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Discussion Starter · #1 ·
I have two broken bolts in my p216G on my 446 that I need to drill out and replace. While I'm at it I thought about just putting in new bolts all around. In any case, I'm finding it hard to find 5/16 all-thread bolts in the 2" variety that I need and it gets even tougher when trying to decide on type of bolt steel. I'll likely order online.

Question is, what is best to use with the aluminum engine? On my snowmobile I fasten things with 316 stainless to the aluminum tunnel or chassis, but that's a less critical environment with little surface contact. I can't find any information on what onan used for their bolts. I imagine I should stay away from standard zinc coated and generic grade-5 bolts from local tractor/fleet supply stores (whatever they put on them).

What have you done? I'm not the first to break some bolt heads off 😅
 

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I am not an Onan guy, so take the following for what it's worth:

1.) Do you really need fully threaded bolts?
2.) I would use grade 8 or better. Zinc coated grade 8 should be fine. ARP makes really nice stuff that is stronger than grade 8.
3.) Use anti-seize to prevent galvanic corrosion and galling.
 

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Discussion Starter · #3 ·
In my case it is nearly fully threaded. The engine block is tapped fully and the cover is thin, so standard partial-thread bolts will not work. The OEM bolts have a slight gap between the threads and bolt-head, but not much.

Right, anti-seize. I have some permatex silver-grade kicking around. Thanks for the reply. I suppose my snowmobiles use a yellow coated grade 8 and 10 bolt for their aluminum engines, so I should be fine here.
 

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ALL of the onan P small block engines used grade 8 2" head bolts.
they are close to threaded all of the way.
that bolt will easily take 35# of torque, only 14-16# needed.
anti-seize is THE KEY,
steel and aluminum are like CATS & DOGS, they do not get along.

if using heli-coil BE SURE they are at least 1" long.

thank you. boomer
 

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In my case it is nearly fully threaded. The engine block is tapped fully and the cover is thin, so standard partial-thread bolts will not work. The OEM bolts have a slight gap between the threads and bolt-head, but not much.

Right, anti-seize. I have some permatex silver-grade kicking around. Thanks for the reply. I suppose my snowmobiles use a yellow coated grade 8 and 10 bolt for their aluminum engines, so I should be fine here.
I did a quick google search and some turned up on eBay. You might also try Bill at Barneveld Equipment.
 

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Personally, I stopped using “normal” bolts on evry engine I build/rebuild a long time ago.

Now I use studs wherever I can. They go in with some threadlocker (normal, not high strength) and then they do all kinds of good from holding gaskets in place while dropping on the part or never having to worry about a bolt breaking off in the case or stripping out the holes. Studs also deal with the clamping forces a little better than bolts becuase you get to run them down and use all of the threads in the hole as opposed to however many a bolt engages when inserted. Studs also tend not to fatigue as much as bolts because of that fact.

Of course, studs are way better than torque to yield bolts IMHO. They’re reuseable and not quite as fiddley to torque as opposed to getting the right “stretch” on a TTY bolt.

The key though is to make sure when using studs that you still have enough room to remove the part while the engine is in place. For example: using studs for head bolts isn’t great if something is in the way and you haven’t got enough clearance to lift the head up enough to clear the studs. Lots of times that’s a problem on V8 cars and things like the brake booster or wiper motor being in the way.
 

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^^^Studs, duh. Ideal for aluminum as it removes the wear and tear from the aluminum threads. (y)(y)
 

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Discussion Starter · #8 ·
Hey thanks for the replies...

Studs are something I will try in the future. I sourced some all-thread grade 8 bolts through Grainger (crazy hard to find, apparently) and managed to track down some long heli-coils. Thanks again for all the input.
 
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