machining

[bigblue91]

thanks for all the rebuilding advice for my 250.

-it was mentioned that 0.050 mm must be removed from the head, due to a thicker head gasket.
what if you just install the headgasket and don't machine the head down, is it bad, catastrophic or what, because 0.050 doesnt sound like that large of a gap?

-also ive been going around locally and checking out machine shops. what kind of price would you guys say sounds reasonable for the 3 angle valve job to be done to these engines?

 

[early ford fan]

the higher the compression,the more horsepower.if your not worried about hp don't cut the head.

 

[bigblue91]

so by removing the 0.050 for the new (thicker) head gasket its just increasing the compression for the engine.

 

[FalconSedanDelivery]

Machining the Head will do 2 things and both are benificial , it will give you back the compression you loose with the thicker gasket ( thats power , MPG , better Squish is the result ) and it trues up the head for a better seal , well worth the 75-100 bucks most places charge

 

[JackFish]

http://falconperformance.sundog.net/compcalculator.asp

You can play with the numbers here.

 

[bigblue91]

that calculator is pretty neat, thanks for showing it to me.

 

[mugsy]

The "0.050" is in INCHES not mm!!! Big difference.

If you can I'd take 0.030" off the block deck and then the rest off of the head. But measure the head first with a cc kit. You'll need exact numbers before cutting.

 

[bigblue91]

The "0.050" is in INCHES not mm!!! Big difference.

yea it is, thanks for letting me know.

 

[bigblue91]

after playing around with the calculator I ended up with 8.5:1 compression ratio.

I'm not looking to make a race engine or nothing :nox: , as stated in my signature. I'm planning a stock rebuild, (1 barrel, exhaust manifold, stock ring/piston sizes, etc). I want to try and do most of the work myself without going to a machine shop, i understand I'll lose some power from the lower compression but is that a reasonable compression for a 250 .

 

[mugsy]

I read a few years back about a rule of thumb on octane and compression ratio. It said that just move the decimal point point over on the CR to get the octane needed. So if the CR is 8.5:1 then it would need 85 octane. Don't know if anyone here on this board has any more info on how octane and CR relate. You want to go has high on CR without the spark knock. I went with about 9.3:1 on my rebuild using midgrade, but have very limited time running it...

 

[Anlushac11]

delete please my post.

 

[CZLN6]

Howdy Back Big Blue:

Yes, 8.5:1 is a reasonable CR goal on a stock 250. But, FYI, machining is a standard part of an engine rebuild. Mating surfaces change over time and heat cycling. So the head and block mating surface may no longer be truly flat, making sealing difficult at best. Milling reestablishes a flat mating surface. You will also need to mill the head surface .025" to compensate for the additional thickness of currently available head gaskets.

Adios, David