Jump Start Engines – part #6


Three groups left to address in the closed case: The Case Group 1200, the Hub Group 2500, and the Rear Oil Case Group 2000.  Let’s take this in order and look at the case. At the bottom of this part I have included several paragraphs on cases from my note book on Corvair engines.  Worth printing and including in your conversion manual if you have not yet passed that part of the build process.


If you are building a 2700, or a 2850, there is no machine work to be done to the case. It only needs to be cleaned really well, checked for damage, and have all the studs in it in good shape. If you would like a 3,000 cc engine, the six cylinder holes in the case have to be enlarged a bit on very good machinery. You can read about this by clicking on this story: 3,000cc Case Modifications.


Below is the simple elements of the 1200 group. If you have a good core, you have nothing to buy, just a fair amount of time carefully cleaning:


Case group (1200)

1201- Case -2 halves with studs-

1202- Main case bolts -8-

1203- Main case nuts -8-




“Once you have an acceptable case, you need to clean and inspect it very well. Pressure wash it after soaking it with a cleaner like Simple Green. This will get most of the big stuff, but in the end, there is no substitute for several hours with plastic bristle brushes and careful scraping. Many people let their local transmission rebuilder give it a first pass at cleaning. The only two things you can’t do are use bead or sand blasting of any kind, nor use any type of scraper or abrasive on the parting surfaces of the crank cases.

The oil gallery plugs can be pulled with a ¼” drive extension. On 50% of engines these cannot be budged. Leave them in and use a gun cleaning kit to brush the galleries out from the end. Get a gun cleaning kit for a .17 caliber rifle, as these are the only size brush that fits in the oil feed holes to the main bearings. Check every oil passage. My neighbor lost a 200 HP Lycoming 2 hours after overhaul. $10K in damage was caused by a mud dauber insect making a sandy plug in an oil gallery. Check, don’t assume.

A lot of discussion goes on about what is acceptable for wear in a Corvair case. I know people who think that a bearing bore that is .0005” out of round makes a case scrap. I have good reason to say that this is an arbitrary number, not reflected in GM’s literature. For all we know they were not this good new. A much more reasonable idea is making sure the bearing bores are within a thousandth of an inch of being round. This can be directly checked with a bore gauge or a set of Ts and a good caliper. You can also evaluate this by looking at the plastigage when checking the crank.

The whole discussion seems like a lot of talk over a very little issue because I have never seen a spun bearing in a Corvair that had oil in it. If they had a big history of spun bearings and every fourth core had one, then bearing bores would be critical. In hundreds of cores I have not seen such an issue, so I am not alarmed. Same concept covers the cam bores and the lifter bores. Look carefully, clean carefully, but don’t let your project get derailed by an unreasonable arbitrary specification. In the first 30 years of Corvair powered flight, no one ever looked at these measurements, they just put engines together and went out and flew them. Over the years I have had a number of people say to me that they don’t want to build a Corvair after finding out that Corvairs don’t have cam bearings (the cam rides in the case). I pointed out to these people that Lycomings are the same way, and engines like the IO-360 use the bare aluminum case for the thrust bearing. I am sure they thought I was wrong or lying, but it is reality.

Some builders get off to a false start by deciding to pull the stock head studs out of the case because “they look rusty.” Do not do this. Studs with surface rust are very rarely damaged beyond use. Our FlyCorvair.net Web page has a good article about how to test head studs that you suspect of being weak. If a stud is very rusty on the top threads or shows signs of twisting or harsh tool marks, it is a good call to change it. If it just has surface rust, clean it off with Scotch-brite and repaint them. If you have questions on a specific stud, call me or send us pictures of it, and I will be glad to help you decide on a course of action.

The base of the studs is a special thread called a 3/8-NC5. It is not a 3/8-16 UNC thread.

Helicoils are 3/8-16 UNC. If you install a helicoil in a case, you must resize the end of the stud with a very sharp 3/8-16 die. Use real cutting oil if you expect this to work. Grab the body of the stud in a vise with soft jaws and clamp it very tight.

If you have a stock stud unscrew cleanly, without removing aluminum from the case, the hole and the stud can be carefully cleaned and dried off, and the stud then screwed back into place with Loctite 620 on it. 620 has awesome holding power. You must remember to put it on clean surfaces, and the bottle must be shaken thoroughly before applying. Keep in mind that half the holes are blind. If you fill one of these up with 620 and screw in the stud, it will hydraulically lock and stop. I drill the bottom of blind holes with a #60 drill to allow the pressure to escape. Do not chase a stock hole with a 3/8-16 tap; you’ll only ruin the threads. If you need to remove a stud or helicoil a hole, use the following method:

  1. Clean the cases thoroughly.

  2. Jam two 3/8-24 nuts into each other on end of stud.

  3. Heat case with propane torch. Put candle wax at base of stud when hot. (Capillary action will pull wax inside.)

  4. Unscrew stud.

  5. Drill cases on drill press for long helicoils. Follow directions that come with the helicoils.

  6. Clean studs with a wire brush; inspect for twisting or tool marks.

  7. Chase the 3/8-16 threads on studs with a split die. The threads on the studs have the NC-5 shape that must be chased with a standard 3/8-16 split die to ensure proper fit with the helicoils.

  8. Test fit into case. When satisfied, Loctite 620 into case.

I’ve often read that a #6 oversize Corvair stud fits in a helicoil. This is a bad joke; it does not. I recommend against using any of the oversize studs. They’re very difficult to screw into the case. They are also of varying quality. Clark’s also sells a 7/16-20 base stud. I would not recommend these, unless you absolutely have to use one. ARP made a batch of super high quality head studs. These cost $300 for a set of 24. They screw right into helicoils. Several of the engines we built and flew used these, including our own personal engine in our 601 and the engine we built for Rick Lindstroms’s 601. These were available in 2003-2005, but they are not made today.

As an alternative to using long helicoils, the case can have time-serts installed. These are available, along with installation kits, from Clark’s. Some people find these easier to install. Again, no stock or oversize stud will thread directly into a time-sert either. If you have a case with a lot of mangled studs, perhaps the easiest route is to find another case.

Always keep cases in pairs. If you change half the case, the motor should be considered junk. I’ve seen them run in cars this way, but it would be ludicrous to consider building a flight engine this way “


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