SBC

[DirtRacer9s]

What would cause both intake gaskets to fail and suck oil into all 8 cylinders. My rotor came loose the last race I ran, and ofcourse it ran like crap. Before the next race we Found that and changed the cap and rotor, fired it up and it ran fine. So I went to pull it on the trailer then it kinda like hic cuped, so I pulled it off the trailer and ran it down the driveway and it was bangin out. I pulled it in the garage cleaned out the carb and fired it up. It ran better but still didnt sound right. So I pulled the plugs and they were fouled out. I put new plugs in and fired it up and started puffing(not consistent more like in little burst) smoking out of both pipes. Then I reved it up to see if it would clear up thinking there was oil in the pipes but it just started billowing out. We pulled the intake off which all the bolts were tight and when we got it off there was oil puddled around the valves. You can see on both of the gaskets where oil had been passing through. I have not got it too hot in long time. The intake is one of the late 60s early 70s, 325 horse cast iron 4 barrells. Got her tore down but plan on running this weekend if we can get it figured out. Its a 355 with stock heads that have never been milled so they are stock cc(we know cause we cc'ed them at 76cc). So the intake sat on there properly. The block has never been decked or resurfaced(we also measured for this) so everything bolts together properly. Any info/help would be appreciated.

[4cylfordz]

that usually only happens when something has been milled. you may want to check the intake because that is commonly cut for compensation. an easy way to check is with clay or plumbers putty, just like checking valve to piston clearance. though u should check the surfaces of the head and intake for warpage (think about it they are hot in the center and cold on both ends) this can typically be fixed with just a small coat of right stuff silicone around the intake ports on both sides of the gasket

[jwmbishop]

X2 - it's the downside to V configurations. You think it's problematic on the SBC - look at a 534 Ford Gas - the deck is not 90 degrees to the bore line just so it brings the heads closer together and a manifold less than 3 ft wide can be used!

 

The only true deck surface references on a head get milled off in other steps before the core even leaves the factory. When milled later the existing surface is the only reference - if it warped and the machinist does not pay close attention it can get milled to the untrue portion of the surface. and even if milled completely true (and especially problematic with older intakes) as the head is milled it gets closer to the line bore - so are the planes for the intake (overmilling to lower chamber CCs will move the two intake surfaces DOWN and CLOSER together and raise the intake requiring milling of the intake manifolds surface - easiest, or the intake surface on the head - very difficult due to valve cover boss). The ends of the intake at the valley can be too close to the block and stop the intake from going down all the way. Rule of thumb is you should be able to bolt the intake down with only a masking tape gasket (both surfaces) and 10ft lbs torque (to avoid damaging the surface) and still have at least a .050 gap at both ends of the intake to valley seals AND not be able to get an .005 feeler in anywhere along the head to manifold surface.

 

SO - most folks remill the intake being as it is so much easier - but then you run into problems when using that manifold on different block or heads. After milling the manifold and then using it on a new block\head assembly the manifold may not be wide enough even if the angles are a match - in which case it will want to be pulled in two directions by the bolts and expansion - basically walk side to side chaffing the gasket until failure occurs (also pulling the dist over slightly adding vibration which can show up as rotor cap screws coming loose)

 

Given the distributor issue followed by the gasket failure it sounds like this is what you have encountered. Add in that factory tolerances are fairly wide here so it is not uncommon to see two slightly different angles between all new block/head assemblies - so once remilled to correct for a previous combo the manifold may be within tolerance on one all new assembly but not another!!). Look closely at the gasket (especially along the bottom) to make sure the intake ports are fully covered. I used to have a source for thicker gaskets (they were bronze shim core coated with paper about .030 thicker than the felpros - but a word of warning here, sometimes it requires a THINNER one (enginetech rebuilder sets are slightly thinner to allow for resurfacing of both head and deck and use of an unaltered manifold - rest assured ET sets have seen lots of racing with no problems beyond machinist introduced issues).

 

Also look closely at the manifold bolt holes - sometimes they crumble just like the head bolt bosses do (and then need to be spot faced or washered). The first indicator is a ring indentation where the shoulder of the bolt sits after that it just crumbles away. Although tight by torque value - when the boss crumbles the force is moved further out away from the bolt main body on the bolt head - so the clamping force felt at the gasket is lower.

 

Best bet is to replace the manifold - but I know that you don't want to as you already have one of the best stock iron pieces ever made! (the late 60's marine application are best). Plan b - glue it.

 

When using silicone as above I have found best results are to start super clean (brakekleen works well for pre silicone cleaning) bead the head around all ports with grey silicone staying back 1/8 inch from the port with a 1/8 inch bead, lay the gasket, bead the manifold, set the manifold without jiggling so that you are going straight down into the beads of silicone, just starting all the bolts - then WAIT about 30 minutes to an hour before snugging and torquing (this not only allows the beads to skin and remain flexible enough to fill the gap issue - but also helps prevent rolled out chunks of silicone from going into the intake and/or sump).

[DirtRacer9s]

So if I had a set of heads milled from 76 to 70cc would help the intake fit better.

[4cylfordz]

flat milling-no....this will only bring the intake ports and bolt holes lower but will retain the geometry of stock. angle milling will change the intake face. though I would suggest figuring out what the exact problem is first before I started whacking metal off them

[jwmbishop]

flat milling-no....this will only bring the intake ports and bolt holes lower but will retain the geometry of stock. angle milling will change the intake face. though I would suggest figuring out what the exact problem is first before I started whacking metal off them

absolutely correct! I probably over expressed - but the point I was trying to get over was that once you modify the manilfold to fit a specific head/block assembly it may never fit another. Trying to replicate that head/block assembly (without it being a blueprinted build) would be a real needle in the haystack!

[JamesHigdon]

I have dealt with some of the same issues on SBCs and maybe it comes down the to gaskets? What are you using and how are you locating them? Unless you are using metal backed gaskets it is possible for the gaskets to sag and causing leakage issues at the ports. Also, if you are using the rubber end seals they can hold a manifold up off a decked block setup. I've had leakage issues before related to using piece meal components that we sealed with different gasket setups and theres a chance you may be able to.

 

James Higdon

[billy5x]

mr gasket ultraseal..thick gaskets...no more problems...