Here's the history:
87 motorhome w/460 (58,000 mi.) backfiring through the carb.. After the normal parts replacing I did what I should have done sooner...I did a compression check'

I found #7 cylinder was dead with 5 lbs. compression.
I've pulled the heads and found what appears to be a bad intake valve seat.

I'm going to take the heads in to the shop but when I put it back together I'm going to put in a new RV cam and do the "straight up" timing set up.

The problem is I don't really know if I should do more, like a short block.

The compression on the good cylinders was 135 to 140.
Does anybody know if that's low...normal or what?

Oh, one more thing. Some previous owner has replaced one of the heads. One of the heads has numbers above the exhaust port, the other one I can't find numbers on though they look identical....Anyone know anything about head ID's

 

the one with the numbers on it is probably d3ve-a2a. theyre the same head as the other one without numbers for all intents and purposes. as for what should be done to the block, does it hold good oil pressure? if so id just get the head fixed and drive it. at 58,000 miles everything on the shortblock should be fine. the straight-up timing chain will help on power, as will the rv cam. a little exhaust porting will do alot of good as well, as will headers. the 135-140 compression is normal, but definitely look at the walls on the dead cylinder to make sure the rings didnt get screwed up. if the cylinders all look good you should be good to throw it back together and drive it when the heads come back

 

only thing I would do is carefully check that #7 cylinder wall, well actually all of them for that matter but close attention to that one. Make sure there is no deep scaring of the wall that would indicate a broken ring etc, or would cause a loss of compression. If the cylinder walls look good I agree with darrin get the head fixed and drive on.

 

Thanks monsterbaby and darrin I feel reassured that I can live with what I have....

 

140psi is a decent number for the later low compression 460's.

 

what about 100 on front cyl on each side?

 

Before you bring the heads in I would do some mild exhaust porting. Grind the smog bump into a vane pointing out of the exhaust port, smooth and increase the radius of the short turn, and slightly deepen the trenches and smooth the bowl area. Beware water is close on the right side trench with chambers up looking through the exhaust port. Takes maybe half hour per port to do a quick mild cleanup and grind the smog bump. Not an all out effort but will give you some more power for passing and hills. Basically free power. If it's feasible get some headers, they will help tremendously if you do a mild exhaust port cleanup. Exhaust cleanup and headers should also help MPG a bit.

I wouldn't bother with the bottom end so long as cylinder walls look good and not big ridge at the top. You're fairly low miles, bottom end should last at least another 60,000 before you lose some compression to worn rings/walls.

 

Hummm, now why does this sound just like what I been through.

#1 thing, you didnt say about the oil presure, what the engine had running down the road or idleing.


Before you go any further or have any work done to those heads, study up some about the 460 so you'll know what the engine is doing & learn what needs to be done to those heads to make them survive.
Be sure the repair shop, servicing the heads know that you have a severe duty engine & its not a pickup or car engine.

You sure it is an intake valve messed up, because normaly its the exhaust.

That motorhome engine, pulling as much weight as it has to do, will cause real high heat in the exhaust area & end up burning the valve seats out.
Being an 87 model carbed engine, the heads should be the E6TE heads on both sides, but you say the engine has been worked on before & one is different.
Different head may not be a problem & the fact your back into it again is saying the first head repair job wasnt done right.

To make those heads survive the job they have to do, you need Stellite valve seats installed in the exhaust side & either sodium filled exhaust valves or Inconal exhaust valves & carbide seats.

Dont let the repair shop talk you into just having hard seats installed on the exhaust side. The nickle based seats will not hold up.

I know this because I've just been through it dealing with my engine.

Another thing you need to know, you need to know 100% that the repair shop is using the right parts in those heads.
A stellite seat, a magnet wont stick to it so check each seat before the repair shop installs them, dont guess because it will cost you again if you do. You have the heads off again, right, so you know that repair shop didnt do the repair right or else you wouldnt be back into it.

SBI, a valve seat MFG, the seats they supplied for my engine had the wrong part number stamped on the seat itself, was a nickle based seat & not stellite, so do the magnet test to be sure each seat is stellite.

Your stock exhaust system is another thing, its to restricktive for what that engine has to do, replace it with larger pipe & muffler or even go to duel exhaust.
As allready said, have the exhaust side of the heads ported, opened up so they can flow that high heat out.
Do everything you can to combat that heat, lower temp thermostat & make sure the air flow through the grill is direct to the radiator, dont rely on just the fan pulling it through.
A cold air intake is good as well & make it big so you get plenty of flow.

Cam, I installed the Summit #3500 cam kit in mine along with the Edelbrock double row timming set, set at 4* addvance.
Neil

 

I would be very cautious porting around the short turns. They are the most sensitive contour in a port and you can easily junk a head if you don't know what your doing & have no flow bench. These heads have a crappy port to begin with.