21 stud flathead

Z

zgears

Member
36 21 stud. I removed the head to repair a cross threaded plug hole, and found that the cylinders have rust damage. Its allready .80 over. What should I do with this motor?


 
great question, how does it run, does it run smooth idle good, does it use oil and smoke, from the pics doesn't look like anything abnormal to me but maybe we cant see the rust, looks like normal cyl. blueing I put head gskts in mine 3 years ago if I remember it looked like this
 
If it was running ok and not using oil before you removed the head, I would say clean the carbon from around the valves, the deck, head surface, etc., and put it back together with a new head gasket. You might be wringing your hands and over worrying needlessly.
If it turns out that a rebore is really necessay, the '37-E'38 21 stud blocks can be bored more than the earlier 21 stud blocks. Walt Dupont usually takes them up to a 3-3/16" bore (0.120" over).
 
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The rust out in 2 cylinders is significant, the motor smokes some. I guess the question should be is .80 over the limit for a 21 stud? can it be sleeved, can it be all sleeved to 24 stud size?
 
JM has answered that question; you 'should' be able to go out to .125" oversize with that particular block
 
If you hadn't cross threaded the plug hole, would you have been happy to continue running it, even though it smoked a little?

Can anyone back up JM's statement that these motors can be bored +.125? How about the distance between the lower stud and the cylinder bore and gasket sealing ring? It looks pretty close to me.

Furthermore, is a gasket available for a 21 stud with a 3-3/16" bore?

No disrespect, JM.

I have one of these motors myself and would like to see that statement confirmed. I know the 59 series engines were produced in a 221 cu in version as a service replacement and those can be bored to 3-3/16" as the castings are the same. Could there be confusion between 221 cu in 59 and 21stud?

Mart.
 
You don't need to remove head to repair plug thread: I used an insert and the tool to put in new plug thread in my aluhead.

Regards, Christian
 
Mart, the castings of the 221ci and 239ci 59 engines aren't the same, the 221city has thinner cylinder walls. And the smaller bore. They won't bore as Big as the 239ci. I think I've said before, I think Glen had a 21 stud bored to 3 3/16". Yes you get close to the bottom stud, but it does go. Theirs also a 255ci 21 stud build in the Tex Smith flathead book. And Don Ferrara had a 1937 engine bored to 3 3/16 + 0.60. There more but they are from the top of my head. As far as I know big bore gaskets went/arnt available for the 21 stud. Stock gaskets appear to be ok for use. If you look at the pics above, that's 0.080" over at 0.125" over the bore edge would still be under the fire ring, the piston edge on stock pistons doesn't normally reach the deck surface, so no problems with them hitting.
Martin.
 
The 36 and older engine have thin walls and I don't recommend over .060 the 37 up will go .125 to 3-3/16. I've done many, Back in the early 50's they were the hot no. on the dirt tracks. Walt
 
Walt, based on your experience, can a '36 LB block be safely bored over 0.060" or is it in the same category as a babbitted '36 and earlier 21 stud blocks? Just wondering.
Also, many thanks for being such a good teacher for us on these Flatheads!! Our visit to your shop during the Maine Tour was definitely one of the high points for me.
 
Zgears, To answer your questions. Yes it almost certainly will bore to 3 3/16", this is what you refered to as 24 stud size. 24 stud came in 3 1/16"(same as 21 stud) and 3 3/16". Yes it can be sleaved back down to std 3 1/16".
I can clearly see the rust marking on the bore. Mut have been sat awhile with water in the bores.Yes is should really be bored out, But if it were my toy, I'd slip the head back on after de-coke, drive it till winter. Then sort it. Or the next winter. I'd get round to it at some point. But wouldn't worry my self over it, depends really on your definition of "smokes some" the valves and pistons dont look realy oily.
Martin.
 
In all eventuallity, the old motors will have to be sleeved at some point. I wouldn't sleeve one until I had to though. It still runs around $100 a bore so it is still less expensive to find a good block but one day a good block may be the only one you have that isn't cracked or repaired in some fashion.

The Tex Smith book build bored the block to fit 3 3/16" sleeves. They also used JB Weld on the sleeves to try and maintain the deck dimension since I'm sure they bored though the original walls to get those sleaves in there. That was an interesting build but I'm sure they spent a lot of money and time to make a bigger motor out of a smaller one. That build was for a Great American Race 35 coupe entry sometime back.
 
I'd put the old head gasket back on too, if it looked like it had half a chance of working!

Mart.
 
Myself if the block was good and crack free I would be looking into sleeving down or back to 221. Being a machinist I never got the concept of boring a size extreme to the original,most Barners it seems are in favor of doing this,you would never do this to say brake drums oversize beyond limits. I would rather have the dependabilty of a cooler running engine, I have a chart that shows a flathead engine from standard to .030 should be in normal heat range,0.060 should run 185F .0.090 over 195F and 0.125 over can get you 210 F.
 
I am new to the Flatheads so I am going to ask a question that may be common knowledge. I don't know if these castings had a issue with core shift but wouldn't it be wise to have the block sonic tested prior to boring to make sure the block had enough material to be board that far safely?
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Yes they can have core shift, lots do sonic test when going real Big. Most flatheads have very thick walls compared to other stuff.
The chart, overbore to heat numbers. How does this work when different flatheads have different thickness cylinder walls? For instance, pump in head is maxed out at about 0.060" over, and 99A can sometimes go up to 3 7/16" ?
Martin.
 
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