21 replies to this topic

[MacFreuden]

Thanks for the info Earwax, seems like that will be the way to go in terms of skimming the head. I will have a proper measure with the vernier tomorrow i think to double check my calculations and then get it sorted at my friends machine shop (luckily for me its all free lol).

I think I will aim for just over 10 CR to be on the safe side and leave some legroom, i can always alter it further down the line if need be.

[podifold]

Compression ratio calculator: https://jscalc.io/ca...RNEglVkKLQir11T

[MacFreuden]

Okay so I’ve done the vernier method, and checked the depth which comes in at a skim of 33 thou or so, as I need a a Combustion Chamber of 19 to achieve my desired CR of 10.5. This sounds quite a lot to me and I measured the heads thickness, have read that standard is 2.750 inches and mine is 2.725, meaning it has already had a skim of 25 thou.

Was wondering what is the max you can safely skim for reliability, I've read in the Vizard book that the face should be a minimum of 0.080” in depth and it shouldn’t be skimmed if this number will be reduced below 80thou. It also mentions measuring this through the oil hole however I’m not sure if I’m being dim but I can’t find an oil hole like the one represented in the book...

Any ideas on how to work this out?

And how would I accurately check the clearance of the valves toward the piston when they are fully open ?

[Spider]

The Oil Hole in Vizzard's book refers to a small bore cylinder head. The Oil Way through the 1275 heads is quite different, it shoots off at an angle, so following a big skim, it's break out location on the deck needs to be checked with a gasket to be sure it's still in the right location.

 

How much can you skim off safely?  You're really governed by the thickness of the deck to the cooling jacket in the head. You can measure through all the holes in the deck to the cooling jack and make your best guess from there. Early 1275 Heads have more meat in this regards, but if yours has had 0.025" off it already and you are looking to take another 0.033", that's an approx total of 0.053" - you should be able to take that off without issue, but check first.

[MacFreuden]

The Oil Hole in Vizzard's book refers to a small bore cylinder head. The Oil Way through the 1275 heads is quite different, it shoots off at an angle, so following a big skim, it's break out location on the deck needs to be checked with a gasket to be sure it's still in the right location.
 
How much can you skim off safely?  You're really governed by the thickness of the deck to the cooling jacket in the head. You can measure through all the holes in the deck to the cooling jack and make your best guess from there. Early 1275 Heads have more meat in this regards, but if yours has had 0.025" off it already and you are looking to take another 0.033", that's an approx total of 0.053" - you should be able to take that off without issue, but check first.

Thanks for clearing that up about the book Moke, makes much more sense now...

When you say measure through all of the holes to the cooling jack, just so I’m sure, you mean through the water holes in the head and measure the thickness of the head face with the pointy end of a vernier, it’s not 100% accurate but I have a reading of 0.250” or so, which by my money gives me loads of meat to skim then?

Regarding the valve clearance to the block/piston, is it literally a case of measuring how much gap they will have to the new skimmed surface and the max lift of the cam ? (I am using standard rockers for my first build)

[Spider]

Thanks for clearing that up about the book Moke, makes much more sense now...

When you say measure through all of the holes to the cooling jack, just so I’m sure, you mean through the water holes in the head and measure the thickness of the head face with the pointy end of a vernier, it’s not 100% accurate but I have a reading of 0.250” or so, which by my money gives me loads of meat to skim then?

Regarding the valve clearance to the block/piston, is it literally a case of measuring how much gap they will have to the new skimmed surface and the max lift of the cam ? (I am using standard rockers for my first build)

 

It isn't easy to get an accurate measurement and in any event, it's only around these holes that you can sensibly measure but the number you've come up with sounds about right. Yes, there's plenty on that.

 

If you are concerned about the Timing Chain breaking or some other Valve Train failure, then sure, make the Valve to Piston Clearance Checks, though, for most Cams and Rockers you'd need something pretty radical to get in to a situation of Valve to Piston Contact. It's a worthwhile exercise to run through, if only to put your mind at rest. You're method is one way to check this.

[MacFreuden]

Thanks for clearing that up about the book Moke, makes much more sense now...

When you say measure through all of the holes to the cooling jack, just so I’m sure, you mean through the water holes in the head and measure the thickness of the head face with the pointy end of a vernier, it’s not 100% accurate but I have a reading of 0.250” or so, which by my money gives me loads of meat to skim then?

Regarding the valve clearance to the block/piston, is it literally a case of measuring how much gap they will have to the new skimmed surface and the max lift of the cam ? (I am using standard rockers for my first build)

It isn't easy to get an accurate measurement and in any event, it's only around these holes that you can sensibly measure but the number you've come up with sounds about right. Yes, there's plenty on that.
 
If you are concerned about the Timing Chain breaking or some other Valve Train failure, then sure, make the Valve to Piston Clearance Checks, though, for most Cams and Rockers you'd need something pretty radical to get in to a situation of Valve to Piston Contact. It's a worthwhile exercise to run through, if only to put your mind at rest. You're method is one way to check this.

Great nice one, I will have a measure when I get up tomorrow and double check but from here on out it looks like it will be plain sailing .... for now at least anyway,

Cheers JB