Valve to guide clearance

[Matt7c]

I'm doing a head rebuild. According to Wilkins in his TwinCam book "wear in the valve guides will be obvious." So far, all his cautionary tales seem to have refered to engines far worse off than mine, so the fact that I am unsure may be an indication that the guides aren't worn. However, I have to ask...

Having stripped everything down and cleaned throroughly, when I re-insert the valves into the guides and hold them about 10mm open I can wobble them by up to 1mm or so from the centre. I'm thinking that this isn't significant. Am I right?

[elansprint]

Have you oiled the valve & guide as they are when operating this will reduce this movement. Manual gives a clearence figure but dont know anyone who can measure a used guide

[Foxie]

Matt,

I've done a calculation based on guide length of 40mm and max clearances of 0.058mm inlet/ 0.076 exhaust. The max rock at lift of 10mm works out at 0.1mm inlet/0.14 exhaust. So at 1 mm it looks like your guides are due for replacement.

Sean Murray

[bill308]

elansprint,

Used valve guides and valve stems can be accurately measured with the correct equipment.

For the valve stems, a good micrometer will measure to 0.0001 inches. I generally look at 6 measurements over the contact area of the valve stem, at the top, middle, and bottom of the contact area and again a 90 degrees to the first set of readings. This will define the taper and ovalness of the stem.

A similar set of measurements is performed on the id of the valve guides. Here, an expanding ball bore/hole gage is set for a slight drag at the top, middle, and bottom of the bore and again a 90 degrees to the original measurements. Upon removal of the ball gage, a micrometer is used to measure the ball diameter. I believe you can get to about 0.0002 inch resolution with this method. The ball gage consists of a split hemispherical end with a conical wedge to expand the hemisphere until contact can be felt. The gage is withdrawn and the measurement taken with the micrometer. The guide measuements can be compared to the stem measurements to ascertain the clearance with good precision. Compare these measurements to the book values to see if they need to be rectified. Variations of this tool can have a split ball or cone end.

An alternative tool to use is a dial bore gage, but good ones with the proper range, are quite expensive. An expanding ball bore/hole gage is relatively inexpensive and not too difficult to find. I did a quick search on ebay and found:

http://cgi.ebay.com/Sears-Craftsman-Exp ... dZViewItem

My local automotive machine shop was kind enough to allow me to perform my own measurements when the new guides were installed and honed to each valve stem and to my specifications. I elected to go for the tight end of the clearance range. The machine shop agreed to open up the guide id in stages. It took about 3 interations to get them just right but the end result was very satisfactory. With the guides honed to size, the valve seats were then cut to my specifications with good confidence the seats were concentric.

Bill

[Foxie]

Bill,

That expanding ball gauge is very interesting, that's a new one for me.

Sean Murray

[bill308]

Sean,

The gages worked supprisingly well. At the time, I purchased all new valves and converted over to the big intake valves and hardened (Stelite?) valve seats.

After maybe 20000 miles, the engine still showed no puff of blue smoke at start up, when used on a daily basis. I was very pleased with the end result.

Bill

[type26owner]

Matt,
The exhaust guides run almost twice the amount of clearance as the intakes. Best guess is your valves and guides are shot though. Have I described to you already on how to remove valve guides and seats from an aluminum head to minimize the galling damage? You should quiz any shop on how they'd do it first and make your selection based on the reply. If you're going to do all the work yourself I've got lots of special pointers to help you. It ain't that hard to do.

First step is to measure the overall thickness of the head to see if it's worthwhile doing at all. The standard head thickness should be 4.600" but some higher compression versions were shaved thinner from the factory. Best to consult the manual for the last word on the subject.

[forfenderjazz]

What is the proper way to remove the guides? Is heat or cold used?

[type26owner]

Heating the head and cooling the guides and seats is only done when installing those parts. Removing both is done at room temperature.

The seats are removed first by mig spot-welding the valve as quickly as possible in two places to the seat. Those two places are flats which you sanded onto the head of the valve so when the valve is placed into the head the sealing surafce of the seat is exposed so it can welded onto. The welding shrinks and warps the seats so they nearly fall out of the head.

The guides are thinned for nearly the whole of length of engagement in the head using a one-piece piloted counterbore from the seat side. The OD of the counterbore is 7/16" and the pilot is 5/16". If the guide has a wire retainer then stay short of it by an 1/8" or it will crack apart at the bottom of the groove. Once relieved by thinning just insert a pin and tap on it gently and it will come out witout any fuss or bother. Make sure you clean off the hard carbon deposits before tapping the guides out because that will gall the guide bores in the head. The guide gets stretched by this process which causes it to shrink ever so slightly in diameter as it's being tapped on. You won't believe how little force it takes to remove the guides this way! Any professional that does not know this stuff should be avoided like the plague. My guy I go to to have the guides honed had several 55 gallon barrels full of valves welded to seats last time I was there. I'm going bite the bullet and buy my own Sunnen P-180 valve guide hone one of these days.

Total time to safely remove all the seats and guides is about an hour's effort. Easy effort provided you have the right tools and tooling.

[Matt7c]

Thanks for all the tips, guys. It seems that the spring retainers and collets will be the only survivors of this head rebuild! You have certainly given me a lot to think about - not least whether I have the ability to do the valve seat and guide renewal - I certainly don't have the tools...yet. I have to go away to Cyprus (on business) for a few days, so I'll order in the parts while I'm gone and think it through some more when I get back.

Meanwhile, when heating the head for replacing the seats and guides, what method do you use? Wilkins stresses the importance of heating uniformly and implies baking in an oven to get the temperature right, which is fine if your oven is big enough!

Thanks again.

Matt

[type26owner]

I use a Coleman 2-burner camp stove outdoors on the driveway which I crank up to high on both burners. Have a 3/4" thick slab of aluminun plate with the footprint size of the head. I place the slab in direct contact with the flames. The head is placed on top of the slab. Loosely wrap over the head with aluminum cooking foil to retain the heat and so it heats evenly. Buy some temperature crayons to monitor this accurately. It's done and smoking hot in about 45 minutes.

Dry ice works ok to cool the guides and seats along with the installation tools. The seats and guides can be installed in under 5 minutes. The guides go in first. The installation tool for the seats is piloted to fit inside the guides so the guide must be there first to do the guiding.

[forfenderjazz]

Wow

[steveww]

I am impressed by your camp fire ingenuity

I feel like I chickened out here by having my local machine shop do the guides and seats for me.

[M100]

Its not ingenuity - Keith just seems to have move useful junk

Just one thing that was mentioned earlier and seems to be incorrect, the head thickness was 4.638"-4.643" on the small valve engines and 4.598"-4.603" on the big valve ones.

The max recommended skim is 45 thou on the standard heads and 10 thou on the big valves.

[rgh0]

While the Lotus recommended limit on head skimming was as stated in the last post this was more to do with selling new heads as spare parts when Lotus was in this business. There is enough metal in the heads to take substantially more off if absolutely needed to get them flat and with the correct surface finsh. You just need to ensure the compression ratio does not get to high and that valve locations versus pistons and piston versus head cavity if you have large intruders remains OK.

Rohan