Crack in Head at Valve Guide

[Fishcop]

I've just discovered two of my heads have cracks where the new valve guides were put in on the intake side. The heads have been sealed and torqued down. Should this have happened? What do I do now?

Thanks

[john walker's workshop]

i would suspect the guides were too big for the bore. gotta measure the guides when they're removed, and be sure the new ones are no more than a couple of 100ths of a mm oversize.

[sayah]

I had a similar problem. The guide was not pressed in using the proper tool and caused part of the boss to crack off. I took it to my machinest with a new guide and he hammered it in. The new guide was snug in the head and he suggested that it probabily be ok since it (the guide) was not loose.

The other option would be to remove the head, buy a replacment, or have the old damaged one welded up, if possible.

[Henry Schmidt]

If the guide is tight and there are no loose pieces that can fall into the engine, there should be no problem using the head/ heads. Although an oversized guide will cause a boss to crack, improper guide removal can also cause cracks in the boss.

[Fishcop]

Thank you Gents, I thought that it was probably okay. The cracks are small and there is nothing loose.

Cheers

[garibaldi]

I hate to be the bearer of bad news, but a crack in the guide boss is NOT ok. The guide is not tight, no matter what you think. When the head gets to operating temp and expands, what do you think will happen to the guide bore? It will grow, and now that there is a crack there, it will take the path of least resistance and open the crack up, especially since it is under pressure from the guide interference. The crack will only get worse, and there is no way to predict what the press fit will be during operating conditions, and after the head has cycled. Guides are installed at around .0015 of interference, and when at elevated temp, the head expands way more than that. You are risking having the guide pull out. Not guaranteeing the guide will move, but if I saw that on an engine I were doing, it would come off and be repaired.

YOU WILL RISK THE ENTIRE MOTOR IF THAT COMES APART

[sayah]

For children, the world is perfect. Perfectly speaking what you say is true.

But, and this is a big but, the boss will expand even when the head was brand new. What holds it in positon then. Are you saying that porsche engineering was so precise that they figuered how much boss material needed to be in place that he guide would remain stationary!!! Preposteroios!!! Most engineers don't know these things, just look as the Seattle-Tacoma suspension bridge (galloping Gertie).

RISK IS PART OF LIFE.
EMBRACE IT !!!!!!!!!

[Henry Schmidt]

it is a judgment call as how bad the crack is, but if the only thing cracked is the boss, you're ok.
We quite often remove the boss completely when doing full race port work. The boss is not structural. It's main purpose is to direct flow.
That is not to say that a guide won't come loose, but most of the late design guides have a size change to locate the guide and prevent a loose guide from falling into the head/ sliding down the valve.

[Fishcop]

Recently a friend of mine (and a Porsche mechanic) decided to take up some club racing. He chose a 1979 Ford RS Escort.... "Why" I asked, "You're Porsche mechanic, working for a Porsche race team! Why would you go and run a Ford?" He just smiled and said "For $2000 I can build a reliable 180hp 2.0 engine for which parts are readily available, and if something breaks I don't loose my house and wife."

I'm beginning to appreciate some of his wisdom....

[garibaldi]

If you want to embrace the risk, then by all means. Im not saying that there are cracked heads out there running in peoples motors. But the unpedicatability is a gamble that the end user must assume. Run it cracked, see what happens, maybe you get lucky, but remeber, there is a big difference between a big block for example and a porsche motor, you can pretty much throw a rod in a big block and it will just shake it off, but bend a valve severely enough in a 911 motor, or better yet, lose a guide and snap the head of a valve off, and the only thing usually salvagable besides the ancillaries might be the wrist pins and possibly the crank, since most of the time, when one of these caugh, you can stand on one side of the block, and look at your buddy on the other side and have a conversation through the holes in the side of the case. I am not saying that that will definetely happen, but it is easier to adress the problem when recognized, and while it still is being built, rather than wonder when that might sneak up and bit you in the ass furhter down the road.

The last set of 911 head I got in that had someone botch up a guide install and were cracked, had that same crack grow by about 1/4 of an inch when I had them heat cycling on the oven to prep them for welding and repair, and that crack grew AFTER I had bored the old guide out and removed it from the head, so there was nothing pushing on the material.

Another set of 911 heads I had that had a cracked intake boss, lost a lima bean sized chunk of aluminum at the tip of the boss when I reahed in there and pryed at it with a pocket knife.

[afterburn 549]

I have been running with a crack in mine for a long time........I think ill know if it starts to come loose....And I am betting it never will.
In a perfect world where $ is no object and time is for sale I would fix it,besides whats the worse thing that can happen??
I would have to take it apart anyway
take my Porsche pedigree away??
Some how I am convinced that all these text book ans. come from those who do not know a left handed monkey wrench from a chisel.
Throw away the text book and live !!!!!!!!!!!!!
If you try too flame me dwn make sure you have at least O.H a briggs and stratton eng. have at least 30 yrs wrenching exp. and know what Dylthium Cristals are used for .
Seriosly I think we can lighten up on how to do it if we just considered our selfs there......It becomes a whole diff. issue

[garibaldi]

[img]

[garibaldi]

[garibaldi]

here is the same set of heads after the repair