1936 Roadmaster broken pistons

[Bill Stewart]

I recently purchased a mostly original 1936 Roadmaster. An inspector said it was "smooth and quiet. It was not. It has four broken pistons where they have hit the head on the rounded area below the plugs.Two pistons have an additional break and one has lost both compression rings. Surprisingly, cylinder walls and valves are undamaged. Seven cylinders had about 80lb. compression and the "ringless" one had 0lb. 

The head gasket seems thinner than normal and does not have the copper on both sides that I expected to see. Can that account for this? I wonder if the head has been shaved considerable to correct a warp. Does anyone know the original vertical dimension of the head? Are there thicker gaskets? can I carefully grind material away where the pistons have been hitting? Will newer design domed pistons fit? Any thoughts would be much appreciated. This is my first(and unexpected) shot at engine work.

[Gary W]

Hi Bill;

I was able to successfully install 1938 "domed" pistons in my '37 that had the original "flat top" style.

If you have time to read through the first four pages of my restoration thread, there is a lot of engine work performed early on in my restoration.

 

I don't know about clearance of your '36 compared to the '37, but we simply bolted one piston to it's rod (no rings yet) and sat the head over the block with no gasket and there was enough clearance between the dome and the head.  I got the pistons from EGGE, but you can also try Terrell Machine in Texas, Bobs Automobilia, CARS, Inc.

 

Like I said, I have a 1937.  You came to the right place here.  These guys are first-class.

 

Good Luck!

Gary

NJ 

 

(Click the arrow on the top right corner will bring you to page three, fitting the pistons.  Or click the main body to read from page one)

 

[MCHinson]

Bill, Welcome to the AACA Discussion Forum. I have moved your question to the Pre-War Buick Forum. I am sure you will find someone here who can offer you some good answers. You might also want to consider joining the 36-38 Buick Club. The club has some very good technical advisors who could also help you with questions about your Buick. The club's website is http://www.3638buickclub.org/.

[Janousek]

I don't think grinding away in the area that hits is a proper fix.   Yes it would probably work but something else is wrong.  I'm with you on the head being decked to much.  It might be an overheater if the compression is bumped up that much.   I measure .060" on a couple copper gaskets I have laying around that have been installed once.  

 

I'd look into the 36-38 club also.  

 

1.  Look into the head or block or both being decked to much.

2. Possible improper hardened seats that raised the valves.

3.  How are the valves working?  Lift and sealing completely?

4. Play with some clay and see what your valve clearances are with a new head gasket.  

5.  Not sure on the "domed pistons" but a little quick research should tell you if they are compatible.  I think they are as my old motors manuals show the early engines bumping the compression a little each year without stroke changes.

 

I would build the bottom end with the new dome pistons and then start your measurements for clearances keeping the possibility of another head in mind.   Or just buy another head.  Cant have too much of this crap lying around.  

[Bill Stewart]

Good ideas. I am a member of the36-38 club where Jon Kanas has been more than helpful to me.

To answer your numbers--

1.  I can think of only two things that could account for the problem--too much removed from the deck and/or head, or, less likely, a very thin gasket.

2. Hardened seats seem to be properly placed.

3. Valves are still in the head. They look perfect so far. Valve action looked fine when running, tappets correct, etc.

4. Clay is a great idea! Valves don't seem involved with my problem, but couldn't it be used to check clearance between piston and head?

5. I've learned that the 38 domed pistons will work.

I do have an extra 1936 head still on a parts car not run since about 1956. My wife thinks its possible to have too much of this crap lying around.

 

[Janousek]

I find the thin head gasket hard to swallow.  Clearances aren't that tight on these engines.  The hot rod guys talk about decking  .25 on a 248 head to bump the compression.   When I built our 31' I had pistons made that were much taller than stock and still had plenty of clearance after the block and head were both decked.  Makes me wonder if the wrong pistons were installed.  

[Bill Stewart]

I'm quite sure the proper pistons were installed, but its obvious they travel too far. The area they hit below the plugs is a flat crescent shaped surface flush with the rest of the head, as though it was originally raised but has been machined flat along with the rest of the head. It looks like it could have originally been rounded. I can hardly believe that so much might have been taken off the head. Maybe someone could provide a picture of an unmolested head showing that area? I do have access to another head but it is likely to have its own issues.

[Gary W]

Hi Bill;

Here's a few photos:  Maybe you can zoom in to see what you are looking for?

 

 

'38 style "domed" pistons fitted without rings to check clearance between the dome and the head

 

 

Installing the pistons.  You can see how much the '38 style protrudes above the block and there is still plenty of room.

 

 

Head.  Back from my machinist with new valves, guides

 

 

The last photo just before installing the head onto the block.  "Graph-Tite" head gasket.

Again, maybe you can zoom in on the head to see what you are looking for?

 

 

Gary

[Bill Stewart]

Thanks. The combustion volume (shape) of your head is quite different than mine. Yours does not have the "bulge" below the spark plug that that intrudes into the combustion chamber. That bulge appears to be partly machined away on my head resulting in a flat crescent shaped area in the same plane as the surface of the head----and gets hit by the piston. Kind of hard to describe----I hope that's understandable. I sure would like to see a picture of the underside of an unaltered 1936 320 cu. in. head. 

[KongaMan]

If you could post pictures of your engine and head, that would help immensely.

[Bill Stewart]

You sure are right. I'm having trouble dealing with pictures on my computer. When i figure it out I'll do that right away. Thanks.

[Bill Stewart]

KongaMan, Piston damage on both pistons (and two others} all where piston comes up under spark plug. That flat area is on the same level as the rest of the head surface as though its been machined. Additional damage in other places is probably from broken rings or piston parts banging around in there. No compression ring pieces from the really awful piston were still in the engine when the head was taken off. So----did someone take way too much off the head? I'd like to know what a head looks like that has not been altered. I'm getting used to the idea that i will probably be getting a different head. I can hardly imagine that the pistons are too tall. Surprisingly, cylinder walls are great and rod bearings are between .0015 and .002 

[Guest]

A

Edited December 11, 2017 by Guest (see edit history)

[KongaMan]

Lessee...

 

If the head and the block are both ground flat, why is there only damage on some pistons and not all?  That seems a bit peculiar.  If there was damage on only one, you might be able to chalk it up to a screw getting dropped down the carb, but this would require several screws (or a 3-year-old). To that end, which 4 pistons are damaged?  Would they all be fed by the same barrel of the carb?

 

Are all pistons identical?  Did you measure the rods?  Did you try turning it by hand with the head off to see how much each piston comes up out of the block?  Are they all the same? 

 

I don't suppose there'd be any chance that chunks could've worked their way into the oil pan, is there?

[Bill Stewart]

KongaMan thank you for your ideas. I've had computer hassles so haven't been able to respond.

I don't know why only some pistons (#1,2,7,8) were damaged. All eight come up almost exactly flush with the top of the block. A tiny amount of carbon buildup could then cause the pistons to hit the head. At any rate, I don't think it makes sense to risk using that head. The block needs to be bored and i am having difficulty finding 36 pistons. 1938 pistons are everywhere in various sizes so I think it seems sensible to use a 38 head and pistons if that does not introduce other complications. I note that exhaust manifold part numbers are the same for 36, 37, and 38 but intake manifolds have different numbers. Anyone know what the differences are? I have a Stromberg carb. 

There were no chunks in the oil pan. Apparently all pieces went out the exhaust valves! 

[Ben Bruce aka First Born]

Bill, to my old eyes, the damaged pistons exhibit classic detonation, pre ignition or whatever one wants to call it. In other words, running with the ignition too far advanced.  

 

 As to intake manifold being different, is the '36 an updraft carb?

 

  Ben  

[37_Roadmaster_C]

I do not know about your 36 320, but my 37 has a new rebuild with 38 pistons from Egge and there were no problems at all. Egge also did the machine work on the rods for insert bearings and they also were a perfect fit.

[Bill Stewart]

KongaMan thank you for your ideas. I've had computer hassles so haven't been able to respond.

I don't know why only some pistons (#1,2,7,8) were damaged. All eight come up almost exactly flush with the top of the block. A tiny amount of carbon buildup could then cause the pistons to hit the head. At any rate, I don't think it makes sense to risk using that head. The block needs to be bored and i am having difficulty finding 36 pistons. 1938 pistons are everywhere in various sizes so I think it seems sensible to use a 38 head and pistons if that does not introduce other complications. I note that exhaust manifold part numbers are the same for 36, 37, and 38 but intake manifolds have different numbers. Anyone know what the differences are? I have a Stromberg carb. 

There were no chunks in the oil pan. Apparently all pieces went out the exhaust valves! 

[Bill Stewart]

Robin, thank you. Now I'm trying to confirm that 38 pistons will also work in my 1936 320. Anybody know for sure? 

[Bill Stewart]

Ben, I was only able to run my engine for a few minutes (and wouldnt have done it at all if i'd known what it looked like inside) but timing did not seem too advanced. The carb is a typical downdraft Stromberg.