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MG MGA - 2 more valve guide questions
|Just removed my old iron guides. Comparing the new bronze ones, the inlets are 1-7/8 long vs 1-5/8 for the old ones. This will mean protruding 1/4 into the port - any problem, and why should it be so long (Moss).|
Second question - the new guides are .0010 tighter fit. Should I attempt to punch them in using the piloted drift I made, or have them pressed? The WS manual says punch out, press in.
|Without looking first for the exact original specifications, my machine design experience says that Force Interference Class-2 fit (FN2 fit) should be about right for this application. The valve guide is 9/16-inch (0.5625") nominal OD. The tolerance dimensions would be:|
Hole: +0.0007 to 0.0000 (0.5632-0.5625)
Shaft: +0.0012 to +0.0016 (0.5637-0.5641)
Interference: +0.0005 to 0.0016
All classes of fits from Running Clearance to Force Interference start by using a standard size reamer for the hole and adjusting the shaft size to suit. That is, the hole minimum is nominal size, and the shaft is oversize for interference fit. (Size references from Machinery's Handbook). FN2 fit in this shaft size should go together okay with a piloted punch and hammer.
If you want a looser fit, FN1 fit would have the same hole size, and the shaft would be smaller for maximum interference of 0.0009. If you want a tighter fit, FN4 fit would have the same hole size, and the shaft would be for maximum interference of 0.0018. The tightest standard spec for Force Interference fit in that shaft size is FN5, giving maximum interference in the range of 0.0023-0.0025. You definitely need a press to get that together without damage.
Now tell me what you measure as the actual OD of the new guides. If it is larger than 0.5650 you're just screwed for assembly, even with a press.
When you install a tube (like a valve guide) with Force Interference fit, it will inevitable shrink the bore a little as it is compressed into the hole. A hollow shaft will tolerate more interference, but more interference also means more reduction of the ID in the process. It is HIGHLY RECOMMENDED to check the bore and working fit after guide installation, and "adjust" the bore accordingly if necessary.
On checking the Workshop Manual I find the original guide OD is specified as 0.5635 (0.0010 oversize). I presume the manufacturing tolerance is on the up side, so the guide may be slightly larger but not smaller. This would make it no tighter than FN2 fit.
Valve guide bore is specified as 0.3438 dia (nominal 11/32").
Valve stem size is specified as 0.342 dia (undersized).
That means 0.002 clearance is intended for original iron guides when new. As shaft and bore go this is a very generous Running Clearance fit, but it has to accommodate thermal expansion that makes the bore shrink and reduces running clearance, so don't skimp on the bore size. This calls for running through a standard 11/32" reamer if the hole is too small after assembly.
The final trick is, if you use bronze guides you need a bit more clearance to accommodate 50% greater thermal expansion of the bronze material. This calls for making the guide bore at least 0.0010 larger (and larger yet is not bad). Since this calls for an oversize hole, you would need a special reamer or an adjustable reamer. In the absence of any special tools you can cut the head off of an old valve, chuck the stem in a power drill, and use it to hone the bore using valve lapping compound. It works a treat, just be sure to clean he grit out afterward. Don't worry about making the bore too large, because bronze guides hardly wear at all.
|Barney, the old guides are .5635. The new ones are more like .5642 on average, so .0007 more. I'm going to have them pressed. Thanks for the honing tip - use jewellers rouge?|
|Use common valve lapping compound to hone the guides, same stuff you use for hand lapping valves to seats, available cheap at any local auto parts store. Jewelers rouge might be too fine grained and not produce enough clearance in the guides.|
|Barney, is there not a danger of the grinding grit getting embedded in the soft metal guide (bronze) and causing valve stem wear?|
|Generally no problem, as long as it is thoroughly cleaned after lapping. Bronze valve guides are very tough alloys (commonly Manganese-bronze or silicone-bronze), not soft like sintered bronze bearing bushings (commonly 660 bronze).|
I have done this myself (in 1994), followed by 100,000 miles of driving with almost no noticeable wear on the bronze guides or the valve stems, then transferring the same head (with new valves) to another engine without touching the guides. Valve stems should also have less wear with bronze guides than with iron guides.
|Job done. Went .344 on inlets and .3445 on exhausts. .3445 by a "ball hone" in effect ramming a ball bearing through the guide.|
|That works too, but where did you find a steel ball 0.001" oversize?|
|The ball hone was part of the machinist's tool kit. He used a .3440 reamer and then the .3445 ball. Not sure who makes them.|
This thread was discussed between 28/04/2009 and 01/05/2009
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