454 Mercruiser Exhaust Manifold Bolts

[Getaway]

I'm replacing a starboard side exhaust manifold on my '88 454 Mercruiser engine. When I removed the bolts from the manifolds, I noticed whoever bolted this up the last time used different length bolts.

Some of the bolts were 1" long and some were 1-1/4" long. They are all 3/16-18 grade 8 hex bolts.

Should I just replace these with all new 1-1/4" long grade 8's or is there a reason for the shorter bolts?

Thanks in advance!

 

[Mike Blake]

If I remember correctly there are 2 that are shorter. with the old manifold off check and see how much is protruding

Just to add I used a half deep socket when I removed them.

 

[Air O'Nautical]

My new Gen VI blocks were all 1-1/4 long.
I did screw all of them in first, to ensure they had the ability to go far enough to torque the manifold down. Then, removed all the bolts and muscled the manifolds in place.

 

[Happy Hour IV]

Look carefully at the manifold and you will find the flange is thicker at some of the holes hence longer bolts in these.

 

[Getaway]

Thanks guys!

After much closer investigation, the exhaust manifold tapped holes are very shallow in places. In fact, a 1" bolt is about all that will fit. My new exhaust manifolds came with studs so I just replaced the bolts with the threaded stud which seems to be a much better design.

The bolts I removed from the existing manifold were all different sizes and I think some of them were bottomed out in the head and not even crushing the gasket.

I'll be 2 days healing from busted knuckels, cramped legs/back and sore arms...LOL.

Thanks again for the help!

 

[rcknecht]

mine are all the same length except the studs used to mount an accessory...

 

[dogturd21]

I have done major exhaust manifold work on Gen 4, Gen 5 and Gen 6 454's and 502's, and they were all 1.25" long. The bolts are 3/8-16 threads. The last one I did to my sundancer, I used ARP stainless bolts purchased at 1.5" and I had to grind them down (mistake because 1.25 are commonly available from ARP). You might find advice to not use stainless bolts as they are not strong enough, and this might be true of non-ARP bolts, but the ARP are actually stronger than grade 8 bolts. the reason I did this is because condensation can sometimes eat away at the manifold bolts, so 5+ years down the road they become difficult or impossible to remove.

 

[SloBurn]

Not quite impossible....I had to literally stand on my head in the bilge and grind/cut the bolts off of my '88 454 when I first bought it and replaced mani's. Bolt heads were corroded to almost nothing.

 

[dogturd21]

Not quite impossible....I had to literally stand on my head in the bilge and grind/cut the bolts off of my '88 454 when I first bought it and replaced mani's. Bolt heads were corroded to almost nothing.

I sympathize- I had the same problem happen to me twice in the last 10 years. first engine I had to remove to get to the bolts, and wound up using many cobalt drill bits to remove them. There are tools similar to easy outs but they fit in the outside of the head bolt, IF you have enough room. they didnt work, so I had to drill them out. 2nd boat was not as bad but abut 1/2 the bolts had to be drilled out. NOW, for about $35 worth of SS bolts this will never happen again. ARP kit for 5 bolts, using 3/8 head (which is fine in this application as torque is 20 ft lbs - part number is [h=2]ARP Kit #: 613-1250[/h]