"Fixing" bent engine valve?

[mmmjboner]

I've got an interference engine that lost a timing belt which in turn bent 7 valves. It's a beater car that I would like to get running for cheap. Recently I have been pondering the idea of heating the bent valves with a torch while turning in a drill press, then use the press to apply force to a stationary ball bearing attached to the drill press base.

My concern is the strength of the valves after the heating, bending, and subsequent cooling. I don't really want to do this if there is more than a reasonable doubt that one of these valve heads will pop off and blow a piston/cylinder away. I am aware of the possible consequences, so please don't tell me not to do this, unless you honestly think there is a high risk of failure.

So, if you think this could possibly be successful, what procedure for heating and cooling should I follow? Is there a strength treatment I can apply afterward?

Also I am aware that this could alter the length of the stem from the valve seat face to the keeper groove, however my rockers are adjustable so I should be okay in that respect.

Thanks!

Jeremy

[tsutt]

go to a junk yard and get another head. you'll never get them straight.
some valves aren't that expensive. Todd

[Geof]

I think it will be incredibly difficult, approaching impossible, to get them running true enough that you would be able to lap them to a good seal. Also you not only have to get the head running true with the shank you have to get the shank straight to fit in the guide; this also I think would be approaching impossible.

[mmmjboner]

$250 for a used head, valves are $10-$15 each. This is for fun.

[solution]

Been there. Dont heat them, just bend them back into shape. Anyone who has ever ground a valve knows the heads are seldom straight after they have been in an engine a while. Just make sure they don't bind in the guide when you are done. The ones I straightened, I clamped in a 5c collet in my Lathe and tapped them straight with a chunk of brass and a hammer. I just eyeballed the runout, I couldnt tell the difference before and after the "accident" in the way it ran. You'd be surprised at how forgiving an old beater can be. I did'nt even lap them either.

Scott

[mmmjboner]

Alright i'll just give that a shot then. I figure if I can get them to hold 50+ psi from the intake runner/exhaust port, it should be fine. I've checked a few heads that way. Place an O-ring on the face around the port, bolt a plate with a 1/8" npt threaded hole in the middle, hooked up to a gauge and ball valve, I can get a decent idea how well they seal.

[NC Cams]

I used to work for a noted company that made engine valves - by the millions for both aftermarket and OEM use.

Valves are roll or impact straightened at the time of manufacture to minimize stock removal should the valves warp slightly during heat treat. Basically, to implact straighten them, you install them in a psuedo guide/seat fixture and rap the head head squarely and to flatten it out with the seat. If done well and properly, you'll minimize the material needed to grind the faces square when you reface them.

HERE IS THE CAVEAT, HOWEVER: if a valve is bent so severely that it visibly won't/can't seat, it is has obviously been overstressed to the point of yielding. Once you take steel, especially valve steel over the yield point, strange things can happen when you try to to back the other way - like crack the material or work harden it which makes it harder and more brittle -not good in light of the operating stresses and conditions that valves see.

Intake valves are typically made of martensitic (iron bearing/based) steel - this is why they are magnetic. Thus they tend to be more ductile at room temp that exhaust valves. The ductility makes them REASONABLE candidates for straightening.

Exhaust valves are typically made of austenitic (non magnetic) steels. In this case, they delete the iron and replace it with nickel or chrome or other alloys that give it much better HOT strength. For this reason, exhaust valves tend to be a bit more brittle when cold.

If the valves were bent hot, and straightened while in a comparable temperature, chances are you might be able to straighten them without creating microcracks in the steel.

HOwever, get them TOO hot to try to straighten them, and you'll screw up the heat treat of the material (valves are typically solution and age hardened - not a thing that the DIY'er can readily duplicate with a torch and rose colored glasses).

Straighten them while too cold and you could micro crack them and they could fail catastropically later.

I've seen guys get away with strange things when repairing engines. Some strange things work and some traditional repairs fail miserably because of simple things overlooked or poorly executed. Caveat emptor.

[mmmjboner]

This is the sort of information I was loooking for. The changes in metallurgy is what I was most concerned with.

[NC Cams]

Here is the executive summary of post #7:

Throw away the bent valves and get some good used or better yet some new valves.

There are simply too many unknowns in play and/or you'll need to incorporate riskly SWAG technology to "repair" the bent valves.

[WA Toolman]

What kind of engine is it? When I was doing race cars we could get valves for 350 chev for $3-4 each. These were for "Claimer" engines. (You circle trackers know what I'm talking about) Anyway, try to cut a deal with an Automotive Machine Shop not a parts house. You'll find it's cheaper than trying to straighten them. Especially when the metal fatigue causes one of the valve heads to pop off and munch the top of your pistons.

[mmmjboner]

It's a BMW M20 B25 2.5L I6 engine from a '90 325i that I got for $200. I'm just going to wait until I can pay for a used head and misc parts.

[greg b]

In the past we have straightend heaps of valves from stainless to titanium and the steels that nc cams has mentioned simply because our customers could not afford new valves, most of our stems are from 3.5mm to 8mm thick.As a general rule if you can see more than 1mm of light between the seat and the valve face it should not be done ,and definitely do not hot bend them .I use a large alluminium block with a hole drilled in the side of it to suit the valve stem and tap them with either a small alluminium hammer or plastic.Try and keep the mushroom part of the valve as close as possible to the block.We use a very small v block with a end piece to stop the valve from moving back to far and a dial indicator to check for truth,when true we lightly grind them to spec.So far we have had no failures at all, but like the guys in this thread have stated if there cheap enough you would be silly not to buy new.I hope this helps greg b