tetrad

I'm pretty sure I've got a blown head gasket. I just got a fresh gasket and am about to dive into it for the next couple of days. My head bolts are not marked, and I know that they can be reused once. But as far as I know the head hasn't come off since the factory.

Auto parts store quoted me $50 for the head bolt set. Should I bite the bullet and just replace them, or are they going to be fine reused?

99 XJ 4.0

greengoblin

when i rebuilt my motor last year i bought new head bolts. i didnt want take the chance. 50.00 is good security vs the cost for a new head if it warps.

tetrad

That was kind of my thought

Are you saying the head could warp from the bolts re-torquing differently or ??

Crispy

New gaskets, new bolts, new whatever else you can do while you're there. Spend the money once or you'll end up buying those bolts at a later time.

greengoblin

i dont like reusing any bolt due to they are streched from being torqued once. when you reuse them your torqueing them again to there max. the head is under alot of pressure. just good to repalce them with new

tetrad

Thanks for the advice guys. You make some very good points. Looks like I'm out 50 bucks

T-VIS

change the water pump while your at it... usually "imho" a little pre-maintenance goes a long way when your already elbow deep into a project.

5-90

The 6-242 head screws are supposed to be a modification of the "torque-to-yield" type, and are only good for two full torque cycles (per various FSMs.)

If the screws have paint marks on the heads, replace them. If they do not, paint them after torquing. If you're not sure if they've been pulled and replaced, then replace them.

I'm curious as to whether or not they can be replaced with conventional SAE8 screws (the head markings on the earlier screws come back to SAE8 pieces, but those aren't usually any variety of TTY screw...) provided you use a quality fastener (read: "not Chinese.") I'd like to replace them with socket head capscrews, but I happen to like them (socket head capscrews are typically H&T to SAE8 or ISO 12.9 specs.)

5-90

Depends.

A "torque to yield" screw is designed to be preloaded to or just beyond the "yield point" of the fastener, while conventional torque specs on screws are actually below the yield point.

The "yield point" is also referred to as the "elastic limit" - this is the point at which a metal part is stressed enough to actually take a permanent change in shape after the stress is relieved (NB: metal is "plastic" in an engineering sense. It is able to be deformed to a limited extent and you can still expect it to return to its original shape - this is what makes springs work.

"Yield strength" is the amount of load a screw can take before it will be permanently deformed, and is always lower than "ultimate strength" - which is the point where a screw is loaded and it fails (ruptures.)

"Torque to Yield" (TTY) screws are those which are designed to be stressed to just beyond the "yield point" - thereby maximising tensile preload. However, since the screws are stressed to the point where they "take a set," they aren't reusable. (TTY screws are commonly used for Diesel engine cylinder heads.)

I'm not sure how they manage two torque cycles for a variation on TTY - but if that's what the manual says, I'm going to listen to it.

tetrad

There are no paint marks, but I'm not 100% that they haven't been reused. I take it that if they *have* been reused, there is a fairly good chance that they will torque incorrectly and cause me major headaches.

I am tempted to pull them, mark them, and bag them for a rainy day.