Lotus Elan

Inlet valve seat damage

PostPost by: JJDraper » Sat Oct 05, 2019 12:00 pm

inlet-valve-no1-seat.jpg and


So, the engine rebuild is moving forward slowly - I was doing a fine lapping in of the valve seats and spotted this. Looks like some grit or something ingested a while ago. Valve is fine & straight. My question is, do you think this should be cut or just use some coarse valve grind paste to try & remove it? Or just leave it and pretend I didn't see it!

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PostPost by: el-saturn » Sat Oct 05, 2019 1:19 pm

if you look at it pragmaticaly, you're loosing (ef) 0.3% surface which lets say would be linearly about 1/100th of total hp loss ( /4) equals 1,5 hp - :roll: sandy ------ if you race, it's 1 big hamburger less before you start!
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PostPost by: jimj » Sat Oct 05, 2019 2:36 pm

Or you could multiply a random number by the length of a piece of string, that`ll give you an answer. I`d try the coarse paste but I suspect you`d be better having the valve seats re-cut. I found it surprisingly inexpensive when something similar happened to me. The symptom was running only on 3 cylinders which reduced power by about a third of my height in inches.
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PostPost by: 2cams70 » Sat Oct 05, 2019 3:10 pm

Have the seats recut. On a valve seat even a very small leak rapidly becomes a big leak. The combustion flame leaks through and melts the metal surrounding rapidly making a small leak a big one. Don't waste your time lapping valves. That's a practice of the past. Valves are never hand lapped in production and you are not likely to get the correct seat width that way. Have them properly cut to the correct angles and widths and you won't have further problems in a long time.
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PostPost by: Concrete-crusher » Sat Oct 05, 2019 5:30 pm

The seats look quite recessed to me so maybe it's time for new seats? What are the shim sizes are they near the lower limit that will tell you

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PostPost by: MarkDa » Sat Oct 05, 2019 10:09 pm

Re-cut is minimum as that's quite a depth to go with paste but as Steve says you need to consider reduction in shim thickness.
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PostPost by: 2cams70 » Sun Oct 06, 2019 2:02 am

If you are going to replace or recut the seats you really should replace the guides too unless there's very little wear. It's difficult to machine the valves concentric with the seats if the guides are worn. Slippery slope...…...
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PostPost by: JJDraper » Sun Oct 06, 2019 7:42 am

Thanks for the replies, Looking at my notes from the last head shimming, all the other shims are in the range 116-122. This one is 109, prior to this it had been 120, which suggests there was a problem at the last seat cut - which was out-sourced so I never actually saw the seats as the head was delivered back to me built up with the valves & springs. Prior to the current tear down, performance or smoke was not a problem, a knackered water pump is the reason.

I have changed machine shop since then and the new guy is well versed in twin cams. He cleaned the head, and stated there was almost no wear on the inlet seats, but the exhausts needed a slight cut. Valves & guides show a little wear, but are acceptable for reuse. No signs of pocketing or valve seat erosion. On balance, I will take the head back to the machine shop & see it he can cut the seat. Interesting that valves & guides from no.1 show less wear than no.4.

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PostPost by: nmauduit » Sun Oct 06, 2019 9:41 am

MarkDa wrote:Re-cut is minimum as that's quite a depth to go with paste but as Steve says you need to consider reduction in shim thickness.


Machine shop will advise according to an eventual recut margin, but the dip looks quite deep : if the engine is in good condition and almost fresh, I would get that seat changed to keep it almost homogeneous (the recut valve will seat lower which impacts efficiency on that cylinder, and may not have any recut margin left for a quick refresh if need be downthe road, unlike the others)
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PostPost by: JJDraper » Sat Oct 19, 2019 11:37 am

Good news from the machine shop. He was able to carry out a slight grind to take out the pit - I will find out how much when I shim the head this weekend. Second opinion from my engine builder was positive - valves not pocketed, shim sizes should be well within range for the cam even after the cut. It is possible that the picture made it look worse/deeper, but as you can see, very little metal taken off. Machinist did not want to use a cutter as there is wear on the valves & guides - minimal and still serviceable, but could cause a cutter to skid/wobble. He's happy, I'm happy, engine builder happy, we're all happy. Nothing beats a conversation with a knowledgeable machine shop & engine builder.

Now my short engine is back together I have no excuse not to finish the job & hit the road (in time for winter).

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inlet-valve-no1-seat.jpg and
Slight damage to seat
inlet-valve-no1-seat-after-slight-grind.jpg and
After slight grind
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