1940 Overbore Question
 

I am rebuilding a 1940 Mercury engine for my traditional 29 Roadster build. I need advice from the experts. My assumption is that since this is a Mercury block with a std. 3 3/16 bore I can overbore safely the same as a later block. It is currently std. I am 90% sure it is a Mercury block. It has the freeze plugs in the rails which make it a 38-40. The center water passages are triangle at top, circle in the middle and trapezoid at the bottom making it a Mercury. Am I correct in my thinking. Thanks for any input.

Re: 1940 Overbore Question
 

Mac Vanpelt is the guy for this info:

http://www.vanpeltsales.com/FH_web/f...85to100mid.htm

Re: 1940 Overbore Question
 

Look at the non machined part in front of the head face in front on No.1 cyl. You should see a 99 there. If you look down the lower hole in the middle of the deck face you might see a 99 cast into the top of the exhaust runner. My 99 block (Merc block) has both these marks.

I attach a pic of my 99 block showing the holes in the deck and the location of the raised 99 casting mark.

Regarding boring it. If you want a normal rebore up to about .060" then go for it. If you want 3-5/16 then personally I would do a sonic test of the cylinder wall thickness.

I did a video series on rebuilding this Merc motor on youtube. If you look for Mart's Garage and then search for crusty flatty you will find the videos.

Oh yeah, welcome to the barn.

Mart.

Re: 1940 Overbore Question
 

Sonic test is the only way to know. Any thing else is Russian roulette.

Re: 1940 Overbore Question
 

Those early ones had thick walls. I think that changed after the 1941 core redesign. Sonic testing is always a good idea. If the water jackets are low on corrosion and the block had proper core alignment during the pour then a person can bore these quite a bit.

I'm more conservative than some so I'd just bore it to clean it up. Put a 4-inch crank in and go. These were good blocks to go big bore with but if it gets too thin then a sleeve or two might be in order.

Re: 1940 Overbore Question
 

Watching

Re: 1940 Overbore Question
 

I will agree. I only overbore what is necessary to clean up the cylinder.

Re: 1940 Overbore Question
 

You are correct