Thread: 348 chevy questions

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hi everyone, this is the first post for me. i am rebuilding a 348 and what is the story on the heads. i have heard that you need to put stainless steel valves in because of the weak water jackets ( how that helps, i don't know ) but i was wondering , do i need hardend seats in the heads or soft seats? do the stainless steel valves eliminate need for hard seats,or do i use stainless valves and hard seats ? from what i understand if you use the original valves with hardened seats it will bust the water jacket and i dont see how stainless valves will fix that.

i was pleasantly surprised at how good the motor is inside, everything looks like new . rings and brgs is all this thing is going to need.

also i plan on putting GM TBI off of a 350 on this thing, i am thinking that it should work good because it is only 2 cubic inches less or does anyone have any bad experiences doing this?

I agree with Tech....

It all depends on how you plan to use the engine....I f it's in a light vehical and relatively low rpm's you'll be ok for a long time...

The heavier the vehical and the higher the rpm range then hardened seat should be in the future ( especially if you race it)...

as far as stainless valves....they are lighter and offer more flow and can only help you ....but can you justify the cost....I would use them especially if you old valves are of 2 piece construction...

as for the Tbi question...I would contact www.jetchip.com the have a 1-800 customer service number on there site and can give you the advice you need....

also how do you plan to put the TBI on there...A custom manifold?

if you are leaving it pretty much stock id go for the TB.

there lighter you say, that explains how they protect the head, never thought of that. the valves aren't that expensive, about $170 for the full set. the motor will be rebuilt stock and put in a 53 chevy pickup but i plan on putting 10k miles on a year or hopefully more.
it was an industrial motor so i hope it has the hardened seats in already, i heard that they used them and i hope it's true. oh well if it doesnt it's nothing that money can't fix ,thanks guys.

as for the TBI you can buy and adaptor plate from square bore to TBI , and the 348 / 409 distributer is different than other chevys so i have that at a machineist right now getting the center to accept the shaft and guts out of the TBI distributor from the donor vehicle.

Whoops!!! somehow double posted,,

You will be able to tell if it has hard seats. After you remove the exhaust valve, clean the area real good and look for the OD of the seat. If you can see where the outer edge of the seat meets the chamber. It has hard seat inserts. But, back then even the hard seats they used (hardened cast iron inserts)were generally not good enuff for hard use on todays fuels. Todays seats are usually Sintered (powder cast) or stainless/stellite alloys.

If you plan on any kind of towing or use other than a driver. Have hard seats installed. Do use the Stainless valves. Lighter, tougher. They will also show less seat pitting and erosion. Common with modern fuels.

PS. If you need this engine bored. Most small machine shops will not have the spacer used to bore a 348-409. Unles they have a diamond head in a CK-10 or CV-616 type machine. They will need the spacer for a regular boring bar.

The commentary about "thrashing", "towing", etc. is accurate. But another way to put it is operating conditions that create high heat (which the above do) is what causes valve seat recession.

Over the years lots of mythology has arisen over unleaded fuel and valve recession. The primary victim is the exhaust valve (heat thing). What is going on is the valve and the seat momentarily weld together in spots. As the valve opens again it "tears" metal from the seat. It's doing this in very small amounts, but over time it "eats" away at the seat. The only thing that the lead in fuel did was deposit lead oxide on the seat and valve face, i.e. it made them "dirty". As most experienced welders will tell you it's difficult to weld dirty metals. All the stories about "cushioning" and "lubricating" are misunderstanding of the dynamic in place.

The reason that a stainless valve works isn't really related to weight or any other mystical property that some might imagine, it's a matter of dissimilar metals (stainless steel vs cast iron or steel) not wanting to weld together very well either.

as for the dist. not fitting a 348 , there is a aftermarket spacer available from ,lamar walden (hes is the guru of a 409 ) for a 348/409 to standard gm dist .
the 348/409 shares many of the same parts as smallblock/or bigblock chev .
oil pump smallblock
drive for oil pump bigblock
lifters bigblock
rockers big block
valves ford /chev
springs smallblock
crankshaft , with some machining makes a stroker motor
and now msd makes a dist for a 409/348
hope this helps

well i checked the seats and it doesen't have the hardened inserts so i know what i have to do now ,and no, its not going to get bored out, it all looks too good, no groove and the pistons show little wear and the rings are still tight in the grooves.

luckily the machinist didnt start on my distributor yet and i checked out lamar's web site for the distributor spacers. didnt see the spacers listed but i e-mailed him about them. thanks alot for all the info, it is making this alot easier for me.

FYI my day job is a mechanic on Case-IH and New Holland farm equiptment, mostly on the tractors. so if your tractor is falling apart, mabey i can help out .

I am pretty sure as far as the distributor goes the 348/409 distributor was/is the same as the chevy big block "tall deck" distributor and these are readily available for TBI....

If you are rebuilding this engine...put special emphasis on the oiling system as these were a little lack lustered and there is a lott of room for improvement....enjoy

Dr Bowtie, actually the 348/409 dist. is shorter than typical sb & BB Chevys. oil system is more than adequate . raced a 409 for years with stone stock oil system in the 1980s

Nice to know thanks for the clearifacation.....

I did a lot of work on these too and can vouch for the week oiling system....especially 450hp and up.....

K...I just never had a problem, same oiling system as BB Chev. ran 11.70s in 4000 lbs. Bel Air, never a oil related faliure

i'm not too concerned about the oiling system, mine will be completley stock with the original cam. i checked the number on the came and according to sources it is a 250-280 hp cam, with the TBI injection i am hoping that it will be in the 280-300 range. that should be more than enough hp for the weight of the pickup an the motor should last a long time. i can't wait to hear this thing run again, i only heard it run for about 10 seconds while i was pouring gas down the carb and then it backfired ignighting my bottle of gas, me , and the three cornheads i threw the flaming bottle at.

take a look at Professional Products POWERJECTION I this is what I'm going to put on my 31 A with a 409