb and then tighten to 40 in the same sequence? 3)Do you normally have to retighten the head at any point after driving the car? Any help would greatly be appreciated.
Kevin

 

[Spridget64SC]

40#'s is the correct value for the stock 948 head studs. 25#'s is the correct value for the rocker studs. Inspect each stud and corresponding nuts to make sure the threads are in good shape and not pulled or distorted. Often the case on the first thread of the 3/8" head stud nuts. The 3/8" head studs tend to elongate in the fine thread area if over torqued.

Look at the top of the 3/8" stud (fine thread end) to make sure someone hasn't replaced the stud set with a 1275 set. Sets with "22" or a "dot" in the end can be torqued to 50#. ARP studs will be a dark black oxide color.

On 948's, the five short studs usually are in an open hole to the coolant. This may be where you were getting your leakage from. Double check this. I like to use Fel-Pro's Bolt Prep product. It is a low tension thread lock and seal. Sometimes these threads can leak past. Should have thought of this when I responded to the other thread.

If you need replacement nuts or washers, the machine shop should be able to help. Pioneer products makes all sorts hard washers and nuts that can be used for rod and head bolting applications on the A-Series.

HTH,
Mike Miller

 

[Spridget64SC]

Forgot the other part of the questions. I'll tighten everything down first using a 3/8" ratchet and the appropriate sockets first. Slow and steady, pulling the rocker assembly down evenly from front to rear. Snug every thing up and then start the torquing process. Back off your rocker adjusters and this becomes a lot easier for the rocker assembly. I torque to 25 #'s first from the snug condition and then go to 40 and then 50 (Note I am doing 1275's or ARP studs). For the stock 948 I would do 25,35 and then 40. With 35 & 40 being just the 3/8" head studs. I like to do good smooth torque pulls. Pause and pull again on the same stud just to check. Sort of a double "Click" with the torque wrench. On the 1275's, I'll use a little motor oil on the head stud fine threads to smooth out the pull. Just a little bit on the threads and in the nuts. A little bit of the oil gets on the washers and helps the nut/washer interface turn smooth too. If everything is too dry, the torque process can be a bit jerky.

Don't use hardware store washers anywhere. They are too soft. Use the thick (nominal 0.100") factory washers or go to the machine shop and get some replacements.

Make sure the rocker arm adjuster ends line up with the cups in the pushrods. After all is done, then adjust the valves using the rule of 9 (one down, adjust 8, 2 down, adjust 7 - etc.).

I do things a bit different in the re-torque department. I warm the engine up, get to operating temp and then let cool. Re-check torques just like it was a cold build again. Back head stud nut off a 1/4 turn and then tighten to spec in same sequence as in manual. Just do this for the 3/8" studs. That oil put on the threads helps with the loosening and doesn't break the seal on the bottom of the short studs in the 948. Then I warm engine and adjust valves to spec. Been doing it this way on my race engines for 30+ years. Then go run the engine.

HTH,
Mike Miller

 

[jlaird]

Agree to all above. Never a problem.

 

[ichthos]

I am not sure how to tell if the threads are distorted. If the nut goes on smoothly, does this mean it is OK? Someone mixed washers and nuts, so I did order some new nuts and washer for both the 3/8" head studs and rocker studs before I read the posts. I did not see evidence of coolant leaking from the 3/8" studs, but how can you tell?
Kevin

 

[Spridget64SC]

A new nut threading on the stud smoothly is a good sign the threads are good on the stud. Like wise the other way around. Thread a used nut on a new bolt and check to make sure smooth engagement. Look at the openings on the nut and make sure the bottom thread on each side hasn't started to pull out of the nut.

If coolant is leaking up the threads at the bottom of the short stud, coolant will be pooled at the bottom around stud and in the head gasket hole. If coolant is leaking at the dual hole passage between the head studs, there will be a coolant trace (discolorization) on the head gasket.

The top thread in the block can also pull up. When I rebuild engines, I'll take a 82 degree countersink and cut out the top thread on the block to prevent this from affecting the seal. Then run a bottoming tap back down the hole to catch the bottom of the hole, chase the existing threads and clean everything up. You can clean up the tall stud holes, but not the short ones due to them being open to the coolant jacket on the engine. Take all the studs out, clean up the top of the block and inspect. You could do this procedure with the engine in the car by using good masking tape and covering the bores, oil passage and the water passages. Clean everything up afterwards and use compressed air to blow out the tall stud holes. Maybe a shop vacuum while counter sinking and taping on the short side?? They make special drilling rigs for industrial applications that have the vacuum connections to hot drill and tap piping systems where you don't want foreign material in the process.

Hope all this extra information helps. At least if you undertake an engine rebuild sometime in the future on this car or another. When I'm doing a race build, all the threaded holes get bottom tapped and all but the main cap holes get counter sinked.

Mike Miller

 

[ichthos]

Thanks for the information, Mike. I will have my parts late Wednesday, and we will see how it goes.
Kevin