Valve seat fint info needed....

[x39]

I'm replacing a valve seat on an Onan engine. The owner brought it to me with the pocket filled with weld (aluminum). I'm machining a new pocket and need information as to the correct fit between the seat and the cylinder. I figured one of the many august members of this board would have the info I'm looking for. Thanks.

[TGTool]

My understanding is that the insert should have a very slight interference fit with the counterbore. Not more than .001 or .002 I think. Press the new seat in with the outside chamfer down so it doesn't peel any chips off the wall that would intefere with seating and subsequent heat flow from the valve to seat to head (or block). Chilling the insert before insertion may help too. I'd peen or roll the top edge of parent material onto the new seat just for insurance. This might require the counterbore to be .015 deeper than the insert thickness. I'm told this is even more important using high chrome or stellite seats since they don't expand at the same rate. IIRC I've also seen loctite used on valve seats but I've got some reservations about that myself on the theory that it might slow down heat flow and lead to burned valves. Maybe Onan doesn't run that hard but I'd try to give any job the best advantages.

[x39]

Thanks TGTool, I went with a .002" interference fit, played a torch over the cylinder a bit first and it slid in pretty as you please. I have two books about rebuilding small engines, one recommends chamfer side down, the other chamfer side up (neither gave any info on the fit), to allow for the metal displaced when staking material surrounding the seat. I went chamfer side up with no problems.

[Tobias-B]

IIRC I've also seen loctite used on valve seats

huh- I'd think that the running temp of the seat would be way too high
for loctite to do any good...

t

[Willy]

I realize this is a little late, but it has been my experience that an .002 interference fit for a valve seat going into an aluminum block, or especially an aluminum head, in an air cooled engine is not adequate.

Most people I know, myself included, would opt for a .004-.005 in cast iron and .005-.007 in aluminum depending on valve seat size for air cooled engines of roughly this displacement.

Also when working on an aluminum head, or valve seats in an aluminum block, preheating the part to about 250-300° F and chilling the seats in the freezer make things a lot easier.
I don't think loctite is required for a proper interference fit, as a matter of fact it may in fact hinder the heat transfer process between the valve seat and the head or block interface.

[A.K. Boomer]

and it slid in pretty as you please. .

Uhhhm - how easy did it "slide" in?


hopefully you meant that in a brutal beating kind of way

Onan's are mostly the aircooled aluminum horizontally opposed twins and they take more press fit than a standard water cooled engine due to higher running temps and therefor expansion of the seat bore.

General practice is to heat the head up beyond normal operating temps,

Also - personally I install a seat chamfer side down (like TG stated) for two reasons - one is that it wont remove metal in the process of pressing (therefor relieving the press fit) and two the metal wont get trapped and hold up the seat from bottoming (again like TG stated)

There's nothing wrong with using locktite if your expecting a press on the mild side, it wont interfere with the metal to metal press or heat transfer - it does not interfere with the contact area's, it simply fills in the voids and valleys and bonds them to the piece.

There's also nothing wrong with anti-seize compound if your expecting a press on the heavy side, its far better than galling/distorting and loosening up the piece - better to have it tight and keep the fit.

Edit; Looks like Willy beat me too it...