Race Engine Forum UK

Good afternoon Guy, i hope youre keeping well.

My question regards a misalignment issue that has now twice reared its head during what should be a straightforward replacement operation.
I have read your article on the site dealing with valve guide removal and fitment operations after also having bought your engine dvd and have followed them to the letter, but i still find this problem arises and can find no mention of them. My first thought was that i had simply done a poor job but now im not so certain as i have kept everything clean and havnt torn up the guide bores in the head at all.

The valve guides are a shouldered item and are designed to be pressed in from the top of the head until the shoulder bottoms out, the guide bores in the head being at 90 degrees to the combustion face, so in theory it should be a very easy operation indeed.
What im finding is that the valve when being test fitted, is angled outwards ( quite a long way ) from the seat, indicating a misalignment that simply cannot be addressed by even a neway cutter .
I have attempted a test cleanup on a scrap head using the neway and the seat would be absolutely miles away from concentric if the method was employed, clearly not good to do.

Previously, i had found that the higher side of the guide boss when internally relieved slightly with a split fork deburrer, to the same height as the lower side of the boss, will reduce the misalignment to a fair degree, but im having a hard time understanding why it should be necessary to do this operation as ive not encountered this "skewing over" before.
I can only conclude that the excess of material remaining on the "high side" of the boss is applying pressure due to the interference fit and skewing the guide in the outward direction.
Additionally I have not reduced the guide boss height on the lower side at all and simply blended it to the port.

Does this sound like a reasonable and possible cause? And am i addressing it in the correct manner by applying the relief?
Have you seen this before?
I am using brand new valves and guides as per your recommendations on the site article, the replacements being made by TRW and FEBI in Germany.

Please see the photos included and many thanks, id be most interested to hear your thoughts on this.
Tony.

A number of causes Tony.

1. Bare guide - bore not accurately parallel or concentric to the OD of the guide
2. Bare guide - OD not consistent diameter all the way along
3. Head bore - out of round or not parallel ie: distorted
4. Interference fit too tight.
5. Guides fitted dry.


Now, bronze (copper alloy) guides are very soft and have a coefficient of thermal expansion higher than the head. They don't need much of an interference fit. 0.001" - 0.0015" is usually quite enough. If you're not sure about the fit (because the head bore is not new, ie: has had guides in before) you're wise to gauge it accurately and if it's not true to within 0.0005" your fit is going to be upset. If the distortion is bad enough your interference fit will be pretty imprecise/localised. That's not good for many reasons: leakage, loosening, misalignment during fit among others. That's clear enough. I'm using these terms rather generally to avoid getting too 'bogged-down' in geometric tolerances. If the bores in the head a really bad it is impossible to design a guide at all and in that case it's necessary to precision ream the bores and start again with new guides. Having a shoulder or circlip on the guide is a real help because you never need to worry about the guides being pulled into the port. There are few firms that can do that ream op to the level of perfection I'd call for without charging the absolute 'earth' for it.

If you fit the realtively soft bronze guides too tight they distort and gall really badly during instalation - a 'bow wave' of parent material builds up at the guide nose, forcing it out of alignment. It causes the guide bores to go tight in the mid region too and can tear the port section as the guide forces its way out. This is more the case with aluminum heads of course, cast iron is much harder and as it contains graphite it's not such a 'sticky' material to press into.
- and too tight is tighter than my figures above. It is vital to have a radius on the guide nose. And if you are cold-pressing (see GC 'How To' data sheet download below) you should lubricate the head bores with anti-seize copper grease. I know of no other lubricant that works so well.

If you encounter shifting as you describe with a nose radius, that lubricant and a modest interference fit then it's a sure sign there is a dimensional/tolerance problem with those guides. And beware - speaking as one who is designing new guides every week to the most stringent dimensional and tolerance protocols and manufacturing QA regime I can tell you there are few guides available in the world (even from 'top' names) that are a) as good as mine and b) technically good enough for me. Some feature stock bar OD which is done on cost grounds (ie: they're cheap) and is quite unforgiveable, some don't have a straight line on them.

Heating the head and shrinking the guides is great if you have liquid nitrogen. However this method - unless a slavish procedure is followed - can cause more problems than it solves. It's very easy to think you've shrunk the guides sufficiently and got the head hot enough only to have the guide stick halfway down. And when you consider that it is out of the question to use lubricant with this method (it will freeze) you're in real trouble, because that guide has then to be dry-pressed into place and that is the cause of many heads cracking. It is a good idea to practice first by shrinking one guide into a piece of aluminum before plunging ahead with the real thing.

Some like to fit guides with an undersize bore and ream/hone in situ, which is OK and indeed used by manufacturers. That's fine in the latter case because the heads are new and the head bores will be in tolerance, but in the aftermarket that method tends to be used when the operator can't be bothered to gauge the head bores and assure their fit-for-duty beforehand. He just rams the guide in way tight and hopes for the best. The trouble with not gauging is that you can end up with a very visible gap at the port end betw guide and head bore. Or, as I said earlier, a cracked head.

GC

Edited 26th Mar by GC due to typos

Many thanks for your comments on this Guy.

Seems I need to get some bore gauges to do some tests with before proceeding further as per your instructions.

I did use copper ease lubricant upon fitting and I had polished the transition between the guide nose and the shank of the guide to create a radius as you've stated is necessary.

After pressing out one of the offending guides last night and examining the bore in the head, I find its unmarked and didnt take very much pressure to remove it, however as you've said, the bores and guides will need gauging to be certain so thats my next action.
I also followed your good advice on cold pressing the guides, you do get a definite "feel" of the tightness as they're fitted or removed.

Many thanks again Guy for your time, I'll let you know how I get on with this.

Tony.

Thanks to Guy for the solutions he posted to this problem, the issue is resolved.

The new replacement guides are oversize by anywhere between 0.02 to 0.03mm, hence the interference fit is very surely tighter than Guys recommendation.
I gauged the original OEM guides I removed and found they were very accurate on O/D, 12.05mm but the new guides had that additional 0.02/0.03mm added on to them, clearly making them too tight.

Although the head I'm working on isn't any kind of high spec item, the misalignment was far too much to address, and once I had hand finished on the lathe and gauged one or two with the micrometer to get that oem dimension, the issues evaporated.

Once again, Many thanks for your superb advice Guy.

Whatever_the_spec there is no substitute for good workmanship and no excuse for bad workmanship.

Well done for pursuing excellence, Tony. Nothing easy is worth having.

G