The Stovebolt Site Forums The Shop Area Engine Shop 261 block and 235 cyl head

Hi I'm new here and to owning a stovebolt engine my 1950 Chevrolet motor was no good when I purchased it I found a add in craigslist for a 235 for sale .the motor rolls over smooth so I removed all the spark plugs and looked inside the holes the cylinder walls look really nice recently cross hatched and the tops of the pistons ate new no carbon .I think I scored a good motor for 350.00 but when I got it home I found that I have a 1958 261 engine block with a 1956 cylinder head from a 235 .I'm kinda confused here because what head gasket do you use are the heads the same .should I look for a 261 head . the guy I baught it from told me he got from a co worker that told him it runs it looks like it will run but will it run properly .

The 261 is a very desirable engine, but the 235 cylinder head will need a little modification to make it work properly. The 261 cylinder walls are "siamesed" together so there's no coolant flow between them. There are holes drilled in the cylinder head to allow coolant to flow from the block to the head beside the cylinders and prevent hot spots. The 235 head needs to have some holes drilled in it at the proper location and angle to avoid cooling problems. If you search the forum archives for "steam holes" you should find much better instructions on how to do the modification than I can provide off the top of my head. The 235 head with "848" as the last three digits of the casting number is the best head to use for that application.
Jerry

My 261 has a 848 head from a 235. Works very well.

Jerry's advice may sound intiminating, but I recently did that process myself on my 235 head mated to my 261. It was very,very
EZ! No problems at all. I just made sure the head was secure to my bench top and I made good punchmarks for the drill bit using the head gasket as a guide.

Drew, I hope you angled the holes properly to intersect the cooling passages. It's possible to ruin an expensive head unless the angle is right.
Jerry

Many years ago before there was this Internet thing I drilled the steam holes in an 848 head.Did it hand holding a drill while securing the head between by feet.This head went on a 261 that ran perfectly for several years.Many other guys I know did the same.
Then somewhat recently old GM info comes up showing the steam holes in the 261 heads were drilled at a 12 degree angle facing the port side of the head.
This is what I think;some castings may vary,perhaps the actual 261 head is cast slightly different internally.Or all the guys who drilled straight got lucky.
Let's take a poll on who here has drilled the steam holes themselves.Ask if they drilled straight in,then ask if they ruined a head.

I drilled mine 8 yrs ago, straight no angle. No problems.

Look here for more information including a section view showing exactly why drilling straight in is a maybe yes maybe no proposition.

Thanks, Curt- - - -that was the thread I was thinking about. As usual, it seems that luck beats skill often enough to make some folks think they're 10 feet tall and bulletproof. I prefer to use a Bridgeport mill with a tilting head for drilling angled holes in expensive cylinder heads. We relocated the spark plug steam holes while angle-milling small block V-8 heads that way.
Jerry

Yes, I did Jerry. I went back and read all the links on this process. Thanks for your concern though.

The link with the photo of the drill angle is great information. One question: if the hole is drilled slightly larger than the steam holes found on the head gasket...will there be any potential head gasket leak or other concerns about that? Someone at one point drilled into the 848 head on my 261 and the holes are slightly larger than the gasket holes and slightly offset.

I'm thinking that by following the correct tightening sequence and ensuring that the spray copper head gasket sealant is applied properly aroud those holes...that there shouldn't be a big concern. ?

Thanks in Advance. Tom