Projects Old school cool 248 Gmc

Maybe, or I am going to dig though my sbc pile.

 

Cam came today. Looks great. Bearings will be here on Thursday Friday.

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Does anyone know how to date the dist? I am still trying to figure out how old this build might be. The last patent number is 1945 but it can't be that old.

 

I recently had my Mallory flat top apart, also with the vac brake, so I was also digging through the resources here on the HAMB. Best I can tell, there isn't a real good way to date a distributor. You can probably get it in the ballpark - Mallory changed part numbers at least once, so if you look through some old Mallory parts books with printing dates you might be able to determine which era yours came from.

I saw earlier you were talking about having it apart to service it and asked about setting/limiting advance. First, I think that the light springs you have in the distributor would be the harder to find ones. For every vac brake Mallory flat top I see, I see 100 more with only mechanical advance, so those heavier springs might be easier to find than you think. On mine, I set the locks out as far as I could on the advance plate when I had it apart. The rate of advance should then be only up to the spring rate on the spring behind the piston. I figured once I had it installed and working the way it should, I would start checking on total advance at different RPMs and then mess around with different piston springs then (which I haven't gotten to yet).

 

I only cleaned mine up and put it back together with no changes. My buddy has a dist machine so I will be checking the advance on the mech and vac brake before it is installed. Since mine has the freeze plug style to hold in the vac advance spring I hand to take it completely apart to pull out the piston and clean up the bore so it would slide nice again. Hope it works OK for what I want without lots of work. The standard style vac advance I had was garbage and didn't fit the mallory body. Problem right now is finding a pin to put the gear back on. The gear and shaft are double drilled so it looks like an 8. Maybe they use a stock gm gear so they have to drill the holes different. I found a piece of tig wire works for the "spacer" above the gear on mine but the gear isn't an easy to find pin. Had one that fit pretty good by filing a pin chucked in a drill but it wouldn't go the last 1/16 of an inch and just folded over. Our big local swap meet is this weekend so I will keep my eye out for flat top Mallorys for parts but we never seem to get much old speed equipment. Mine is different from some of the parts diagrams in that both weights go on one side and fit back to back.

 

Ah, I didn't realize yours had that style cover for the piston, don't think I've seen that before. Is it a regular nipple, or does the vacuum line thread on? I wonder if you could drill the distributor body out and adapt a later cover? Or maybe that's more trouble than its worth?

Agreed, putting it in a distributor machine is the easiest way to get it dialed in and ready to run, if I had access to one that's what I'd be doing as well.

I'm having trouble imagining the pin arrangement - you mean someone drilled the gear in a different spot to make it line up with the hole in the shaft, so the gear has two sets of holes, one on top of the other?

 

it is double drilled, not sure if you can see it in the first pic. I don't think I can change this vac brake to the newer style.

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it is hard to see but the hole was drilled once, then drilled again in the same starting hole but not on the same axis.

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I am adapting a Pontiac V8 Mallory Double-Life with vac-advance (# Z 312 E) to use on a 153 Chevy four cylinder. The gear is the same as Chevy V8s. I used a bronze gear. It is drilled at 90 degrees for two different pin sizes. The shaft is too short but I had an older Chevy distributer that uses a 2 piece shaft. I used that idea. I turned the body down to fit the Chevy. The later Chevy sixes are shorter than yours and won't work in the older engines but the old distributers can be spaced and used in the newer engines. I thought I was all ready and then it dawned on me that the Pontiac distributer turns the other way. I started looking for innards to swap. While looking for numbers on my parts I found that the point plate has the Pontiac # stamped in one side and the Chevy V8 # stamped in the other. The tabs for the screws just need to be bent the other way. My point here is that there is a lot of interchange in these. The biggest difference is how they fit the engines. The vac advance is not nearly as common as only mechanical. Double-Life is dual points but only half the lobes( V8=4 lobes). Each set of points fire 1/2 the cylinders. The theory is that the points and condenser will last twice as long but the reality is you have to buy twice as many.I have an early six Double life (YCM 76 HP) that looks like your but there are no vacuum parts inside. I have a couple newer ones that don't use the cool old cap but not the red caps either. Dating? After the GMC six came out and before the one piece caps. '39-?

 

Just a little note on 6 cylinder distributor gears. This is for Chevy but should be the same for GMC. They're not the same as SBC. What's your cam made of?

 

You can drill and tap a hole in the block to run an oil line to lubricate the distributor gear. Just a pinhole. You can see the copper line in this pic (Taylor Style Special).

 

I was worried about the gear. It is definitely for a steel cam and my cam is cast I have 2 other dist and will look that those gears. I planned on adding the dist gear lube line possibly this weekend.

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You normally don't need to drill a passage to oil the dist gear. Not sure on a 248 but the 270/302 had a 1/8" pipe thread hole about 2" down from the dist mount. It was the return for the bypass type oil filter. You get a 90* fitting, solder the pipe threaded end, drill a .050-.060" hole at about a 10* angle in it and tee off from the line feeding the cylinder head rocker oiling system to squirt oil on the gear. The stock big trucks did this from the filter. You can see the line in the photos of my sons engine I’m installing in our 1940 Chevrolet.

 

Yep, you are right they are not the same. Don't know where I got that must have been thinking of the old and new sixes. Here is what I have. Crane 20990-1. It's for the 153-292 but since the old distributers can be used in the newer engines it will probably work and take care of the steel cast iron issue. Not cheap though.

 

Jimmy Six, that's a nice looking engine!

My '47 270 has no hole. It must be a later modification. The 270 GMC in the Taylor Style Special is from a WW2 Staghound armoured car, manufactured by Chevrolet with Chevy style 4 bolt rear mains. I'm sure the owner said he had to drill a hole in that block.

Mallory bronze gear shows it fitting 235 Chevy as well as 230-250-292, so it must fit GMC.

https://www.holley.com/products/ign...tor_accessories/distributor_gears/parts/29428

 

The boss for the dist gear oiling is missing on this 270 and the original 248 I scrapped. Pretty sure this 270 is a 1941 block. I do have a later 248 to measure off of and steal the fitting from. The original 248 has the Isky LDB ground on a steel forged core and this Mallory was on it. The other 2 dist I have had cast cams, so I ass-u-me one or both will have a proper gear for cast. I will swap gears and wire this one to the Isky for proper compatibility if I use it later. Brass gears don't generally like cast cams at least according to comp, I only used one on my billet roller in my 427. The rest of the time I used a mellonized gear from gm, but that is all BBC stuff Now you guys have me worried about lifter compatibility again.

Good thing I bought every GMC I tripped over in the last 20 years. Makes my parts pool larger. So far my only new parts are cam, lifters, cam gear, gaskets and bearings.

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Your NORS milk can tappets will be just fine ... don't worry ! Recommend break-in with stock springs before using high rev replacement items.

 

Well my NORS bearings came. Pretty cool boxes and likely as good as modern bearings.

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Mallory has a groove buy the bottom of the gear, good for cast iron cams

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had the valve cover powder coated "chrome". Not perfect but not to bad for $40

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Those are one of the nest looking stock valve covers ever.

 

Agreed

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Sealed one no vents extra hole had copper tube an fittings. Best looking one as others had non-cursive letters and no circle. I believe these were used on Military also on the sealed engines.

 

this one is off a 248 bus engine.

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Didn't get a lot done. Installed the crank and used one rod and piston to determine deck height. #2 was .210 and #6 was .200 in the hole. We evened up the the deck and ended up taking .160 of the front and .150 of the back to set the piston .050 in the hole leaving. This comes out to 9.75 to 1 compression. The deck is still over .300 thick even after the cut. The second ring on these pistons is weird.

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Good ol' CK 10 Cylinder King …

 

Did a few things this morning. I am still a machining noob so I watched while my buddy honed the block to 3.960 which gives us .009 clearance, right in the middle of the Jahns 8-10 thousands. Then we spun the crank to check balance. It was at 2 gram inches in the front and back and we got each to under one by drilling the counter weights. Then for fun I weighed a couple pistons and found them to be with in .4 of a gram. If they are all that close I will be impressed.

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Actually it is the next model. Which is supposed to be green but when a big shop originally bought it the owner said "I ain't buying an ugly green one to set next to all my red CK10s! If you want me to buy one it better be red!" So it is red. Basically a CK10 with more hp I am told.

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Small update, Worked on getting the piston weights within 1 gram, about 1.3 gram spread currently. Compared some stock SBC valves to the valves that come out of the GMC 302 head. They are within .020 in length and should work perfect. The valves currently are approximately 1.84 in and 1.45 ex, going to 1.94 in 1.60 ex valves. Intake seats will be going to a 45 deg angle. I will try and flow it with the stock valves and the improvement with the bigger valves. It has a mild port job but that will probably get little clean up. Sorry no pictures this time.

 

Stopped by the shop for an hour. My rod bolts came in. The are FE Ford bolts. Took a small grinding modification to fully seat. Length is just about perfect. The bolts go in with a easy tap from a dead blow. Looks like I will now need to resize all the rods.

The 35 Chevy is getting worked on. Sat since the mid 60s but the engine wasn't stuck. Owner heard a bad noise cracking that turns out to be a big mouse best in the bell housing. Head isn't cracked and they said the bottom end looks good. Probably get cleaned up and honed and reassembled.

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