Question for the that use an oven to heat heads

[Datsunworks]

What temperature do you heat aluminum heads to when installing seats and guides. I'm using liquid nitrogen to shrink FYI. I'm getting quite a bit of warpage even as low at 185. These are brand new straight six castings that I manufacture and I'm trying to narrow down what is happening. Not causing any real problems since I surface the top and bottom again after it's done. Wondering if it might be an internal stress issue in the castings. They are A356 heat treated to T6.

Thanks
Derek

[PackardV8]

Any casting moves with heat and stress; a straight six head more than most. Installing twelve seats and letting the head shrink tight around them induces stress.

FWIW, every aluminum head with miles, years and heat cycles on it will be warped when it is removed for rebuilding. You know they were flat when installed, but it's just the nature of the beast. If you mill yours flat after installing the seats, they won't be flat the next time they're removed.

[BillK]

Ok, I will go on a limb. I very, very rarely heat up a head to install guides. Only if the originals are very tight coming out. I have never done it to install seats. Not sure how the oems do it but I doubt that they heat anything. I have never had a seat or guide move afterwards. I stick the guides in the freezer but I am not sure if it makes any difference. I have never done anything with seats other than push them in.

The few times I have heated a head for guides I have done it to around 250. Most of them run at close to that temperature anyway. Never had a problem but I cant say I have ever done a 6 cyl.

[mag2555]

Makes one want to size guides and do a valve job at close to the actual running temp of the head!
Also I know for sure that Brodix does not heat any of the heads they make to install seats, it’s just a .003” press fit at room temp!

[Geoff2]

Hope you did it when the wife was out!!

[Aussie Dave]

I worked at a oem engine plant, the heads were aluminium and the seats and guides were shrunk in liquid nitrogen before installation but the heads were not heated though.
Read a Coventry Climax book were they heated the heads to fit seats and guides and the heads grew in length because of this. So yeah a lot of induced stress. Think the key is the machining after to correct seat and guide fitment, which you are obviously doing.
On a side note I worked with a cylinder head guru and he wanted to fill the floors of a aluminium 4 cylinder head, so took all the seats and guides out had the intake floors welded up got T6 heat treatment and the thing was 10mm shorter. We didn’t use it but I often wonder if we had heated it up and put the seats and guides in if it would of got back to close original length.

[Datsunworks]

Thanks everyone for the input. It was helpful for sure. For me as a machinist anytime you press one harder metal into the bore of a softer metal it is effectively a broach. So in my thinking the closer to a slip/shrink fit the better. Especially putting guides in a brand new casting. I just finished seats and guides in 6 heads and my observations are that the liquid nitrogen makes the biggest difference vs the heat. A head that was 250 didn't feel much different than 185. How long I let it soak made the biggest difference.

All of the heads warped in the same direction so that tells me that most likely it is an internal stress by the design of the casting. On the next batch I'm going to stick the raw castings in the oven and heat soak them at right around 200 for an hour or so and see if that normalizes the castings a bit before I start machining them.

Thanks again,
Derek

[jed]

When I was doing this type work I would heat aluminum heads to 225’/250’ if the guides had more than .002 press and seats had
More than .006 press. It sure made it easier for installation. I am like billK and have tried freezing guides and seats but never thought it helped. Trying to Remember but I think they shrunk .0008/.001.
As far as your manufactured head warping I would bet the head needs to be heat cycled and or stressed relieved by cryo treated or
Metal-lacing (vibratory stress relief).
As far as factories installing seats and guides I am sure they have a very sophisticated pressing process.
Also in the aftermarket where we usually drive/hammer seats and guides in the more solid your working surface is the easer they drive in. I have seen machinist driving in guides on tables or benches that are bouncing off the ground while driving in guides.