The Stovebolt Site Forums The Shop Area Engine Shop 235/261 Main Cap

I just got my engine back from being rebuilt and was bolting my crank in to check bearing clearances and I noticed I had a Cracked Main bearing cap! Does any ine have 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? This sure put the breaks on my engine rebuild plans in a hurry.

Just a word of caution, but I don't think you can just replace a bearing cap. As I understand it, the caps are machined (bored) in place and are aligned specifically for the block that they are used in. If you try to replace just the cap, the bearings may bind on the crank. Hopefully someone more knowledgable will chime in and let you know if I am off my rocker. I had pretty bad binding when I failed to match the caps on the rods correctly. Bound to the point that I could not turn the engine over. Once I matched the caps to the rods it spun pretty easily.

I am aware that the main cap may need to be lined bored, but right now I need to find a replacement for the one that is broke. I am hoping that the one that I find might just work as is, but we all know how "Murphy's Law" usually works.

Hey Cujo8,I'm in Webster, we met ,right? I don't have one, but I'll check around. Is there is still that old time junkyard on Redman near 104?Rd ,never been there myself.Or take a ride to Smacks Street rod shop on Ridge Rd just a little west of 350, they might know guys that have 235 stuff.

I'm afraid you're blowed up.

The #3 main cap is also the end thrust main. Failure to have a shop properly fit the new cap to the block WILL result in crankshaft journal failure.

I had the exact same thing happen to me. I had a '54 235 block bored with new cam bearings and freeze plugs installed. When I got the engine back, I noticed a crack in the #1 main cap. I planned on using the #1 cap from another engine, but it would bind the crank up. I tried to have it line-honed, but no shop in MS had a line-bore bar that could do a 235. Since a 235 has "tapered Mains" each journal is a different diameter, unlike 99.9% of the engines currently being rebuilt. So I had to throw the block away and rebuild a '60 235 instead.

I'll look thru my pile of junk and see if I can dig out a good #3 cap. I do hope Im wrong, but only a machine shop can measure the fitment and tell for sure. BTW, does your engine use shims? That complicates the issue even more. Some say that shim motors don't need line-boring, and some say its more critical. Again, consult your local quality machine shop. Which yours doesn't sound like a good shop, any half-decent shop magnafluxes the parts for cracks before working on it.

Geez, I keep hearing all these crumy machine shop stories, the first 235 I had machined went to a local shop who pretty much does general work on newer engines and of course 350 Chevys.I asked him if he needed engine clearance specs, he looks under the counter and pulls out a big manual. And there is all the info,bore centers,crank grinding specs,clearances, maximim overbore,torque specs,all that stuff and even illustrations .Seems to me,all shops should have this info and they should be experienced enough for proper handling procedures, it ain't brain surgury.The customer always bring the block with the main caps in place, if one is broken as Chip says , then the shop is at fault.But they might try to pass the buck. Cujo, what shop did you use?

Hi Tony, yes we have met before. I just keep getting deeper and deeper into this truck restoration. I have the whole front clip off of my '54 GMC and I feel I am always taking things off of the truck and never putting things back on. I went over to my machine shop and he says we may be able to repair the cap be putting in a sleeve and welding the cracks. I hope that I can salvage this situation and not end up trashing the block due to this main cap issue.

The machine shop I used is called Three-S Engine and Machine located in Palmyra NY. I had used there services 22 years ago when I was building a 350 chevy engine and had no problem then and many people recommended them to me again this time as well. I had noticed the crack in this Main Cap when I brought it to him and I specifically pointed out the crack and asked him for his opinion on what to do about it. His opinion was that the crack was due to the fact that Chevy did not use harded washers to spread the load and that the bolt had probably cracked the cap when it was torqued down the last time. I asked him if I should get a new cap and he said that if I did get another cap the engine would have to be line bored, which be expensive. He then went onto say that if I just use the proper harded washers that it would be OK. He gave me what he said were harded washers and I went home to give it a try. But the washer trick did not work and the cap cracked worse when I tried to torque it to spec. As it turns out the washers he gave me were not hardened either, since torqueing process tore the washers up. I talked to a friend of mine who is a professional machinist and He is looking into the possibility of welding and inserting a sleeve to repair the cap, but he is not too sure that this will work for the long haul. He did say that if I was to get another cap he could machine it to an exact match of my original (minus the cracks of course). I think that this is my best option as long as I can find a good used Main Cap, so please let me know if you know where I can get one. Thanks.

I understand how they shave the caps and align bore but how do they redo the thrust surfaces?

THE FACED SURFACSE OTH THRUST BEARING IS MOST THE TINE THE SAME YOU NEED TO FIND #3 SHAVE ALL THE CAPS DOWN AND LINE BORE THE CRANK.
THINK ABOUT IT THE thrust bearing slipes between the caps not a problem

Cujo8, I would follow Chiphead's lead on this one- he's been there and back again (even has the T-shirt and ball cap)- To trust a local machine shop to "line bore" is one thing but for "align bore" you are talking somethin out of their reach. If you had a rare engine that couldn't be replaced there are a couple of ways to go- get a cast made of you orignal bearing cap and have it brought back to specs- send the whole nine yards to the place in Washington (My wife Sleeper55 found it) and have them repair the faulty cap and restore it to new condition. Either way is mega dollars and it is much simplier to get another 235/261. You could try to weld(?) the cap or use another one but that wouldn't be advisible because it will probably cost you "ALOT" more in the long run. It's a bummer I know, but the best advice is to listen to what Chiphead has to say about it.

I worked at a local machine shop, one that has a great reputation. It scared me to see the $5 an hour cleanup man haul blocks in & out of the hot tank by a single hook on a main cap! When I saw your post this is the 1st thing I thought. Brian

Cujo-

What year is your engine? I have a #3 main cap from my infamous '54 full pressure shim-style 235. It uses the earlier dowel-type bearing inserts. It has the threaded boss on the top for the early-style oil pump pickup. If you want it, I'll send it for the cost of shipping if I can find it in my shop. If you can save the engine, I also have a good '54 oil pan and a '54 reman water pump I never used that I'll let go cheap.

email me

chip

I got the replacement main cap I was looking for from EVILDR235 (Inliners memeber) 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 (Inliners memeber)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 **** 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.

cudo's cujo!!!

I have been watching your story and all the good input you have been getting over on the inliners site. Glad to hear you got your cap problem behind you and it sounds like you are well on your way to having your motor ready to go back together! Bet you feel a lot better about things!

cujo - you should also look at your torque wrench. We design robotic assembly machines for a living and I used to use my torque wrench to double check how accurate some of our screwbots and fastener tighten heads were . . . what I found out was that my torque wrenches were sometimes as much as 20% out of spec. That is huge when it comes to properly torquing bearing caps and headbolts.

Where do you go to have your torque wrench checked for accuracy?

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?

Cujo8,
there is definitely something amiss with all the bolt problems. The very early Chev engines, maybe in the 20s had problems with the steel and bolts could not be reused like today. Your engine is much too modern for that. Age does not cause problems like that. I would suspect that they were over torqued by a previous owner and that would fit well with the cracked cap. The other possibilty is that your torque wrench is bad, really bad.

You gave a perfect description of a stretched bolt, almost a textbook description! I was reading an Engineering textbook on bolted joints a while ago and would you believe that the force is all taken up by the first few threads and all the rest are just along for the ride. To actually use more of the threads, the bolt and the joint have to be designed to distort, which these are not.

You could only check your torque wrench against another that you know to be within spec. Basically a torque wrench has to cost quite a bit of money to be any good. Then they get dropped and messed up. Also, you don't want to use one too big for the job as the bigger they are the more they deviate.

The best value in inexpensive torque wrench is the ones that bend with the arrow. The clicking ones are generally more inaccurate unless $50 and over. IME

cujo-I have a 60 261. The front 3 main bearing caps (6 bolts) have a M in the center with 3 hash marks. The rear main cap (4 bolts) have a O in the center. The engine was professionally rebuilt.

From Rusty46's post it seems that the Main-Bolts with the M would appear to be original to the engine. I only have two of M marked bolts out of the ten necessary for my engine. Like I said earlier the only bolts which gave me trouble were the four that were marked with TR (Two stripped, One Stretched, and One was even slightly bent). The torque wrench I used is a Sears Click type wrench w/ 1/2" drive w/ a torque range of 25-250ft-lbs and now sells for ~$99.00. I have owned it for at least 15 years and I have never had it calibrated. I just read Ken's post about Torque Wrench Calibration and I now know I have not been storing my torque wrench correctly ever. After using my torque wrench I just put it back in my tool chest at what ever torque it was set at last. I never loosened the torque knob to the low setting unless I was setting it for use at that range. I live in Upstate NY and I never thought that extreme temperatures (High or Low) would effect the calibration of the torque wrench. Here in NY we do not have a problem with the hi temperatures, but we sure do have the cold temperatures. I am going to have my torque wrench calibrated and it will be interesting to see just how far it is off.