The Stovebolt Site Forums The Shop Area Engine Shop Educate me on a Chevy 261

I picked up a 1962 Chevy 261 engine off Marketplace today. My plan is to replace my 235 in my 1953 Chevy 3100 pickup with this. The 235 engine runs ok, but I wanted to have a fresh rebuild in the works. Not sure if the engine is sound or not, but the oil is golden and looks brand new.

All I know is that the 261 were very similar to the 235, but they have more stout internals, full oil filtration, and slightly more power.

Any tips or advice on doing a 235 to 261 conversion would be great.

should be a bolt in. i have a friend that did it and also installed a late model 3 spd with an overdrive. What a sweet rig to drive, it runs down the road perfect.

*should* be a bolt in -- if somebody already boogered the upper radiator shroud when they moved the radiator forward to accommodate the 235's water pump. And changed your engine mounts... and you already have the adapter plate for your water pump ...

After 1954, the 261's were built to go in the Tack Force and later trucks, thus they have a lower water pump than on the engines built for Advance Design trucks. You will probably need to get the adapter plate to raise your water pump to get better cooling (otherwise half of your fan disc will be below the radiator.

Also -- the Advance Design trucks had the one engine mount at the front (under the timing gear cover) and two at the bell housing. That '60's 261 will most likely have engine mounts on the sides near the front of the engine. Not too bad of an issue if you are going to rebuild the 261 anyway -- just use the plate off the front of your 235 to switch the 261 to an AD engine mount. Otherwise you will most likely need to fabricate some sort of an engine mount.

For more info on rebuilding a 261, check out our tech tip we wrote when we rebuilt mine for my '49 3804.

Rebuilding a 261 for use in an Advance Design Truck

But once you have all this laying flat, its a great upgrade.

John

Thank you John and Jay.

The 235 in my truck now has the lower water pump (it’s a 1955 235 engine), meaning the fan isn’t centered very well. So either way the water pump relocation will need to be done.

I’m excited about having a good 261 core to rebuild while I still have the 235 to tool around in. I haven’t rebuilt a Stovebolt personally, but I’m looking forward to learning more.

I did see that this 261 has side engine mounts and a few other things done that makes me think it was possibly a power supply unit for a generator. The bellhousing looks very custom. It also “might” be stuck, but I haven’t tried with any leverage yet. I have it soaking in some marvel oil right now. Will report back if I tear into it more. I don’t see any visible issues on the outside of the block to indicate anything major, but the inside will tell more of a story. The good news is that the oil is pure golden and appears very clean. Hopefully it’s been sitting in clean oil for awhile…

There's a better (and less expensive) "free up a stuck engine" chemical than MMO. Pick up a quart of acetone and a quart of cheap, house brand automatic transmission fluid at WalMart. Mix them together. You'll have half a gallon of penetrating oil that rivals KROIL for rust busting ability. Put several ounces into each cylinder, let it sit a few days, drain the residue that soaks through to the oil pan and reuse it. While you're at it, pull the valve cover and soak the valve guides- - - -they can get stuck badly enough to bend pushrods if you force the engine to turn. Obviously, you'll need to drain the crankcase first. Save that "clean" drain oil to use in a pump oiler around the shop.

The 261 is basically a 235 on steroids. Chevy increased the bore size to 3 3/4" from the 235's 3 9/16" bore, added some beefed-up connecting rods, and on some engines, they used a bigger, high flow thermostat with a bypass system to avoid hot spots in the cylinder head during warmup. The crankshaft is pretty much identical to a 235- - - -same stroke and firing order. 261s ran a solid lifter camshaft, the same one used in the 53-54 Corvette six. It's slightly better than the 235 cam, torque-wise. A good substitute for the discontinued 261 cam is the Melling CCS-1- - - - -same intake timing and slightly more exhaust duration for better cylinder scavenging without causing a lumpy idle and erratic intake manifold vacuum at idle. With a few custom parts and some serious whittling on the crankshaft, it's possible to get close to 300 cubic inches and almost 10:1 compression from a "261"!
Jerry

I freed up a stuck 235 using acetone and transmission fluid in the cylinders and around the stuck lifters.
First thing I did was pour several ounces of straight acetone in each cylinder and at the top pf the lifters.
I did this to let the super thin acetone have a head start to penetrate and make an easier path for the 50/50 acetone/transmission fluid to get into.
One unforeseen benefit of adding the straight acetone first was that the two really stuck cylinders were slow to let the acetone to drain through so that I could focus on gently pounding those two pistons down first. "Gently" needed to be gradually upgraded until the stuck ones finally moved.

Once the cylinder head is off, getting stuck pistons moving is a lot easier if you make up a jackscrew from a rectangle of 3/4" steel plate that bolts to the head bolt holes, with a piece of 3/4" ACME thread rod and a nut welded to the center of the plate. Put a piece of round steel plate on top of the piston, and tighten the jackscrew a little at a time. Then hammer on the top of the jackscrew. "Rinse and repeat"- - - -just like shampoo!

www.ebay.com/itm/206353197611?


Jerry

The 1960-62 Chevy 235's and 261's used a different style molded one piece front motor mount in the same place as the earlier multi-piece Art Deco & AD front mounts. It bolts to the front engine mount plate and cross member with a different bolt pattern than the AD ones and also stands a good bit taller than the AD series front mounts.

Here's a thread that discusses the 1960-62 235/261 front motor mount to help see what it is.

Here's a really on-topic thread discussing what is needed to convert the 1960-62 235/261 front mount to the Art Deco & AD style mount.

I'm going to have to do the same process when I go to install my 1962 261 (heading out for rebuild next week) into my '51 3100.

Hope this is helpful.

Had a little time over the weekend to dig in to the 261 engine I have. First thing I noticed is that the rocker assembly and top of the head are very clean - not sure if this head has been gone though in the past, but I was pleasantly surprised. Push rods in great shape. They have a black coating on them, and look brand new.

Cylinder #1 appeared to be the only cylinder where the Marvel oil didn't seep down. I will be soaking it more over the next week with the acetone/ATF as suggested by a few on here. I noticed that there is no crank bolt on the front of the pulley, and this engine also has what I think to be a custom bellhousing. Therefore using a flyweel turner and/or crank bolt method is out for the time being. Does anyone else have ideas on how to possibly rotate this engine?

Another thing of note would be that cylinder #2 looks to have a little scoring from the valve on top of the piston. Is this normal for its age? Either way the pistons will be coming out - so not necessarily a huge issue.

Just an FYI that the head casting# 3835913 isn't an original 261 head. Listed as fitting 1954-55 235's with the small temp sender hole and 4 bolt valve cover according to our own "tclederman".

It's actual the same head I have on a '54 235 out of a passenger car with hydraulic lifters that's about to be replaced with a '62 261.

They can certainly be used on 261's with the proper holes drilled to open the coolant passages into the head.

Just as an FYI, an original "261" head would have the casting# 3836850 for US built engines. I believe some Canadian built engines used the 3836848 casting number heads.