496 Head Bolt Length
 

Guys,

During my tinkering this winter, after pulling the engine to fix a coolant leak I couldn't get to while the engine was in the boat, I discovered that I was getting some coolant past the head bolts. I ordered a new set of Fel-Pro TTY head bolts and set to swapping them out. After snapping one of them off before it ever reached torque (I got it out easy enough, and I've since been informed that Fel-Pro stuff has been crap the last couple years), I acquired a set of ARP head bolts (not studs) from Raylar. I didn't really notice until I was in the install process, but they are measurably shorter than the TTY bolts that came out (and the Fel-Pros, for that matter). I contacted Larry at Raylar, who indicated that he'd never encountered a failure, but he also indicated he doesn't deal much with the iron heads, and I can't seem to get a straight answer if the aluminum heads use shorter bolts or not. Near as I can tell, at "finger tight", I've got ~10mm of thread engagement with all but the two mid-length bolts that are at either end of the heads, which are probably close to 15mm. They torqued just fine, but that thread engagement is scratching at the back of my neck. I don't think the bolts themselves are going to fail, but I'd really hate to have them rip the threads out of the block. For reference, originally a Merc 496, now 503ci (by my math), Raylar BP211 cam, roller rockers, M1 ProCharger pulleyed to an estimated ~6psi. Thoughts?

Thanks. Brad.

rule of thumb,bolt dia wants at least that much of thread contact.

Sutphen,

At least. My rule of thumb has always been 1-1/2X the thread diameter, depending on thread pitch. But I wanted to get some insight from some who’ve built engines. I certainly don’t fit that description.

Upon review of my emails with Raylar, I believe it’s only the longest of the three bolt lengths that are short. I’ll need to pull one of each of the three different length bolts out and Make sure. My thinking is that I received 90mm length bolts instead of the 100mm bolts that ARP sells. I can also compare them to the FelPros and OEMs I have here. This block has the tapped head bolt holes breaking into coolant passages (which is how we got here, BTW), so a little longer than OEM (103mm is spec) is OK, and I can use spacers if the “grip length” is too long (threaded length is too short). I’d sure like to use the entirety of the threads in the block if I can make that happen.

I spoke with ARP today. They sell these bolts in 10mm increments in 5packs. I should be able to get what I need if I don’t already have it.

Thanks. Brad.

I've lost count on how many engines I've built.fun fact,arp bolts on a 454 salt water motor in 1 season would be eaten away to have 1/2-5/8" of thread left when pulled.the stock gm bolts,they would be fine for 3 seasons.

Sutphen,

The OEMs were plated with nickel or zinc, probably.

And this is why, when we were looking for our boat, if it was anywhere near salt water, it was an immediate swipe left. Just…. Nope.

Thanks. Brad.

Guys,

So.... Some discovery....

I pulled one of each length head bolts and measured them. The long ones, which make up the bulk of them, are 100mm long, which is as long as ARP offers. The mid-length ones are 90mm long, which is actually slightly longer than they need to be. The short ones are 50mm long. They all have washers under them that are all but 2mm thick. The Fel-Pros I have here are spec'd at 103, 86 and 52, respectively, but don't include the washers. I am going to call ARP and see if they offer 110mm long bolts, but I'm doubting it, since they aren't in their catalogue. Assuming they don't....

What's the best course of action? Remove the washers and go with it? I'm inclined to order two 5packs of the 60mm long bolts to replace the short ones. I can always double up the washers if I run out of threads, and I don't have to worry about the threads bottoming out, since the holes break into coolant passages. I'm fairly spooked by the TTY bolts, having snapped one of them off already; I don't want to have to pull the heads again. There's just too much disassembly that goes with that if I can avoid it.

It was also suggested I go with bolts from Caterpillar, 110mm UHL, which spec to 170K tensile strength, vs ARP's 180K; still plenty strong enough. I'm thinking I'd be willing to give up minimal tensile strength to gain thread engagement in cast iron.

Thoughts?

Thanks. Brad.

Guys,

I made an executive decision. I ordered 60mm long bolts from ARP, and 110mm long bolts from Caterpillar. This will satisfy my concern with head bolt thread engagement. They will be in tomorrow. So I have questions….

The engine is all but together. I have to remove the exhaust rockers to gain access to the head bolts under them, but the intake rockers can remain. Which is a good thing, because I can’t really get to the intake pushrods to feel them as I “zero-lash” the rocker after putting them back on. My plan is to replace the bolts, one at a time, torquing each one as I go. Is this OK? Or should I just commit to it, pull the intake and all the rockers, and go through the whole process over again?

Thanks. Brad.

just change one at a time, shouldnt be an issue with it. I have done that before and not had a problem.

Just curious what you were torquing them to ?? I have never had a problem with a Fel Pro head bolt.

Brad, can you describe what and how and where the head bolt broke?

Bill,

Per advice from Larry at Raylar, 65ft/lbs for the long and mid-length ones, and 60ft/lbs for the short ones. He said he was not a fan of the <X> ft/bls + X° of rotation theory. 33 of the 34 came to torque just fine. But that one never even got close. A more experienced builder would have gave up and stopped before it snapped.

The funny thing is that, when I asked him about the bolts he sold me being shorter than the OEMs I’d just pulled out, Larry told me to just put it together and run it. I don’t doubt him, but I don’t have a wall of bare blocks to go through when/if I rip the threads out of mine.

Personally, I don’t understand the concept of tightening something to the point it’s ready to break. I’ve read/heard it explained a dozen times. Still doesn’t compute with me. A friend of mine who used to build drag engines on a hobby basis said they used to call them “torque-to-fail”. The term seems accurate in my mind.

The guy that’s been doing the Holley tuning for me is who told me that Fel-Pro’s quality has gone to sh!t in the last couple years. Both he and his dad (who builds the engines in their shop) categorically refuse to use anything from Fel-Pro anymore.

Thanks. Brad.

Tartilla,

Just below flush of the deck. It snapped off pretty clean, even if not flat/level. I was able to drill into it and get an easy-out to get a bite on it and got it out pretty easy with no damage to the tapped threads.

https://cimg4.ibsrv.net/gimg/www.off...4e354777a.jpeg
https://cimg5.ibsrv.net/gimg/www.off...ac25114ed.jpeg
https://cimg6.ibsrv.net/gimg/www.off...4357aed2c.jpeg

I’m fairly well convinced it was just a faulty bolt. The problem is that that one faulty bolt made them all scrap, if you follow the “one use” rule for TTY bolts.

Thanks. Brad.

Guys,

For the record….

https://cimg5.ibsrv.net/gimg/www.off...36822ab9c.jpeg
https://cimg8.ibsrv.net/gimg/www.off...6e18b6676.jpeg

This shows the difference between the ARP (100mm UHL) and the CAT (110mm UHL) bolts. Nothing wrong with the ARPs. Just not long enough. You can clearly see where the engagement stopped on the shorter ARP bolt, and how much more engagement I’ll have with the 110mm long bolts. Same deal with the short bolts; 60mm as opposed to 50mm.

Thanks. Brad.

With the updated tensile strengths—CAT bolt at 170,000 psi and ARP bolt at 180,000 psi—the difference is relatively small (about 6% higher for the ARP). This puts both in a similar high-strength category, comparable to premium Grade 8 or better fasteners, and well above typical OEM specs in many applications (often around 150,000–160,000 psi for standard high-stress bolts).

Key Comparison Factors

• Ultimate Tensile Strength (UTS): ARP edges out slightly at 180 ksi vs. CAT's 170 ksi. This means the ARP can theoretically handle a bit more peak load before failure, assuming identical diameter and material quality.
• Thread Engagement:
  • CAT (110 mm length, 22 threads): More engagement (~1.57 inches if ~14 TPI typical for 7/16" BBC heads), spreading load over more threads → lower stress per thread, better resistance to stripping (especially critical in cast iron blocks), and improved fatigue life under thermal cycling/high cylinder pressures.
  • ARP (100 mm length, 15 threads): Still solid engagement (~1.07 inches), exceeding common minimums of 1–1.5x bolt diameter (~9–12 threads for safety in performance builds). ARP's manufacturing advantages (e.g., threads often rolled after heat treat for better fatigue resistance and grain flow) help compensate for fewer threads.
• Overall Clamping & Durability:
  • In a high-performance BBC setup (e.g., boosted, high-compression, or high-RPM), the ARP's slight strength edge + superior metallurgy (better fatigue properties, precise tolerances) often makes it more reliable long-term, even with marginally less engagement.
  • The CAT's extra threads provide a safety buffer against thread shear or pull-out in the block, which can be valuable if the block is older/worn or if you're pushing extreme loads where gasket sealing is marginal.

Is More Thread Engagement Worth It?

Yes, more engagement is still worth prioritizing in most cases—especially if your build involves high cylinder pressures (boost, nitrous, big cam) where the block threads see the highest risk of failure. The extra 7 threads on the CAT bolt distribute stress better, reducing the chance of localized yielding or stripping, which can happen before the bolt itself fails.

However, the ARP's ~6% higher tensile strength, combined with ARP's typical quality advantages (e.g., rolled threads post-heat treat for ~10x better fatigue life in some cases, tighter tolerances), makes the shorter ARP bolt very competitive—and potentially preferable—for reliability in demanding applications. Many BBC builders favor ARP precisely because of these engineered benefits, even if engagement is slightly less.

Recommendation

• Go with the ARP (100 mm, 15 threads) if you're building for max performance/reliability, following ARP's install specs (proper lube like Ultra-Torque, accurate torque/stretch, clean threads). The small strength bump and overall quality likely outweigh the engagement difference.
• Choose the CAT (110 mm, 22 threads) if your block/head combo benefits from maximum thread depth (e.g., to avoid bottoming out concerns or for extra margin in a cast iron block under sustained high loads), or if cost/availability favors it.

Either way, both are strong options at these tensile levels. Measure your actual bolt diameters, thread pitch, and hole depths to confirm fit—no bottoming out, and adequate stretch for proper clamping. Always chase/clean threads and use the recommended assembly lube/torque sequence for best results. If this is a high-horsepower build, studs (if available) would be even better than bolts for uniform loading.

my .02
 

Why not studs??
I would not trust some CAT bolt.
Never seen an ARP bolt break.
I`d use ARP not play guinea pig with a CAT bolt.

Dan,

I don't disagree with your sentiment at all. But.... It’s not a guinea pig experiment. It’s been done. The CAT bolts are fine. UTS is plenty for my relatively low boost application. It wasn’t a bolt failure I was concerned about. I agree: Nothing better than ARP. If ARP had had 110mm UHL bolts, we would be having THAT discussion instead. It was the block. With only 10mm of thread engagement, it was the cast iron block that was going to give before the bolts. And that’s worse than the bolt failing. I don’t want to think about what would be involved with correcting threads ripped out of the block. I don’t have to worry about that now. With ~20mm of thread engagement, assuming there’s that much thread in the hole before it breaks into the coolant passage, we are at maximum thread strength, so I’m getting all of that UTS, instead of a third of it (it’s not a linear scale). I am 100% confident in all of the things I’ve done this winter, and this one snag had been eating at me. Now it’s a problem solved. It’s kinda like this: I wonder what the UTS of the TTY bolts are, but I KNOW the CAT bolts are stronger, even if not as strong as the ARPs. If the TTY bolts are sufficient, the CAT bolts are a huge step in the right direction, and I don’t have to worry about the inadequate thread engagement undermining the UTS advantage, leaving me with perfect bolts and a scrap block.

FTR…. I looked into the studs. ARP offers a set. They are just expensive a farq, and I’m running on a pretty thin, and quiet :silenced:, budget.

Thanks. Brad.

The block is the issue.. got it.
Sounds like a good plan then .

Dan,

https://cimg6.ibsrv.net/gimg/www.off...1aba8a56c.jpeg

This is scary.

Thanks. Brad.

All these issues on a new rebuild? How many hours did you put on it this year?

are these bolt holes blind or are they open into the water passage?I see you gooping them for open but that bolt you showed,kinda looks like it bottomed out in the hole.

Sutphen,

They are open into coolant passages. Which is how this all started. Sorta.

What that pic shows is how deep the bolt was in the deck of the block. Where the “goop” ends is at the deck. :eek:

Thanks. Brad.

Don’t take this the wrong way because it’s not meant with any disrespect, but do you feel maybe you’re causing your own issues? I have been doing this a very long time and very very familiar with the 496s. I’ve worked on many of them and owned many of them. I never had the issues that you are trying to correct, is the builder you’re using a professional builder or is he doing it on the side as a favor out of his garage? He should’ve been the first one to tell you about the bolts not being long enough when he originally put the cylinder heads on, again you’re about the only person that I know having all these issues with a 496.

maybe I am missing something

Wildman,

the only “issue” was a coolant leak we fought all summer. After trying to get to it in the boat, the decision was made to pull the engine. Initially thinking it was a bad circ pump, I ordered a new one. Upon disassembly, I found this:

https://cimg8.ibsrv.net/gimg/www.off...559f3c883.jpeg
https://cimg9.ibsrv.net/gimg/www.off...de1294192.jpeg

The connecter between the crossover and the circ pump. It’s my assumption that this was leaking. I also decided to replace the T-stat with a 120°, and, while in the process, discovered that the T-stat seal was all but crushed out. It was hardly proud of the top face of the crossover. So I ordered a new seal. All that’s back together.

While it was out, I decided to swap the cam for a Raylar BP211, so the exhaust manifolds, intake and valve covers came off, along with all the other crap that has to come off for that project. While this was all apart, I noticed what looked like oil pooled around the external head bolt heads. I mentioned it to my tuner, who indicated it couldn’t be oil, as there’s no oil there internally to leak. During discussions with Larry at Raylar about the cam, I mentioned it to him. He enlightened me that it wasn’t oil, but coolant. If the head bolts aren’t properly slathered with sealant, coolant will seep up the bolts, leak out, dry and cook into a slick, brown substance that resembles oil. Going through my texts with my builder, I remembered that he’d told me he’d reused the head bolts, so I decided to replace the head bolts, and ordered new FelPro TTYs. While replacing them, one of them snapped before coming to torque. So off came the heads, to get to the bolt stuck in the block. While the heads were off is when we did the rocker stud work, which allowed for options on rocker studs, leading to a much better rocker geometry. While doing all this, Larry convinced me to get a set of the ARP head bolts, which ultimately turned out to be too short. It’s my understanding that they work well with the aluminum heads, but my guess is that the iron heads are a bit taller, at least at the head bolt bosses. So here we are.

Clear as mud? :angry-smiley-038::ernaehrung004:

I just wrapped up replacing the head bolts with the CAT bolts mentioned above. I can start putting it all back together now. Again. :faint2:

Thanks. Brad.

BoatNT,

Read my response to Wildman. And no offense taken. But, honestly, would you be OK with the depth of thread engagement shown in that pic above?

To your question,

The builder from last winter is a service tech at a local m dealership, and has been building (mostly) high HP marine engines for a very long time. I do not doubt his ability to build. I think he made some decision on my behalf, trying to same me some money, because he knew the nickels and times were starting to sting. I wish he hadn’t done that with these couple issues, as I’d have sprung for new head bolts and that crossover connecter. They were cheap. But…. On the bright side, I got a new Raylar cam in while I had the engine out. So, all good.

Thanks. Brad.

can you put up a pic of the long bolt w/ the next shorter bolt that goes in the end of the head under the valve cover.I don't like how little thread is engaged.

Sutphen,

If I understand you correctly, I don’t have a pic with the 90mm ARP bolt. They are all installed and I’m not pulling one of them to take a pic. I can tell you, though, that they are ~4mm longer than the TTY bolts are spec’d as. I have plenty of of thread engagement there.

Thanks. Brad.

CAT bolts have been some of the highest quality bolts produced. With the current outsourcing potentials, who knows today.

A 170k bolt is strong. A 1/2" bolt would hold up 33k pounds...and 20k pounds in shear.

Dan's suggestion for studs is valid for a single engine offshore. Twins makes it difficult to remove heads with the engine in the hull. But typlically easier to temove tue engine to do major work anyway.

Tartilla,

I’m doubting CAT has settled for substandard components. Outsourced or otherwise. Just too much liability.

Yup. I don’t disagree with the suggestion for head studs. They are just too far out of the budget, and not really justified for a build of my level. Especially considering we are still dealing with M10 threads. But this doesn’t mean I don’t want to wring as much as I can out of them as I can. Which I think the CAT bolts satisfies. It does amaze me, though, that ARP doesn’t have 110mm long M10 bolts. It also surprises me that Raylar shipped 100mm bolts when TTY spec bolts are 103mm long.

Overall, I am very confident in what I’ve put together at this point, as Frankenstein as it is.

Thanks. Brad.