Exhaust pipe to manifold Bolt size

91 XJ 4.0 HO

Anyone know off-hand what size the bolt heads for the collector are? Going to pickup a new XJ tomorrow and the owner claims he didn't tighten the bolts down and it is pretty loud under the loud. I'd like to know what to bring with me for tools.. Although I suspect it's going to be a cracked manifold.

91Red4.0 said:

91 XJ 4.0 HO

Anyone know off-hand what size the bolt heads for the collector are? Going to pickup a new XJ tomorrow and the owner claims he didn't tighten the bolts down and it is pretty loud under the loud. I'd like to know what to bring with me for tools.. Although I suspect it's going to be a cracked manifold.

Click to expand...

Probably 3/8"-16, 9/16" wrench.

If I take them off, I'll replace them with brass - brass won't seize on steel, so it's easier to take to bits next time (always assume the next poor schmuck to work on something will be you - with a Swiss Army knife and a rock... Plan, engineer, and replace parts accordingly.)

I know they're 3/8"-16 through 1990, and I'm fairly sure that didn't change (the 6-242 stayed an "inch" engine through its production life, and most of the direct bolt-ons stayed SAE as well.)

If it's possibly ISO, you're looking at M8-1.25 (13m/m wrench) or M10-1.5 (15m/m wrench, I believe.)

And there usually is a "donut" gasket in that join, which may be what is burning out and causing all the racket...

I need to replace those bolts on my '87XJ... last time I replaced the downpipe those bolt threads were seriously degraded... rusted off to where the nuts caught on only a few of the threads.
But when I put a wrench on the top of those bolts they wouldn't budge...
so my question is:
assuming they are the OE bolts should PB Blaster and a stronger twist on the bolt heads break them loose,
or do those bolts have splines on their shafts and they need to be PB Blasted and then with a c-clamp and oversized socket on the top force them up and out by tightening down on the c-clamp ?

Mine weren't splined at all, but they were also loose to begin with. On my last XJ I sprayed them down with PB a few days in a row then loosened them. It did take some upper arm strength.

Thanks for the note
- I'm going to proceed as those OE bolts are not splined... will use lots of PB Blaster over a few weekends and then try to twist them out via a hand wrench on the bolt heads.
I'm gonna have to watch out that I don't twist to the point of maybe damaging the exhaust manifold on my '87.
I replaced the exhaust manifold on my son's '99XJ 4.0L last year (that and replacing the motor mounts got rid of the P0171 code from that '99... so far so good a year later).
I want to not create an exhaust manifold-replace job on the '87XJ just by trying to replace the down-pipe bolts... but I need to track-down the noticeable exhaust sound that is coming from that area on the '87.