Heating to install valve guides

Wed May 07, 2014 3:58 pm

I read Joe's post below. I understand completely how/why freezing a guide works - the metal shrinks in all directions becoming smaller. And I know that the torch is applied to heat and expand metal -- but why does that work with guide installation? I would think the heated metal would try to expand in ALL directions. Outward would "push" against the rest of the unheated block metal and, I would think, go nowhere. Inward there is nothing but air to resist inward expansion and, in addition, would absorb the outward expansion thus increasing the movement, in effect doubling the inward expansion. So, it seems to me the hole to receive the guide would then be smaller. That said, it would seem the best thing to do is to freeze the guide and not torch the block for easiest insertion. What am I missing? Al D

Re: Heating to install valve guides

Wed May 07, 2014 5:10 pm

It does seem a bit counterintuitive - but think of it this way: Picture the inner diameter of the guide bore as a circle of people standing shoulder to shoulder. if the people want to move farther apart, the only way is to expand the circle.

Re: Heating to install valve guides

Wed May 07, 2014 5:14 pm

bythepond88 wrote:It does seem a bit counterintuitive - but think of it this way: Picture the inner diameter of the guide bore as a circle of people standing shoulder to shoulder. if the people want to move farther apart, the only way is to expand the circle.

But I believe what Al is getting at is in your example, there will be a cooler/colder wall encircling the people, giving them no where to expand the circle to.
It has been my experience that cooling the part you want to insert is all that is necessary.

Re: Heating to install valve guides

Wed May 07, 2014 6:46 pm

The fault is in thinking that the metal will expand in all directions. If unconstrained, it will do exactly that. Imagine a cube of steel, every dimension (length, width, height) will grow under heat. However, a bore in a piece of metal IS constrained. The molecules move further apart when heated, so the bore simply cannot grow smaller. I like Raymond's version of a ring of people, they cannot get closer. That is the constraint.

So, as counter-intuitive as it may be, heating the block in that area will make the valve guide bore from larger. Since the entire block will act as a heat sink to pull heat away from that zone, it will only grow very slowly, maybe not even noticeable to our measurement capabilities. Give the size of that block and the power of a propane or MAPP torch, I wouldn't bother with that aspect. It also just adds a safety hazard into the equation. Freeze the guides, use dry ice if available (handle with care), and they will drop right in.