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We keep blowing head gaskets.
https://slantsix.org/forum/viewtopic.php?t=58698		 Page 2 of 2
Author:  wjajr [ Sun Nov 15, 2015 6:19 am ]
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I agree one or two bolts are prematurely bottoming disallowing full clamping pressure on head.


Use a grade 8 washer, it's harder, able to hold its shape and thickness better. Chase hole with blind tap to clean last couple of threads at bottom, then evacuate all the smutz from hole. Lastly measure each bolt threaded in until it bottoms without head making sure all can be fully seated before bottoming out once head is in place.
Author:  SpaceFrank [ Sun Nov 15, 2015 7:11 am ]
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We always put the bolts back in the same spot. First blow was between 1 and 2, second was between 3 and 4, third was between 1 and 2 again. Based on how the bolts feel when torqueing down, I don't think they're bottoming out. Especially since they all gave a bit more on the recent hot retorque.

Definitely will check block straightness. Head should have been fine, but it will need to be checked again after blowing two more gaskets. Good point about spark advance. We really need to.look at that.

What are the signs of detonation when looking at piston tops? Nothing looked unusual when we had the head off recently, but I'm not sure I'd know what to look for.
Author:  Badvert65 [ Sun Nov 15, 2015 7:25 am ]
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The more I read this, the more it sounds like SEVERE detonation. That can break stuff, like pistons, rings, spark plugs,etc. If this is what is happening in your case, you are lucky and it's blowing the head gasket. But, have the head checked for straightness just in case. It would have to be warped in multiple places and just blowing out in different places by coincidence.
Author:  Doctor Dodge [ Sun Nov 15, 2015 9:01 am ]
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You really should deck the block to make sure it is dead flat and has a nice surface finish...

I know it's a lot of extra work but if you pull out the head gasket / head locating pins and then use a stiff "long board" to flat sand the block/s deck surface, you will quickly see many high / low spots, left behind from the factory's machining.

The block shown below has high spots that tend to line-up with the areas between the bores but this engine would let water thru the low spots, where the small 'steam holes', go up thru the head gasket & into the head.
DD

Image
Author:  SpaceFrank [ Mon Nov 16, 2015 11:50 am ]
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Well, the car ran the rest of the weekend as a slant 4. I think we finished nearly dead center in the pack, something like 40th out of 79 cars. I guess it wasn't making enough power to blow the gasket in another location. I look forward to seeing what it looks like when the head comes off.

We did a rough timing check during a pit stop. It looked like about 25 degrees advance at 3000 RPM, and about 30 degrees at 4000, but I got these numbers from extrapolating distance by eyeball off the stock timing tab that only goes to 15, so it could be +/- 5 degrees. What's the best way to check this accurately? Attach a ruler to the timing tab? This engine never sees anything above 4500 RPM, FYI.

We are planning on upgrading our ignition to GM HEI soon. My understanding is that this mod uses the same Chrysler electronic distributor. After I get a better measure of our total timing, I'll see if we need to make some adjustments to our distributor.

I am comfortable with the accuracy of the 8.8 figure we have for SCR, although I'm sure that will go up a bit after getting both deck surfaces cleaned up. I'm not as sure about the DCR figure since I don't know exactly how our cam is degreed, which is something I'll plan to actually do properly when we re-assemble this block again.
Author:  ProCycle [ Mon Nov 16, 2015 11:57 am ]
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You can buy a timing light with an adjustable advance dial on it.
Author:  CNC-Dude [ Mon Nov 16, 2015 1:39 pm ]
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Your making a huge assumption to even think the timing tab and balancer mark accurately represents a true value between each other without first verifying they do. Also, its extremely hard to tune an engine with an unknown or unreliable timing window of as much as 10°. That much variation can create an overly rich or an overly lean condition in carb tuning alone. This could be the reason you keep popping head gaskets. You need to narrow the timing variance down to 2° or less, and then tune the engine around a known or true parameter. As ProCycle said, an advance style timing light is your best friend, and they are not expensive.
Author:  SpaceFrank [ Mon Nov 16, 2015 2:34 pm ]
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Right on. I just ordered an advance timing light, so we'll be able to measure that soon. We installed a new harmonic balancer a year ago, and I'm pretty sure we verified the timing mark was accurate at that time. If I can dig up my piston stop tool I'll check that again.
Author:  Dart270 [ Mon Nov 16, 2015 7:02 pm ]
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I wonder if your torque wrench might not be right and you just have low torque? Just another thought. Glad to hear you finished the race.

Lou
Author:  SpaceFrank [ Wed Dec 23, 2015 5:31 pm ]
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I finally got around to measuring the ignition advance on the race motor. Took a bit of cranking with a 100% charged battery to get it fired up cold with only four cylinders compressing. I let it run until it got up to typical operating temp before I took this data.

The mark was very jumpy up to 1000, and still a little bouncy through 1500. After that, it's +/- 1 degree. I was afraid to rev it any higher than 4500, in the garage by myself with one hand on the timing light and one on the carb.

RPM - Advance (deg. BTDC)
600 - 2
800 - 2
1000 - 2
1250 - 6
1500 - 9
1750 - 12
2000 - 16
2250 - 19
2500 - 22
2750 - 25
3000 - 27
3500 - 34
4000 - 35
4500 - 37

The chart can be seen here: https://docs.google.com/spreadsheets/d/ ... sp=sharing

Based on what Lou and others have said, this looks like too much total advance. Now that I have the data, I'll start pulling the head off. I still intend to check both deck surfaces for flatness, but now that I know we've got a timing issue I might skip the machine shop if everything looks nice and flat. I've got a machinist's edge on order.
Author:  Joshie225 [ Wed Dec 23, 2015 5:36 pm ]
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So you have the initial timing set to 2°?

Yes, 37° is too much. 30° total is about right which for most anything with a decent street cam or hotter you run about 10° initial timing and 20° mechanical for a total of 30° BTDC. And it should be all in by 3000 RPM or a little sooner.
Author:  SpaceFrank [ Wed Dec 23, 2015 5:43 pm ]
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I seem to remember having it set at 5 degrees, but god knows what we did in the paddock during this last race. Normally it seems to idle best around 7-8.

I've got some reading to do about re-curving distributors. That's on the agenda for this winter. I'll shoot for 30 degrees total. Even then, from now on we'll be racing on premium gas just to be safe.
Author:  DusterIdiot [ Wed Dec 23, 2015 6:09 pm ]
Post subject:  It's...
Quote:
I was afraid to rev it any higher than 4500, in the garage by myself with one hand on the timing light and one on the carb.
Much easier to use a screwdriver and tighten the idle stop screw so you can keep the rpm constant and pay more attention to the tach, timing light and anything that might get caught in the fan...

Let us know what you find when you pull the pickup plate...you should only have seen 30 degrees of advance mechanically if it was a 1973-1975 15L governor.... (hopefully the damper hasn't slipped a bit and timing is a little off)...anything above that and the governor would be a for a V-8 (at 17R for 34 degrees of mechanical advance).

If this was a highway car, the numbers aren't that bad and would produce good mileage even in a 9:1 SCR engine...but if you have to hammer it out of the corners or a lot of part throttle on the uphill grade, then you will sacrifice some pistons if the head gasket doesn't blow....

As Josh said you will need to respring to have the mechanical all in by 3000 rpm...(on highway cars I tend to 'peak' my recurves a couple hundred rpm above the rpm at the highway speed limit...so there is some room when passing and the vacuum advance won't be heavy handed at part throttle cruise, typically this happens at a lower rpm than 3000, unless you are running a set of 3.55's and some 13" tires... or a Feather Duster with 4.56's 14" tires and leave it in 3rd...LOL).

If you are targeting your advance with 8-10 degrees of initial you will have to limit the governor, either by replacing it, welding the slots up, etc...so you have the equivalent to a 9,10,or 11 stamped governor.
Author:  SpaceFrank [ Thu Dec 24, 2015 7:01 pm ]
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I used the idle screw at first, but once I got over 3000 or so I got worried about being able to bring it back down quickly if something happened. I was able to keep it pretty steady, though. The higher RPM numbers are still +/- 100.

I'll check back in once I have the distributor apart. This is an FBO unit I bought several years ago when I converted to electronic ignition, so I've got no idea what guts are in it.
Author:  440_Magnum [ Tue Jan 05, 2016 12:09 pm ]
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Quote:
Well, the car ran the rest of the weekend as a slant 4. I think we finished nearly dead center in the pack, something like 40th out of 79 cars. I guess it wasn't making enough power to blow the gasket in another location. I look forward to seeing what it looks like when the head comes off.
Actually, I view it the other way around. The working cylinders were running at *higher* mean pressure to keep the engine running and haul the car around with two dead holes. The fact that you didn't blow gaskets on them makes me somewhat doubt the detonation theory... but that's just speculation. Were it me, I'd focus most on block and head flatness and clamping force. Might be time for new head bolts "just because." Washers as suggested wouldn't hurt either.

By the way, I follow you guys on farcebook and the LeMons site. Good luck this year!
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