The Stovebolt Site Forums The Shop Area Engine Shop 261 block numbers help

Well I had an oportunity to drop down to Arkansas and work on my 56 3100 last weekend. I took a scope with me this time. I checked the cylinders on my 261 and saw rust and scoring so I am going to have it rebuilt. I also took photos of the block numbers.

Above the starter CON 4. D 9 8
below the vacume advance 3733813. (1956?)
flat spot by distributor is blank.
Head 383685. If there is a number after 5 I could not make it out.

Wondering which year it is since the flat space is blank.

Have a good day

I found a better photo of the head and was able to zoom into the casting number.

Head casting 3886850
OR. 3836850

to me the third number looks more like a poorly cast 8 than a 3.


See attached.

Your block casting number more likely reads 3738813, which corresponds to a 1958-59 Chevy 261 block.

The block casting date code D 9 8 decodes as D = April / 9 = 9th day of April = 8 = 1958 model year.

The serial number pad being blank is a sign it was a replacement block (warranty or purchased to replace another engine)

The head casting number reads 3836850 from what I can see. That indicates a 1956-62 261 original head. The key to know what year is the heads date casting code, which is found under the valve cover, just inside the gasket rail on the driver side by the carb location.

Hope this helps clear things up.

Note: This info is provided if you find that the head and block dates are not consistent with the same year of manufacture. The head build year is important as there were some changes to the later 261 blocks and heads that could significantly affect engine operation. I learned that when we finally got down to the nitty gritty on my 261 currently being rebuilt. I have a 1962 261 replacement block which has the later oil passage ways for feeding the top end, but an earlier 1957 261 head that does not have the same passages. Due to changes in the internal oil passages for the cam and top end oil feed, there isn't a corresponding path for oil to go directly to the earlier head internally. To provide top end oiling, it requires an external oil feed line from the oil pressure feed port on the drivers side of the block, all the way around to an external threaded port into the center passenger side of the head to provide lubrication to the rockers. This was also a "fix" used for earlier 261's that had problems with blocked oil passages to the head which starved the rockers and caused early wear out of components.

Gdads51,
Thank you for the additional information.
I took the photos of yesterday morning before my drive back to KC and did not take the valve cover off, I will do that next time to verify the #s.

In the photo I attached showing the block numbers 3733813. The fourth number apears to match a 3 more than an 8. which from the numbers I found indicated a 1956 261.

Is it possible they used a 1956 mold to cast the block in 1958?

Though it could be a red haring, the oil fill hole in the valve cover is in the front of the cover like those from 55-57.

Fourth number on block looks a lot more like a 3 to me as well, rather than an 8. The radius of the circle to create the top part of the number 3 is larger than the radius of the circle that makes up the top of the number 8. As a number 3 it is consistent with the other number 3s rather than the other 8 which has a smaller radius on the top circle than the bottom. Number 3s are the same top and bottom.

After relooking at your pictures and diving through all the casting number resource links I have saved over the years, I agree that it does in fact look to be "3733813". My apologies as there is also the very similar casting number 3738813 I mentioned, which is where my brain thought it need to go.

With that in mind, looking at the following casting number resource lists, all of them indicate that this casting number "3733813" is for a "1958" 261, which corresponds with the block date casting code.

Jim Carter 6 Cylinder Casting Number Data

Greg Wapling's 6 Cylinder Casting Number Data

Dave Dawn's US and Canadian 6 Cylinder Casting Number Data

Big Dog Garage 6 Cylinder Casting Number Data

I'm interested to know what source you found that indicated it as a "1956" block???

I did try and confirm through various Chevy Master Parts Books, for both of these block casting numbers, and couldn't actually find it listed in 1957 or later 1959 books. I don't currently have a 1958 version to check, but plan on try to find one eventually.

Bottom line is either way, you have a 261 and that's a great thing IMHO.