280 engine noise

Dear friends;
('74 280, AT, M110)

Have I made a mistake simply bolting my visually good, 46K cylinder head and cam/valve assy. on top my new short block that I got from Adsit?

Mechanics who saw my original head/camshaft/valve unit said it looked like new, so get a new gasket set, a new tensioner, properly torque, and then adjust valve/cam clearances, and you'll be good.

I just read some stuff in an old thread about needing to ascertain a good piston/valve clearance (plastigauge? modeling clay?)

What I am still trying to figure out is this rising and fading, harsh, metallic knocking noise that seems to eminate from the rear top end. I can feel it as I put my hand on the valve cover near the firewall.
Clues from that head's history:
It came from the car's original engine, which apparently threw the rod bearing in the rearmost cylinder, seizing the whole deal.
This noise seems to come from the area that used to serve that same cylinder when it was on the old engine.
Could this incident have damaged a valve or guide? Could the piston, in the brief time when the rod broke free from the crankshaft, banged into a valve, making a condition where it catches at regular intervals?

Doing the 'pull the plug wire while it's running' test suggests that it's not running in that cylinder. "Do a quick compression check before you dive into it", my aircraft engine rebuilder brother says. .

I appreciate your 2 cents.

Will

[Mark DiSilvestro]

With your latest information, I'd pull the sparkplug from the affected cylinder, check it for damage, then do a compression check. Possibly a bent valve.

Happy Motoring, Mark

[Tomguy]

If the engine blew a rod in that cyl, there's a damn good chance that one or more of the valves that service it are bent. The piston may have smashed into the valves if it were on the upstroke, the rod could have been on its way out when collision with the piston caused utter failure, the piston could have stopped in the cyl and the valves struck it when opening, etc.

The valves on small engines (such as my tractor had) rotate when they close. I don't know if the same is applicable on a larger engine, but I would imagine it's likely (to avoid uneven wear). It's possible that the valve sticks when it rotates to this certian point. Check the valve guide as well - if the valve is bent, even not visually (but measurable with a straightedge) then the guide likely has been worn oblong as well.

[300SDog]

It sounds like you're worried about too much metal having been removed from your twincam carrier and/or cyl head by whoever put the engine back together....... or maybe Adsit suggested this as likely problem, i dunno.

Did any machine shop plane the cyl head, or worse yet the camshaft carrier?? If so your engine may be completely screwed........ you just cant shave metal beyond tolerances from soft alloy MB top-ends without driving compression into oblivion and valve/piston clearance into negative integers.

Otherwise, if you suspect crummy Adsit build of the engine (search "Adsit" on this forum) - best way to isolate any knocking sound is with mechanic's stethoscope.

Btw, plasiguage and silly putty is something intended to measure valve clearance (notso critical)..... whereby you slip plastic between camshaft and rocker - then can tell measurement of compressed plastic, sort of like using feeler guage in reverse.

Otherwise, something weird about MB shortblocks - bores are tapered and common rebuild mistake is actually installing sleeves upside down..... in which case the sleeve comes loose and knocks like hell, assuming the piston sleeve was even properly fitted to block in the first place. Basically the only sane way to rebuild the Stuttgart engine is to mill the bore straight and then install aftermarket staight cut sleeves...... which begs another question - did Adsit put straight cut sleeves into tapered bores?

No, the head and cam carrier were basically just wiped off and re-installed on top of the short block. Everything seemed flat and ducky. I never separated the the cam carrier and the block, and considered them as a single unit.

Does anyone know for sure if these valves rotate? If so, how is this induced? I'm intrigued with the notion that if there is any deformation, any at all- a rotation along the valve stem's length would be easily affected.

Will

[ctaylor738]

I am not making this up.

I just received this neat 1969 brochure on the 250C entitled "Sales Features" that is aimed at car sales types. It lists various features of the cars and how they benefit the buyer.

On Page 4 is a discussion of how "The Rotocap valve rotation device prevents burnt spots between the valve seat and the valve head." The illustration shows an arrow in the chamber between the valve seat and the guide indicating valve rotation but no further details.

I did a little more research and learned that this is a basic necessity in all types of internal combustion engines to keep the valves sealing. In engines with split valve keepers (collets) like the M110, a bit of intake or exhaust gas pressure is used to rotate the valve (as it closes?). If there is a one-piece keeper, somehow the valve spring is slightly wound as the valve is compressed and that tension used to rotate the valve.

I am also very curious as to exactly how this works.

Dear Community:

I believe, after long last, I have discovered the 'smoking gun.'

I decided that that there was just too much evidence that points to the 'damaged and thus sticky valve' theory, so I went ahead and avoided a bunch of variables and hassles by buying a whole new rebuilt head (with assembled valves) from Adsit.

Upon removal of the old head, I saw some telltale marks on the top of the affected piston, a new, shallow crescent shape. On the corresponding valve, there were marks that suggested contact with the top of the piston, but only around, say, 220 degrees around the perimeter; an interesting illustration of a valve trying to turn.

Also, the thick blackening of both ports suggest that things weren't closing.

I feel happy that I finally see the problem so clearly... I think. :^) too much frustrating experience with this car to jump for joy quite yet...

I am certainly glad that the engine gets to stay in this time- hopefully for good!

As soon as I get the new head bolts and gaskets, the whole thing goes back together. When it's a driver (maybe this weekend!) I'll write another update, maybe a picture as well.

Thanks for your technical help- as well as your encouragement.

Will

[Tomguy]

WillN: Possibly a bent valve that still rotated but since it was bent, part of the face was too close to the piston (thus colliding with it). It also would undoubtedly not fully close!

[psfred]

You can check the valve to piston clearance by installing a dial indicator on the head so that it's in line with the valve, set on the retainer, then set cam to correct spot (don't remember the degrees on the crank at the moment), then pushing down on the valve until it makes contact with the piston. Should be something like 35 or 50 thousandths.

Dollar to a donut you have bent valves on #6 from the piston contact during the failure.

ALWAYS completely check out a head on an engine that has tossed a rod -- it's always possible the piston got thrown up. End caps usually come off as the piston goes round the top of the exhaust stroke when the crank is yanking it back down, so if the rod stays on the crankpin for the rest of the revolution, it will stay up, then get hit by the crank on the next round and fly up out of time. When it bounces down, the crankpin the smash it through the block. With the canted valves on the M110, they will bend any time the valve contacts the piston while running, just like the V8s. The old vertical valve sixes and fours usually take the impact with minimal damage until it's so hard the cam or cam towers go.

Peter

Yep, by replacing the head with the rebuilt one, I bypassed any possible stuck valves, guides or whatever that was present from the old situation.

With careful assembly and valve gapping, the engine sounds normal now. A bit of adjustment via jets in the Weber 38DGAS to smooth out the idle, and I should be in good shape. Does anyone know which jet combination works best?

Thanks to all,
Will