Can someone tell me what grade of bolt the 235 Chevrolet used on its main caps? I read on another forum about using hardware store grade 8 bolts for the mains. About 1/2 said no problem, the other 1/2 said no way! Now the question is, what did GM use? We know replacement kits are available, but he question is what did GM use? None of manuals or parts books list grades, just size. Joe
I don't know about your engine, but main cap bolts are typically #8 fasteners. The 1/2 that told you not to use #8 was the group that believes #8 bolts are brittle and prone to failure. It's a lot more complicated than that. Think of a bolt as a clamping spring. Although there are cases where #8 bolts are not appropriate, basically, those are situations where the joint is not stable, or the fastener can't be tightened enough to make it act as a clamping spring.
That shoulder is were this gets interesting. Hardware store grade 5 and 8 bolts do NOT have that shoulder. The shoulder is critical since the diameter and material of the bolt determine it's clamp load at a given torque value. That may be a grade 5 bolt, but it will have different clamp loads then a hardware store bolt due to the differences in geometry. It is always best to use bolts with the same geometry and properties as the originals. ARP has a stud kit that will replace the main cap bolts for the 235. Part #132-5402 Good Luck
The reason it has that shoulder is because it is an automotive grade bolt made with a specific purpose. I suoopse that in a pjnch one could get away with a bolt from where ever but if you can buy or find the proper bolt why would you use anything else.
That shoulder is there to locate the main cap properly on the block. A 216,235 and 261,other than the rear main, have no dowels or reliefs cut into the block to locate the main caps. Replace the bolt with anything different and the bearing half will be misaligned. anyone who says different has never looked at a one of these engines.
A grade 5 bolt is more forgiving that a grade 8 for many applications. The bolt in the above post is a grade 5, which should be fine when torqued properly. The shoulder diameter is a pretty close fit with the bearing cap hole, not the standard 1/64" and up for general automotive bolt fit clearances. I always use the right bolts for mains and rod bearings (as well as any other engine internals), too much room for error using whatever is laying around. Bob
The shoulder on the bolt can keep the main cap in the general vicinity of where it needs to be. It cannot locate the main cap to within the less than .001" accuracy needed. Although not the best design, on some engines the crank is used to align and position the cap. The one of these engines I had apart was decades ago, but according to what you posed it is one where the cap is aligned by holding it square against the crank during initial tightening. In any case, the shoulder on the bolt cannot precisely locate the cap relative to the crank. I agree, if it had a #5 originally it is obviously adequate.
IMHO, I would rather have a 2 bolt main with the grade 8 ARP bolts, then a 4 bolts with stock ones. NEVER had an ARP fail me....EVER!
I'm not trying to give you a hard time, but think about it. Even if there was NO clearance between the hole in the cap and the shoulder on the bolt, the threads themselves have more looseness than correct cap location requires. That's the case even on old relatively crude and loose fitting engines like this one. If you installed the cap on the block without the crank in place, then run down the bolts so they have no play but aren't tight, it is going to be possible to move the cap. Whatever than movement is, it's going to be more than the main bearing clearance. So, that means the crank, and not the bolts, is locating the cap. And, for the threads and bolt shoulder to be concentric, both the shoulder and threads would need to be precision ground, which I guarantee they are not.
The shoulder has nothing to do with locating the cap. Like many head bolts, this bolt has a relieved shank, so that the od of the shank of the bolt is no larger than the minor diameter of the threads. The idea is that the bolt stretches along its entire length as it is torqued, not just in the thread area. For cyclic loading, this is intended to prevent the bolt from breaking where the top thread engages into the block. Ron Dupree
lets discuss it.I'm not a pro engine builder,just a hobbyist that's kinda familiar with assembling 261's Your statement about the rear main locating the crank; The block is upside down with the four lubricated main bearing insert halves in place. You lower in the 75 pound crankshaft.It's seats in the bearings,other than rotation and a few thousand back and forth on the thrust,you cannot move the crank from side to side.I say the doweled rear main won't line up the crank at all,it is where it is. On the last 261 I did about a year ago ,the build in in the "dedicated 261 thread" here, there was a problem with one main cap.The machine shop explained how the main are registered by the shoulder on the bolt.If you run the bolts on the main cap with no bearing in place and stop just short of tight,leaving a few thousand gap,you can't move the cap around. It's captured by the bolts.When assembling the engine at home I installed main caps on the bare block.Stop short a few thousands and you can't move the cap at all. It's captured by the bolt shoulders.Lightly tapping the cap didn't move it. The thrust surface is on number three main,undoweled,and it lines up perfect... I understand what you are saying,you have more experience than me.But with my hands and eyes I felt and saw how the main caps mounted. At any rate,it amazing the 235 engines are durable with such a shitty main cap arrangement.
I'm not a pro engine builder either but wonder if the block required line boring how would it be possible if the crank is what aligns the caps? I also have doubts about moving the bolts radially more than the clearance if at all when fully engaged.
I agree. The shoulder also serves to locate the head squarely in the hole and maximize maximum engagement surfaces between bolt head and cap. Basically the cap locates the bolt in the hole rather than the bolt locating the cap.
The 1948-1951 factory Chevy truck manual describes a procedure to align the intermediate caps. Tain't the bolts.
For whatever difference it makes, I have had a lot of different auto related jobs in a variety of fields. Depending on your point of view you could say I have a wide diversity of experience, or, that I can't hold a job very well. Not that it makes me any smarter than anyone else, but engine builder is on my resume. Road cars, classic cars, hot rods, race cars, trucks, boats, motorcycles, stationary/industrial..... I have built a lot of engines both hobby and professionally, in shops that range from good to outstanding. With the crank laying in the block it doesn't seem like it can move any way but lengthwise, but there is bearing clearance. Although it's only a couple thousandths on an inch, it's there and it has to be there. If you put a dial indicator on the cap you will find it can be moved around considerably more more than the bearing's clearance. The reason that matters is; it s possible for the crank to be all the way to one side while the cap is all the way in the opposite direction. That means ZERO bearing clearance side to side, and reduced clearance up and down. Or, the cap can be twisted the bearing interferes at opposite corners. You obviously don't want either of those conditions, or any degree of them. Modern car engines and well-built race engines require very little break-in. That is because dimensional tolerances are precisely maintained and any irregularities are very small. In older engines that wasn't so. Because of that, break-in on older engines and sloppily built engines is very important. General rule for street car engines; the older the engine, the more important break-in care that's needed. There are other factors, but that's another discussion. In the situation I described above, the tight portions of the bearing would need to be worn away without failing the bearing during break-in. When that doesn't happen the engine fails prematurely or it is never as good as it could/should have been. When a main cap isn't properly located that can happen. It's also why modern engines have some way of accurately locating the main caps.
The best "waisted shank" bolts have a shank that is smaller than at the threaded and head ends. Those are routinely used in race cars and aircraft engines, but they also show up in diesel engine head studs and other places too. Some also have stress relief/distribution holes drilled through the center of the head at one end and through the center of the bolt and past the threads at the opposite end.
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