The Stovebolt Site Forums The Shop Area Engine Shop Possible 261 block with the mystery "M" casting mark

Hello, I went to look at what I thought was a 261 today sitting in a Canadian-built 1960 C10. Based on the story from the owner, the previous owner replaced the original 235 twice with GM replacement 261 blocks. I had a heck of a time identifying it as it has what appears to be 3836848 head for the 235, but the block has captains bars above the starter. I couldn't find the other set of bars on the driver side but it was dark and the manifolds were blocking my view. Where the block casting number should have been I found a capital "M" inside a square and that's it. There is a lot of years of gunk on the block but I just could not find the number where it should be between the distributor and fuel pump. It also appeared to have the full flow oil filter. Since this is a 1960 truck I believe it would be almost impossible for this block to be the '53 Powerglide 235. So is this a 261? Does the "M" mean anything to anyone? Thanks. James

Look for these number between the fuel pump and starter, they signify a late model full pressure 261. 3739365 3769717

Aren't all 261's full pressure?

I know which casting numbers I need to look for but they just aren't appearing in the usual places. There is a lot of junk on the block preventing a good thorough inspection though. I wasn't able to clean off the other usual casting number spot just in front of the fuel pump. My dilemma is I had arranged to come this weekend to pull the motor out and take it home, but I can't know for certain what the dang motor is! I am worried that I will do all the work to pull it only to find it is a 235 haha. I guess the better question is do the presence of captain's bars somewhat confirm what the motor is? How likely are those markings to appear on a 235 other than the powerglide motor? Anyone form the Canadian side with experience with Canadian 261s out there? Thanks.

If it has the low mount water pump and captain bars, it is a 261.

To be absolutely sure, pull the cylinder head. A 261 has a 3 3/4" bore. A 235 is 3 9/16" They both have the same stroke but the 261 crankshaft is forged, not cast. Jerry

I think I read somewhere that the Square with a "M" was the logo for the Mansfield Foundry in Canada...someone will correct me if I'm wrong. Mike B :) 1953 Panel straightened me out...the Square with a "M" was the logo for McKinnon Industries...Thanks, Leo

The Canadian built 261 motors that were installed in the passenger cars, to my knowledge, all used the by pass style oil filter but you never know, nothing is ever absolute. Both of my 59 Canadian blocks are bypass. I also understand, although I have not seen one myself, that the Canadian built 261 pass car motors did use an 848 head, not the more common 261 heads. The Canadian Pontiac 261 Pass motors also used a valve cover that was identical to the later 235 chevy valve cover, but no script. I have one of those valve covers if anyone is interested in seeing one and would be willing to post a photo for me.

With a full flow block, it would appear that you have a later 261 truck block but really the proof is in the pudding. You will need to get the casting numbers, date and as above, really the only fool proof way is to pull the head and look for the steam vents and measure the bore.

Ok I think that is a good plan. I was hoping to take the whole thing home, clean it up and test run before sinking any more money into it. I am guessing the head gasket would be toast once I pulled the head off?. I am a little concerned about whether the shop that put the 848 head on there actually drilled out the extra water holes. If not we may have some cracking from overheating. Mike B: that is interesting info on the "M". I did a quick search and found nothing on this facility. More info from others on that would be appreciated. Thanks.

If the head is removed you will know immediately if it's a 261 by the 3 pairs of small holes in the deck but bring a caliper to determine if it's been punched out. 3.830" would be .080" over and the end of the road for boring. All Canadian Pontiac's with a 261 had the 848 head drilled by GM of Canada (note that the holes are on a 12 degree angle into the water jacket), bypass oiling, hydraulic lifters, and a fiber timing gear. Look for green paint. Some are alpine green like US engines but I've seen several with a dark olive color. Good luck.

Even if it's bored way oversize, a 261 block is worth sleeving back to standard bore. A 3/32" wall sleeve doesn't require a great deal of material to be removed from the block, and the sleeve is usually better quality metal than the original block alloy.


Jerry

Not in this neck of the woods...besides myself I know of at least a couple other guys in Alberta with multiple 261's stashed away. 55-62 6 cylinder Pontiacs are plentiful around here.
.

What are they selling for? It might be worth a road trip to acquire a trailer load of them.

Not sure what you are thinking here Jerry, I have never seen a cast crank in any 235.
1954-63 235 and 261 cranks are "forged" 3701488.

Forged yes. Although they do look sort of cast at first glance. Even as far back as 36.



Look at 12:28 through 13:06 or so.

Those are some real hard working men manhandling those forgings around.

That is amazing. I’ve often wondered about crank machining and forging.

I ran a Canadian 261 in my truck for many years. The "M" you speak of stands for McKinnon Industries, which was an engine plant in St. Catharines Ontario. My uncle worked there for many years.

http://hotrod.gregwapling.com/chevrolet/mckinnon.html

What are the head and block casting codes, and, what are the head and block date casting codes?
http://1954advance-design.com/Web%20images/casting-code-photos/codes-ser-nums.html

Thanks Panel for the info. I noticed the same M on my differential housing as well. Very cool to get the history on that.

The head is an 848, haven't had the valve cover or starter off to find the rest. The block casting code is elusive. I am pulling the motor tomorrow... if it is a 261 code I will take it home. If not, it goes back in the truck for the next guy. I will update once I get that far. Thanks all.

James

I agree with those who have said that the (late 50s? to) early 60s Canadian Pontiacs used 261 with bypass filter, captain's bars, low mount water pump and 848 heads and plain valve covers. Many seem to have Rochester carburetors but I've seen at least one that had a Carter. The Carter car had a powerglide but I don't know if the manuals got one carb and the auto the other. I believe the Carter was original to that motor and certainly some of the Rochesters were retrofits (modified bolt pattern and linkage).

Just wanted to follow up, I found the block code tucked behind the starter and under a heap of engine gunk, just below the first set of captains bars. 3759365 (52-62 261). The date code, found on the other side next to the other set of captain's bars specifies '59. It has the bypass oiler set up.

So I guess the lesson is the codes can be in many different spots!

Thanks for all the help.

James