Milling head and cam timing

[jeb358]

I have found it necessary to remove the head on my '78 1800 to replace a broken exhaust stud. I now plan to true and mill the head approx. 1mm, I don't have plans to replace the pistons. What effect would this have on cam timing and would it be significant. I don't plan on buying adjustable timing gears until I replace the cams, which would be regrinds. Would reducing the valve lash or milling the cam boxes help? What are your thoughts? Thanks.

[majicwrench]

Certainly it changes cam timing but not much. Use stock valve lash and do not mill cam boxes.
I'm a huge fan of NOT milling head unless absolutely neccessary. Is it flat?? From your description doesn't sound like a head gasket issue, why would you surface it?? FYI, you can take a perfectly flat head into a machine shop and probably 90% will tell you it needs surfacing.
Keith

[Kevin1]

Why would you advise against milling a head? Even if it is flat, if you were after a bump in compression wouldn't it help like it has on other engines, especially since the head is already off?
Or is there some other reason it isn't a good idea that I am not aware of?

[BEEK]

as a machinist, i do not mill heads that do not need milling. on a fiat i would allow .010 warpage before i would mill on my own. for a customer depends on what they want. i have run fiat heads .025 out and not had a problem, wouldnt recommend it, but i have done it. for heads that are way out, i have a head straightening oven, where the head is bolted to a thick piece of steel, and shimmed and torqued thin the opposite direction of the warpage, the baked for a few hours at 500deg,

[TX82FIAT]

I think you are both right. Milling the head will increase to compression ratio. You would need to do the math to calculate the volume of the combusion chamber before and after the milling to come up with a % change. Back of the envelope math will allow you to figure a 499 cc cyclinder volume might decrease volume by 10% from the 1MM shave to change the comp ratio to something like 8.9:1. (I could be compelte off on this as I did not break out the geometry book for cylinder volume). I'm pretty confident you would need to adjust the timing but agree that you would not need adjustable cam gears. The valves would need some adjustment.

I'd like some other folks to chime in on this. If the head is flat i would leave well enough alone. The changes could lead to unintended consequences in the long run such as a pining engine. I'm just not sure you gain that much from the increased CR when considering all factors.

[BEEK]

actually compression increase is the largest power gain in the engine, you just have to feed it properly

[divace73]

Beek is right, both on compression and milling. Mill only if you have to. Best to have as much meat as possible for the future, you never know when you HAVE to mill the head.

[FiatMac]

......I now plan to true and mill the head approx. 1mm, ....

Milling 1mm off the head will likely get you significantly into the inlet valve seat! Then in order to be able to get a proper seat you would have to cut the seat deeper into the head, which will recess the valve, which will increase the volume of the chamber, thus negating most of what you hoped to gain by shaving the head. It would also increase shrouding of the valve which would decrease the flow potential of the head, and possibly cause difficulties in getting the valves shimmed (closes up the clearance with the cam).

As to compression gains by milling the head, you have to remember that the head chamber does not extend all the way to the bore diameter due to the quite large quench/squish areas on the head, so it is not an easy calculation.

In my opinion, shaving the head on this engine is not a good way to increase compression.

[majicwrench]

Beek, I love you!! Usually eveyone slams me when I preach against milling heads. I've been fixing cars for a living for 35 years, and I often let 4cly motors go with some warpage with no issues. Heck you have an aluminum head bolted to a cast iron block with 8 (or more) bolts, it is gonna suck it flat.
Your not in the LIbby MT area are you?? I could send you some work. The last machine shop I sent a head to for a valve grind MILLED THE FRICKIN HEAD even tho I told em not to. I seldom yell at people, seldom call people names, but.....
Keith

[Kevin1]

Thanks for the explaination why it can be preferential not to mill the head if it is not needed to seal well, or to keep the option to last another rebuild. Coming at it from the perspective of performance on a budget, would milling ( for the smaller combustion chamber) along with porting and matching the seats to the valve diameter be worth the effort?