Inliners International Forums General Discussion Engines 235/261 Main Cap

I am looking for a 235/261 Main Bearing Cap GM# 3837009 it is the third one back from the front marked w/ a "R" and has a threaded hole on the top. Would anyone happen to have one of these? Thanks.

Jim Hauenstein

I will check out in the garage and leave a answer tomorrow.
EvilDr235

------------------
Two types of people drive old cars.Rich people because they want to and poor people because they have to.
Remember EvilDr235 has a cure for Your sixness.

Dear Jim;

Try Tom's Truck Parts in Miami, Ok. There # is: (918) 542-3871. That's the 'control' main, right?

Good luck, John M., #3370.

------------------

Yes, I believe so, it is the main with the thrust bearing on it.

What year 235-261 engine are you working on ? Is the casting number off your old cap and if not, where did you get it from ? If you are working on a 1953 Powerglide engine,a 1954 or 1955 first series truck 235 or 261 or Corvette 235 i have that cap according to the 1961 Chevy master parts catalog.Mine is cast with the number #3836538 and does have the 3/8 x 16 NC inch threaded hole,has the letter R and is for the thrust bearing.--No 1956 or newer engine has that hole because it is no longer needed due to a change in the oil pump design.I can get most any part you need if i have enough information to go on.
EvilDr235

------------------
Two types of people drive old cars.Rich people because they want to and poor people because they have to.
Remember EvilDr235 has a cure for Your sixness.

What I have is 1959-62 261 (Block C/N 3769925), it is not original to my truck which is a 1954 GMC 1/2-ton. The casting number I gave earlier is off my cracked main cap. The cap you describe sounds like the same part to me. Please email me so we can work out the details. cujo@rochester.rr.com
Thanks.

cujo,
I have sent you a email.
EvilDr235

------------------
Two types of people drive old cars.Rich people because they want to and poor people because they have to.
Remember EvilDr235 has a cure for Your sixness.

My '60 261 has the same type of cap (extra hole in it). '55 and earlier caps won't work because they use different bearings. I think you need to be looking for a '58 or newer one. My '56 235 didn't have the extra hole, and I believe the caps varied dimensionally from the later ones, but I'm not sure if this would ultimately matter. You will need to get it line bored as well, which may be dificult/expensive as I believe the bearings are not all the same size.

That's right they are each different. I had it done a while back & it was under $200.

I would surface the top of the block first, that way the 'crank center' is parallel to it.

Good luck, John M., #3370.

------------------

After looking in my 1961 Chevrolet master parts catalog I found the cap cujo needs.Catalog says the cap fits 1956 to 1962 235 and 261 engines.When i first read his first post,he said it had a tapped hole in it.Reading that i assumed he had a 1954 or first series 1955 engine.There is no reason for that hole to be there after 1955 and up passenger car engines and second series 1955 and up truck engines.I also informed him about needing to line bore the engine and suggested he install studs in the block,instead of bolts for the main caps.I also located the correct cap with the part number he requested.Maybe someone could help him find a set of studs to mount his main caps.Somebody must be making and selling them for old Chevy sixes.The newer main caps that are drilled still has me puzzled.Maybe military stuff?
EvilDr235

------------------
Two types of people drive old cars.Rich people because they want to and poor people because they have to.
Remember EvilDr235 has a cure for Your sixness.

for $200 you should be able to come up with a complete 261 and save yourself the trouble of line boring. Just a thought.

As for studs, fiqure out what size you need and call ARP. They will sell one or a thousand, whatever you need. Or, save yourself some money and talk to a race shop in the area and see if they have any good used ones laying around. Thats what I did for mine.

Gearhead, when you picked up the used studs for your application, what application were the studs originally meant for and were you building a 235/261? I have been on the ARP website and they do not list main bolts/studs/rod bolts for my 1960ish 261 inline six and they don't seem to list them by measurement specs either. I was particurly looking for new rod bolts and they did have two listings for chevy Inline sixes, but I know that one of them does not fit my application, since the machine shop I used had some of the rod bolts they said would fit the chevy I6 and they were too short. The SBC and BBC were too short as well, the closest match we found was for a Chrysler 440 engine and it looks like these might just work. So if anyone has gone through this game of matching currently available parts to your old I6 parts, please let us know what works. This would be a good technical reference for all of us if we could pull this information together. I would be willing to put a spreadsheet of this information and post it on this site if people are willing to email me the information they have about interchangeable parts.

There was no "application". I just knew how long and what diameter/thread I needed, and called around to some of the local speed shops until I found one that had what I needed. I only studed the rear main cap. Probably SBC,BBC or SBF. I can't help you with the rod bolts.
I had also called ARP directly and told them the size I wanted and they sent me what I needed. Make sure you ask for the nuts, as I think I didn't and had to make an extra purchase to cover this oversite. There may be other sources out there, but I haven't looked.

To all: I needed to replace the main bearing bolts in my engine ('61 261) when they started stretching as I tried to torque them to spec when measuring the bearing clearances. After making sure that the threads in the block were clean, deep enough, etc. several of the original bolts still started stretching before reaching spec. Not trusting the old bolts anymore, I called ARP. They had a main bearing stud kit for the Inline 6 '54-62 as part # 132-5402 and listed for $75.57. This is what I ordered, along with their recommended lubricant. The fit was perfect, except for one thing. The nuts on the front main cap required a socket that was too large in diameter to fit in the recesses of the cap! I solved this problem by ordering a set of 12 point nuts from ARP. They used the next size smaller socket, problem solved!
While this seemed an expensive solution, the peace of mind makes it worth while (to me).Also, my invoices indicate that the hardened nuts and washers come with the main stud kit, as I only had to buy two of the smaller 12 point 9/16-18 nuts.

I have a question about the stud kit from ARP. The original Main Bolts have a 9/16" shoulder diameter that extends 5/16" below the 3/4" hex-head, which then tapers down to a 1/2" for the rest of the length of the bolt. I was told that this shoulder was necessary to keep the main caps aligned properly, since the bolt hole through the cap is just over 9/16". The guy at my machine shop told me that the main stud kits he's seen do not take these shoulders into account and they just send 1/2" studs, which could allow the caps to move. The main caps on 235/261's do not use any built in positioning studs, so there is nothing to prevent cap movement except the original main bolts 9/16" shoulders. I have not seen the ARP main stud kit myself, so I am hoping they did take this issue into account and that the upper portion of the stud measures 9/16". Chiefoldephart can you tell us the dimensions of the studs in the ARP kit you purchased?

I guess I did not read Chiefoldephart's post very well when I wrote my own! I now see that he needed to purchase a couple of 9/16" 12-point nuts to fix the clearance issue on the front main cap. So it does look like ARP's main stud kit should work great for our 235/261 engines. Thanks Chiefoldephart for the good input, I will be placing my order to ARP ASAP.

To All: I recieved a question concerning the ARP stud kit's ability to accurately locate the main bearing caps. They do this quite nicely. The stud diameter is actually 9/16 of an inch. The end that threads into the block is necked (rolled) down to the 1/2 inch thread required. IMHO, this is an improvement over the stock system, because it puts the locating metal right at the parting line where the movement forces are the strongest. The stock bolts actually locate the caps quite some distance away.
For what it's worth: When I started the preliminary assembly on my engine, I wondered if I would have to align bore or hone the mains after I substituted the studs for the bolts. I did not, and here's the method I used to determine that. First, I measured the bearing clearence of each main with plasti-guage, one at a time, and recorded it.This is done without the rear main seal in place! All the bearing caps were then removed, and the bearings were re-lubricated with WD-40. I then torqued the main cap with the thrust bearing to spec., while moving the crankshaft back and forth to align the thrust faces of that bearing, and measured the torque to turn the crankshaft with an inch/pound torque wrench. In my case, 15 inch/pounds were required. Next, I torqued the no. 1 main cap, and measured the torque requirement again. Now it was 20 inch/pounds. Torquing the remaining main caps and measuring the torque required between each one showed no additional increase. Loosening all the caps and reversing the order of tightening showed the same results. I was satisfied! This trick was tought to me by an old time mechanic, who assembled several NHRA and APBA record holding engines! (Thanks, dad).

I got the replacement main cap I was looking for from EVILDR235 and it worked perfectly as received! I compared it visually and all the important surfaces seem to match up. I then tested the cap with plastigage and I got the 0.002" clearance I was getting on all the rest of the mains. I then removed the cap and cleaned and lubricated all the journals with WD40 and then reinstalled the cap and torqued it to spec and gave the crank the old spin test and it spun freely. I then installed the next main and again found that the crank spun freely without any binding. When I then tried to install the front main and I ended up stripping off the threads off of two different main bolts just trying to get to the first 30ft-lb torque step. I guess these 44 year old bolts have seen better days. I am glad I ordered the ARP Main Stud kit that Chiefoldephart told us about. I had already stretched one of the rear main bolts earlier, so I knew I had to replace these old bolts now or I might regret it later. Do you know what the difference between a stretched main bolt and a broken main bolt? You don't need an "easy-out" to remove a stretched main bolt! When I was torquing down one of the rear main bolts, I was just trying to bring the bolt to the final spec and it seemed like the bolt was no longer increasing in torque like it had been, so I backed it out thinking I might have ruined the threads in the block, but they seemed OK. On closer inspection of the bolt I found that section just below the head appeared thinner in diameter and the threads in this area where more widely spaced when compared to the lower threads. Later when I was checking the rod bearing clearances, I also broke one of the rod bolts, so now I had lost all confidence in the integrity of these 44 year-old bolts in this engine. I decided I would replace the rod and main bolts as well, so I gave ARP a call again. They did not have a listing for my application at ARP, but they were able to come up with replacement Head bolts and hardened washers(p/n-ARW-78, $0.70 each)that would work for the 235/261 application. We were able to replace the longer 4-9/16" bolts with 5" bolts (p/n-6AR5.000-1 $8.99 each). I measured the combined depth of the head and block w/o gasket to be 5-1/16", so with the washer and gasket the 5" bolt should work fine, as long as you make sure the threaded holes in your block are clean (I did have some packed dirt/grease in my center bolt holes I needed to clean out even though my block had been hot tanked). The 3-3/16" shorter bolts were replaced with 3-1/4" bolts (p/n-6AR3.250-1L) $4.70 each). They did not have a direct replacement for 235/261 rod bolts though, but we did come up with something close. The closest replacement turned out to be a rod bolt for a Ford 351 Cleveland (ARP p/n-154-6003, $45 for a set of 16 bolts and nuts). These were the correct diameter and length, but the head design was football shaped and the orginal bolt head is D-shaped. I was able to grind one side of the football shaped head flat with a bench grinder and it fit nicely in the rod after it was pressed in. The cleveland bolt was not threaded down as far as my orginal and we determined that we only had one thread below the rod cap surface, so we decided to add a ARP 1/16" thick harded washer (p/n-200-8505) under the nut. All total (Main Stud kit, Head Bolts w/ washers, and the rod bolts w/ washers) I spent ~ $250.00, but I feel that it was money well spent in the end. After all the money I've spent on the machine work and all the other new parts, I don't want to find out I ruined the engine due to $9.00 dollar bolt I had fail to replace when I had the chance.

Hello again.
Is there any chance the main bolts were replaced by a former owner at some time in the past.I have taken apart more early Chevy six engines than I care to remember in the last 35 years.The only time I saw a damaged or broken main bearing cap bolt was when the engine had broken a crankshaft.I would compare the bolts to someone elses for heck of it.Goodluck on your motor,
EvilDr235

------------------
Two types of people drive old cars.Rich people because they want to and poor people because they have to.
Remember EvilDr235 has a cure for Your sixness.

Cujo,
Exactly the same situation happened to me. The only reason that I didn't replace the rod bolts is that I purchased a set of new, narrowed, OEM GMC rods (with bolts) from K. Patrick Smith, along with the custom pistons required. Several people I have talked with agreed that the new GMC rod bolts would be more than adequate, especially since the new pistons are considerably lighter than the 44 ounces!(each!) of the stock GMC pistons.
Can anyone enlighten me as to their experience using these rods? chiefoldephart

I bought my '54 GMC just over a year ago and the 261 Chevy engine came with the truck and I do not know what had been done to the engine in the past. I do know that the main bolts were not not all from the same manufacturer. All of them had grade-5 markings though. As it turns out the two that stripped and the one that stretched all had the capital letters TR in the center. The four longer intermediate front and rear cap bolts, which I have not had any problems with yet, all have a smaller font capital F in the center. I also have two other bolts on the front main cap, which also have not been a problem and they are marked with a capital M in the center. So those people who thought I wasn't playing with a complete/matched set, were correct. I guess I bought a Frankenstein engine; 261 block, 235 head, and they got the bolts to put it together from a guy named Abe Normal. Does anyone know what markings are correct for main bolts in a '58-61 261 Chevy engine? My head bolts all have the same markings, a capital A inside of large circle and no Grade markings, are these original to the 261?