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Block pocketing, 998 1098

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racingenglishcars

racingenglishcars

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Many people want to make their 998 or 1098 a-series go a bit faster, but the 12G295 heads appear to be rather expensive. The alternative is to install a 12G940 head from a 1275, which is a wonderful head for the price (used of course). Problem is, that the exhaust valve hits the block, just a little bit. There are some ways to get around this. Sink the valve and use a standard cam is one way, but for any good performance one needs to use a high lift cam and pocket the block. This can be rather expensive as normally the engine must come out and sent to the machine shop. Another alternative here is to grind it while in the car... by hand?? NO, but maybe this tool I drew up would do it. Just use a bit of oily cloth in the cylinder to catch the dust and a 36mm face-mill and grind it by hand.

https://i183.photobucket.com/albums/x288/frogeye61/Links/BMCTOOL1.jpg
https://i183.photobucket.com/albums/x288/frogeye61/Links/BMCTOOL2.jpg
https://i183.photobucket.com/albums/x288/frogeye61/Links/BMCTOOL3.jpg
https://i183.photobucket.com/albums/x288/frogeye61/Links/BMCTOOL4.jpg
https://i183.photobucket.com/albums/x288/frogeye61/Links/pocketblock.jpg

Would anybody like to make it and try it out? Then you can lend it to all us needy people who want to pocket our block.
 
I've done this type of thing on 948 race motors with high lift racing cams, but not with that big of valve. A few things to think about, will the 1098 head gasket with a 1275 heed clear the valve, or do you plan on using a 1275 head gasket, and does that work out as well. Then another thing to think about with valve notches in the block, is you need to make sure the first ring land is low enough that the top ring doesn't make it up that far, you may need special piston, in the 948 race motors we made custom pistons with this change made to them. Also I would look for about .100" valve to block clearence as a minumum.
 
Hello Don,

1098 Morris Minor.
I made up a mandrel to which I fitted a grindstone, machined to the correct diameter. The mandrel went through the exhaust guide and with the head in place I ground the pockets using an air powered die grinder.

As this was with the engine in the car I carefully sealed the bores with rags and grease. Worked fine and I do believe I used a 1275 head gasket.

Alec
 
Gesh, you guys are technical marvals and I only want my rings to seat. sigh
 
Relieving the bore for valve clearance really screws up the gas flow. What you MIGHT gain in a larger ex. valve you WILL loose in the screwed up gas flow.

That said, and using an exhaust valve that needs bore pocketing, you MUST register the cylinder head to the case as you want just the bare minimum of valve edge to case distance. The stud holes in the case have (usually) two register holes and the others are a lot larger than the stud diameter. But.....the head still walks around ...try it when you lay the head with the studs in the case.

The answer is to dowel the head to the case but thats a pain to do a match dowel. Easier is very thin wall sleeves (2), one in each end of the head and a slight interference fit in the cyl head hole.

The 948 has the bigger bore and is not so problematic. But still, using a custom piston with a high top ring placement your dead in the water. Moving the exhaust valve guide over toward the intake side is the better alternative. You still have to deal with rocker/pushrod angularity and geometry if you move the valve. At the end of the day a larger exhaust won't get you much out of the ol' horsepower slot machine.
 
Hap sez.............

"" A few things to think about, will the 1098 head gasket with a 1275 heed clear the valve, or do you plan on using a 1275 head gasket, and does that work out as well. """

And Haps right of course.....and think you must with that combo of using a 1275 h.g. with a small bore motor. You will have a ton of dead air with the larger 2.875" (+ -) bore h.g. on a 2.540" motor. More horsepower lost.

Just another thought....offset bore the case toward the exhaust side.....
 
Hello all,

the gain from using the 12g940 head outweighs any of the theoretical drawbacks mentioned particularly over the 12g202 fitted as standard to the 1098 engine (Minor). A 12G295 requires no modification but are not as plentiful.

Jerry, the amount of walk you refer to is negligible in view of the task at hand. Offsetting the bore, apart from the expense is not practical as the valve overlaps by about (and this is from memory a few years ago) 0.250" to 0.300". I doubt that there will be that much metal.

Alec
 
How does one identify what head(s) they have? i.e. where is the number found?
 
Mike, the casting number is found on the upper surface of the head, under the valve cover. I can snap a pic for you when I get home tonight.
Jeff
 
Just posted this in another thread...Jeff, you might recognize this one:

12G295.jpg


Ray
 
Looks familiar, Ray. /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
The 206 head I have handy has the numbers in the same spot. The 202 has them between #1 & #2 intake valves.
Jeff
 
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