GrahamPaige29

Connecting rod cap bolts.

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Hi guys.  My latest frustration.  My connecting rod caps are held on with "bolts" but when I was tightening them on to spec, the nuts stripped on several of them.  You can see that in the pictures. These are not standard size bolts.  They're more like pins with threading.  Since it's a Graham Paige 612 and parts are scarce, I'm thinking my only solution is to take slightly larger diameter category 5 bolts (in picture 4) and get a machinist to turn the shaft to diameter, then cut new threads. Do you think this is the best option?  The shaft diameter is .404 by the way. 

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Con rod bolts are usually made of extra tough steel. Suggest you look in the ARP catalog for bolts of the correct size and modify the heads if necessary.

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I wouldn't use grade 5 bolts under any circumstances. When I went through the ARP catalog some time ago I'll be damned if I could find anything that would work with my engine since everything is cataloged by the car rather than size. It is worth a look but if you can't find something usable, I'd go for grade 8 bolts with an NSF thread. The con rod bolts on these old cars did not have the extremely high tensile strength of ARPs bolts, mainly intended for racing or big horsepower, modern street engines. You are lucky they stripped... they are clearly well beyond use. If they had failed after you put them together, you'd have a much bigger problem. That is a strange size... I'll see what I can find that is close. 7/16 is a nominal .4375 although the actual diameter of the bolt is somewhat less.

 

Do you know what the thread is and can you measure the inside dimension of the hole?

 

Edited by JV Puleo (see edit history)
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A 7/16-20 grade 8 bolt has a diameter of .4335. You would want to ream the holes to about .4345 or .435, a .001 or .0015 clearance. You would need to get a special reamer ground which isn't terribly expensive. You can get them from Victor Tools in NY. You'd also need someone to grind off one of the flats so it was flush with the shank of the bolt. Just "turning down" another bolt is much easier said than done, especially as the cap bolt you show appears to have a ground surface. The measurement is strange though. .404 is  letter gauge drill size Y. You could get drill rod that size but it won't thread well and you still have the head to worry about. You'll also want to get grade 8 nuts if possible.

 

Edit: .435 is a 7/16 dowel pin reamer.

Edited by JV Puleo (see edit history)

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Cadillac used for a very similar bolt in there cars that is available - http://www.classicandexotic.com/store/p-1297-1923-27-cadillac-connecting-rod-blade-bolt.aspx 

 

They maybe able to make you one up with the correct size.

 

Having said that, my car (at least until I salvaged a couple of proper ones from a spare motor) had a regular bolt in a few of the rods where someone had just hack sawed off a few of the edges. It looks like it had been that way for a very very long time without incident. 

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Measure the distance from the inside edge of the hole in the cap to the inside of the cap. You would have to enlarge the hole by about .030 so the edge would move .015 closer to the bearing. If there is enough room, I'd consider this option. The Cadillac bolts are too small... I'm betting that, to make them from scratch, the cost would be in the area of $50 each. And... you need to replace all of them. If even one was bad enough to strip that's a huge red flag. I wouldn't trust any of them.

Edited by JV Puleo (see edit history)

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Oh man even the pre made ones are expensive. It think you guys are right that turning down a bolt may be a bad expensive option. I'm thinking now that I should buy existing bolts and team the holes to proper size. 

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JV Puleo thanks I'm going to explore your idea. If I can get cat 8 bolts and nuts and ream out the holes to fit them, I think it's a good option. Excuse my ignorance but what type of reamer am I looking for and do I use it in a drill press?  Is it tapered to allow it to enter the hole?  Thanks.  

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You would probably want what is called a .435 "chucking reamer." If I read their web site correctly, Victor Tool will make one for $20.50. You can also look on ebay. I saw at least two there, both more expensive, but Victor has a $25.00 minimum order in any case. I'm not sure if you can just ream the holes. .030 is about 1/32. I regularly ream that in aluminum but I generally like to get closer with iron or steel. I don't have my charts at hand but you may want a drill about 1/64  (.015) smaller than the finished hole. 27/64 would give you .4219, leaving you with .013 to remove with the reamer.  As to doing it in a drill press... a guarded yes. If you are using a modern, pedestal drill of the type commonly sold to hobbyists, no. All of these run too fast for that size hole and depth. If you have a big, old fashioned commercial drill that runs at slow speeds, yes as long as you can work out a way to hold the rod firmly in place and perfectly vertical. You should drill and ream both the cap and the rod at the same time. This calls for bolting on the cap, removing one of the bolts - drill and ream... then replace the bolt with a new one, remove the other and do that side. To do this well, you will have to be able to hold the entire big end in a vise so the two parts can't move. You may very well be advised to take the drill, reamer and bolts to a machinist and see if he can do it. You'll want one of the flats on the bolt heads removed in any case.

 

The reamer will have a slightly chamfered point that allows it to enter the hole. You want to drill & ream without moving the work piece... NOT drill all of them and then ream all of them.

Edited by JV Puleo (see edit history)

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3 hours ago, LI_BENTLEY said:

.404" is = to 1 cm  should be easy to find

 

Isn't 1cm equal to 10mm? If so, my on line calculator tells me it is .390". That's .014 too small – much too big a clearance for a rod bolt. I did think of metric bolts but couldn't find anything close to the original measurement which is why I think it is extremely odd.

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Thanks Puleo!  I'm going to go with your plan. I'll get the tools an let a machinist drill/ream them. This car is driving me nuts with all the odd size parts but at least I'll have something "unique" when it's done. Thanks again!

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I guess you just have to like, or learn to like, the challenge. If all that was available was a Model T or A, I'd quit the old car world altogether. It's the challenge that keeps me going...but, to each his own. There are plenty of people who aren't up to it and just want to pick up a phone and order whatever they need.

 

Oddly enough, I think the first old car I ever looked at was a 29 Graham Paige... a 4-door sedan with a big ding in the roof where a tree had fallen on it. Of course I couldn't afford it and my father would have had a kitten if I could... he had no use for old cars. He wasn't interested in cars at all, aside from getting him to work. In his mind, old cars were almost a criminal waste of time.

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Haha I love that my car is different. Graham Paige was a good company and the car is very solid. It'll be great when done. Thanks again sncerely for the assistance!

 

Geoff

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Out of curiosity, what practical difference would it make using a regular hex head bolt rather than the a circle with a side cut off?

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Good question. I believe it keeps the bolt from rotating so you don't have to try to get a wrench on the head while in the engine. That would be impossible. 

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17 minutes ago, GrahamPaige29 said:

Good question. I believe it keeps the bolt from rotating so you don't have to try to get a wrench on the head while in the engine. That would be impossible. 

 

if it slots down the side of the bearing I wouldn't have thought it made any different, I suspect that normal bolts dont just fit down there without a little bit being trimmed off the head

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On ‎8‎/‎5‎/‎2017 at 2:38 PM, JV Puleo said:

I wouldn't use grade 5 bolts under any circumstances. When I went through the ARP catalog some time ago I'll be damned if I could find anything that would work with my engine since everything is cataloged by the car rather than size. It is worth a look but if you can't find something usable, I'd go for grade 8 bolts with an NSF thread. The con rod bolts on these old cars did not have the extremely high tensile strength of ARPs bolts, mainly intended for racing or big horsepower, modern street engines. You are lucky they stripped... they are clearly well beyond use. If they had failed after you put them together, you'd have a much bigger problem. That is a strange size... I'll see what I can find that is close. 7/16 is a nominal .4375 although the actual diameter of the bolt is somewhat less.

 

Do you know what the thread is and can you measure the inside dimension of the hole?

 

 

I agree.  Someone used a $0.10 grade 2 bolt for the rod wrist pin clamp bolt on a previous engine repair on my truck(unknown to me) and it broke causing a rod to break and a $2.500.00 engine repair.

 

Grade 8 minimum.

Edited by Larry Schramm (see edit history)

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3 hours ago, hidden_hunter said:

Out of curiosity, what practical difference would it make using a regular hex head bolt rather than the a circle with a side cut off?

 

I believe GrahamPaige29 is correct. The flat keeps the bolt from turning when tightened. I think the shape of the head is arbitrary. In the twenties and earlier, high quality bolt heads were often made by "straddle milling", two cutters that the head passed between. Even then this must have been highly automated but, for a con rod bolt, it wasn't necessary as long as there was a single flat. Connecting rod bolts were not always as strong as we expect them to be today but they were still commonly made of nickel-steel by the teens which was when designers started to fully understand the stresses they were under. When I did the calculations for my Mitchell con rods, I came up with 3/8" diameter bolts as sufficient but, since the originals were 7/16", I went with that dimension simply as an added margin of safety, discounting the fact that grade 8 bolts are much stronger than the originals. Stresses were calculated at "racing speed" i.e. as fast as the engine could possibly turn. Thus, people could get away with softer bolts, often for a very long time as long as they did not over stress the engine. I also suspect that most con rod bolts were made to much closer tolerances than standard bolts so that there was minimum clearance between the bolt and the holes it passes through in the rod and cap. This is why I suggested that Mr. GP29 get a special reamer for his 7/16" bolts as the clearance between bolt & hole for a true 7/16 hole, even reamed, will be larger.

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59 minutes ago, GrahamPaige29 said:

That's right.  They fit up against a flat edge to prevent them from spinning. 

 

It's the same setup in my 22 cad and someone had been using a filed bolt for what looks to have been a very very very long time

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From the Franklin factory drawing for connecting rod bolts for 1922 to 1927.   .375-.373 diameter, 3/8 - 24 thread.  Material is  N.S. 21 steel.

 

Starting in 1928 they added the following note to the drawings.

 

"Note:-To be made from cold drawn N.S.21 steel showing the following physical properties:

Yield point 90,000 lbs. per sq inch minimum.

Reduction of area 50% minimum.

Elongation in 2" - 20% minimum.

Brinell hardness   196 - 230. "

 

Starting in 1929  they changed "N.S.21" to "S.A.E. 2330" on all drawings,  but kept the same physical properties requirements, so I assume SAE 2330 is the new number for NS21 as they were switching over to SAE standards.  And it seems that, going by later drawings, they kept that requirement until end of production in 1934, only adding a larger diameter shoulder to the length of bolt that is in the rod half of the journal. The bolt length in the cap half was still 3/8 inch diameter, still with a 3/8 - 24 thread.   

 

Paul

 

Edited by PFitz (see edit history)

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"This is why I suggested that Mr. GP29 get a special reamer for his 7/16" bolts as the clearance between bolt & hole for a true 7/16 hole, even reamed, will be larger. "

 

I bought a few category 8 7/16" bolts and they are indeed .4335" diameter.  If the holes are drilled out and reamed to accept this size, it should solve the problem.  It's a lot cheaper to buy standard cat 8 bolts that try to match the odd size originals by machining something else.

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