Bronze or steel guides

[RichardL]

I'm trying to cure an oil leak into one cylinder. I thought it was guides, but now that I have the head apart I can't tell if the guides are good or worn or if they are, in fact, the cause of the problem. The new guides I bought are iron, as are my intake guides, but my exhaust guides are bronze. I've seen one comment from someone, I think Muskrat, preferring bronze. Why might this be? Anyway how could I tell a bad guide from a good one if it is not jiggling in its hole and the fit of the valve seems the same as for a new one. I'm rather certain I have no broken rings and quite certain the pistons fit well in the bores. The head gasket showed no obvious signs of oil moving from oilways to bores, at least I don't think that is an issue. Your advice and ideas greatly appreciated.

Richard L.   

[muskrat]

G'day Richard,
                   if it were guides you should see signs of oil in the port. A loose one may not be obvious when cold. Heat the head in oven to 200 C and turn upside down. A loose one will most likely fall out. You can buy oversize ones as every time one is replaced the hole tends to get bigger.
I prefer bronze in alloy heads as their expansion rate is closer to alloy than cast iron I was told. I have found they last longer.
Cheers

[RichardL]

Muskrat,

Thanks. My wife always loves it when I turn the kitchen oven into the shop oven. I'll be giving it a try tomorrow. The expansion rate issue makes sense and could explain why someone replaced only the exhaust guides.

Richard L.

[RichardL]

Muskrat,

First, yes, I did notice some oil around one of the guides on the offending side. This was really good news because it verified the diagnosis. In the process of washing off the head before risking the wrath of family due to fumes in the kitchen, I tried to wiggle all the guides again and noticed that the seam of water, around the base of the oily guide, revealed some movement. After baking, - Daughter: "Dad, what are you doing!  Ovens are for food!" - I shook the head (i.e., cylinder), as you said. The offending iron guide did not fall out but, sure enough, it had moved just a bit and the others did not. Both iron guides offered little resistance to being drifted out, but the bronze exhaust guides were quite firm and showed no signs of loosenes.
For now, I'm leaving those in place.

Thanks, kindly for the good advice.

Richard L.

[RichardL]

Next questions: (Thanks, in advance, for any help.)

     My exisiting guides measure 0.563" diameter and one hole is very close to that but the other is 0.001"-0.002" bigger (cold). Is 0.563 the original
     standard size?

     Do the holes start out smaller than the guides, then expand to let them be press fit?

     Should I hone out to some uniform diameter close to accept an available oversize diameter for available guides?

     Should I buy each guide according to its hole diameter?

     How much will the hole expand with heat to fit an oversized guide?

I appreciate any help on this and please don't feel that responding obliges you to answer all the questions. I'm happy to take advice in dribs and drabs and the advice is not wasted.

Richard L.

EDIT: OK, I'm, right now, heating the head again and freezing a guide to see if I can determine the changes in diameter.

[muskrat]

G'day again Richard,
                            I have never measured the hole size but if you have found one hole slightly larger it is a candidate for an oversize guide. I am not sure of the grade interference fit but would be in the region of a few thou. Heating to 250 C and freezing the guide they fall in. In your case I would go for a 2 thou oversize in that one and a std in the other. Make a note of it in your manual.
Got me thinking now. I will investigate as to the grade fit today.
Cheers

[trevinoz]

Richard,
               Have you measured your valve stems?
They need to be a good fit also.
On the subject of guide material, I prefer iron. Over the years I have found the bronze guides appear to wear the valve stems more than iron.
I may be corrected on this but I have been told that iron has less friction on the stem due to the carbon content.
Trev.

[MikeN]

Hello richard
  Iron valve guides are usually fitted into iron heads. If you try to fit a bronze guide into an iron head you can end up scraping a couple of thou off the OD of the guide as it goes in (The iron head is much harder compared to the bronze and you cant get it to expand enough in the oven) and end up with a loose fitting guide.
  As  already stated ,bronze  guides are usually fitted to alloy heads due to the closer rate of expansion. I would like to see about .0015"-.002"  interfearance fit on a bz guide in an alloy head .I would not personally use an iron guide in an alloy head . Also I think bronze gives better heat transferance to the head ,so assisting cooling of the valve.
  I also always use a drift with the end turned to accept the guide so i dont have to hit the guide itself when fitting.
  And , if the holes in your head are worn differently then you need buy or make guides to suit them individually.
 Ive made them succesfully in the past from SAE 660. A material that has been discussed in recent threads .
Mike

[muskrat]

FN2 or FN3 grade fit =  1 1/2 to 2 1/2 thou, I'd go 2. I think Trev is correct about cast not wearing the valve stem as much but valves are easier to replace and were only talking a poofteenth (didn't see Josh at the gay mardigras last night). I agree with Mike as to the rest.
Cheers

[Toby]

I may be corrected on this but I have been told that iron has less friction on the stem due to the carbon content.
Trev.

Toby comments. Cast Iron is self lubricating providing it has been cut with an very sharp tool.
It's the graphite in the Cast Iron.

[duTch]

When I pulled my guides partially out to do some port work, I used a 5/16"unf (allen head) bolt from inside and a sleeve over the guide to pull it out-with heat, rather than hammering, and kind of reverse for back in?. maybe it doesn't matter much but worked well and seemed less stressful?
   cheers ,duTch

[BSA_54A10]

I may be corrected on this but I have been told that iron has less friction on the stem due to the carbon content.

In a cold application the graphite can be self lubricating but this is not normal as the flake graphite is not a phase that is naturally slippery.
However in a hot application the graphite can burn out leaving voids that fill with oil.
In any case the grain structure on a cast iron guide will be larger and go perpendicular towards the center thus leaving a lot of intergranular voids that will also hold lubricant.

Bronze guides will always be made from extrusion where the grain size will be much much finer and run axially leaving much smaller and fewer voids to hold the oil.
Leaded gunmetal will have an almost continious matrix of lead in the spaces between the grains which is self lubricating and when it gets really hot melt out leaving a lot of space for oil to fill.