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 1275 Engine Refresh 
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This brutal Winter combined with other projects at work and at home have kept me away from working on #909. But I knew I needed to get back into the engine before the '14 race season started.

For one thing, the 1275 is pushing a bit of oil out of the rear main scroll "seal". I haven't really changed anything in terms of crankcase venting so I've been assuming crankcase pressure has increased for some reason; probably blowby. The engine has about 50 hours on it, so it's due.

It's important to note here that the engine was running OK and still able to pull almost 7000 in 4th gear. But the oil leak spoke to emerging, related problems. I've been decent with maintenance; new oil / filter every event (20W/50 racing). Valves checked/adjusted regularly (although these are stock rockers so they are hardly ever off at all).

Last week, with reasonable temps, I decided to do a leak down test. A normal compression tester last Fall indicated things were OK, but I never really trust them. The leak down test confirmed my guess: 60% leak down in cylinder #3 with others in the 40% range.
During the test I also ran about 60 psi directly to each cylinder and used a piece of 3/4" plastic pipe (as a sort of primitive stethoscope) to listen for leaks. Cylinder #3 was allowing a lot of air into the sump (I could hear it via the dipstick hole). And cylinder #2 was allowing a lot of leakage via its intake valve (it was audible from the from carbs).

So, off comes the head.

None of the valves actually looked too bad. Margins are a bit skinny (I ground them 2 years ago) so I will replace all with new ones.

Head gasket looks like it may have been compromised between #2 and #3. I'll have to look at that.

Ring end-gaps were all pretty loose. Cylinder #3 was over 0.035" (it should be around 0.010" or so). One ring came out cracked but that may have been my fault. There was a large built-up of crud at the top of the bore and I may not have cleaned it up as well as I could have before knocking out the piston.
One thing that I always notice is the excess build-up of crud on valves, pistons and the upper cylinder bore. I would guess these are lead deposits because I never see this much gunk on cars running unleaded street gas. I'm also running a slightly rich fuel mix, but this looks more like lead than carbon.

I think the heavy bore deposits may be related to the fact that the 21253 AE pistons place the upper ring pretty low relative to normal pistons. This make the upper part of the piston stronger but leaves a space for deposits to build up

When I had my first MGA back in the '60s, this sort of heavy lead deposit was common (I don't really think it's carbon). Build up of lead on plugs was very noticeable even after 1000 miles. The owner's manual for my old TR-3 used to advise that you should "remove the head and de-coke every 10,000 miles".

Two of the pistons I removed have scuffing on the skirts of the pistons (and not just on the thrust side....both sides). I'm not sure why; the other two are perfect. So I'll buy two of the 21253 pistons (+.040) and re-ring the old pistons. Then I'll balance the piston/rod assemblies, run a dingleberry hone up and down the bores and hope for the best.
These are pretty mild, dished pistons designed for 9.75:1 compression.
Combined with a "fast road" cam (276 Kent), stock carbs and a non-LCB header the engine is overall, pretty mild. I'd guess maybe 75 horsepower.

Crank bearing journals measure good so I'll just replace them with new tri-metal bearings (-.010" in my case).

Right now, I'm waiting for parts from Moss and 7Enterprises. Then the fun part begins......

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Mon Mar 17, 2014 8:40 am
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Good stuff Nial. Scott is tearing down the Turner 948 today, I'll see if I can wrangle some pictures from him and put up another thread.

We've been eyeballing this rear main seal conversion, to address the issues you note with oil making it past the scroll seal:
MED Midget Rear Main Seal
but as you note, this isn't really much of an issue when there isn't blow-by. It appears in our case, the pistons and a couple of rings have given up the ghost, and I think the leak-down was in the ballpark of your own.

Off to the machine shop to see whether the cylinders are salvageable...hopefully they are, otherwise, we're going to have to dust off one of our spare blocks and prep that.

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Mon Mar 17, 2014 11:48 am
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You can sleeve those cylinders but it's often not worth the cost and effort.

I have a 948 street Midget that was leaking out the rear main too: new rings solved the problem (at least to an acceptable level).

By the way, I've read many comments on the 'net about the various rear main seal kits. From what I've read, the Moss kit isn't very good.

I'm not familiar with your's. The split seal is interesting.

There is a kit from Gerard's Garage that is pretty well-regarded but I think you have to line-bore the main bearings first to be sure it works; obviously a lot of work.

http://gerardsgarage.com/Garage/Tech/SealKit/SealKit.htm

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Mon Mar 17, 2014 2:35 pm
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Nial,

Not sure if this applies to the A-series engine, but on the B-series, the correct clearance for the scroll requires a SMALLER i.d. on the cap/block than the i.d. for the main bearings. There's more detailed info on the MGA Guru site. This is a pretty stupid situation. It means that whenever a machine shop line-hones the bearing saddles (to the correct i.d. for proper bearing crush), they end up making the block/cap "too big" for the scroll. The factory said the diametral clearance for the scroll should be .003". When you line-hone an MGA or MGB block, you end up with about .005 or .006" clearance at the scroll -- and it leaks rather profusely. I never figured out a good solution to this, although my expert local machine shop has recommended that I paint the block/cap scroll area with something like POR-15, which gives a fairly large film thickness and (according to them) stands up to engine oil and other solvents. The idea is that the POR-15 would bring the i.d. back down to proper size, and if it is a little too tight, the scroll would wear away "just enough" of the paint ... that's the theory, anyway. I'm a little skeptical but will probably try it on my next rebuild.

I think the other fundamental problem with the scroll is the RPM. I suspect it was never designed for sustained high-speed running. It can probably pump enough oil back into the pan at 2000 - 3000 RPM, but probably not at 5000 - 6000 sustained. Just speculating.

I spent some time in the industrial gear industry and we used "labyrinth" seals on very high speed gears -- non-contact, close-clearance, multi-walled caps (speeds were too high for any type of contact seal like a lip seal). We always had a flinger (which the MGA engine also has) and large drain ports were key. But one of the biggest things is whatever part is right next to the "seal", on the outside of the gearbox (or in this case, the crankcase). The flywheel acts like a huge centrifugal fan! It pumps air outwards, which effectively creates a vacuum at the crankshaft just outboard of the crankcase -- a very significant vacuum! So even with large-diameter breathers on the engine, I believe there is still a very large pressure differential between the crankcase and the center of the flywheel/crankshaft hub. This may be why some attempts at a lip seal have failed: the vacuum may be strong enough to suck the lip seal down hard against the crankshaft surface (or Speedi-sleeve, or whatever it is riding against), prematurely wearing out the lip seal -- again, just speculation on my part. But I still think a lip seal is worth a try, and I have heard some good reports on the one developed by Barney Gaylord (MGA Guru) for the MGA. Maybe something similar could be developed for the A-series?

Finally -- your comments on piston scuffing. You probably know all this, but I would investigate further, it sounds like insufficient piston-to-wall clearance. For my engine and my pistons, Venolia (manufacturer) recommends .0035" piston-to-wall but my trusted experts say this is too little, and I have always used .005 to .006". If you have scuffing on both sides of the piston, I think your clearance is too tight. You said it only occurred on two of the pistons: which cylinders? If it was 1 & 2, or 3 & 4, then I'd suspect you were running a little lean on that carburetor, causing higher combustion temps in those two cylinders. Or it is was #2 & #3, maybe cooling isn't as good there due to siamesed cylinder bores? Vizard says that with a normal cam, two of the cylinders (I think it is 1 and 4) get "robbed" of some intake charge because of the firing order & siamesed ports, so that could be a factor -- in other words, it sounds like something may be making those two cylinders run a little hotter. (or the machine shop was a little sloppy and bored those two cylinders a little tighter than the other two, or somebody honed the other two more, or ...) Anyhow you get the picture, I'd check piston-to-bore and try to correct it during the rebuild. (Knowing Nial, that probably means just turning up the speed on the drill while dingle-ball honing!)

Good luck ... interesting stuff ... keep us updated!
Mark Palmer


Tue Mar 18, 2014 3:59 pm
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One of the things I was going to mention about this engine is the brass cylinder head plug between #2 and #3. You see this problem in all 1275 A Series engines. These plugs are pressed in and the fire ring on the head gasket overlaps them. It's a terrible design. After years of use combined with heat and pressure (plus two dissimilar metals) the brass plugs start to recede and you get a breach between the two cylinders. I've fixed two of them before this.

I fixed this one the same way. Drilled and tapped the brass plug and yanked it out. Then drilled the plug hole to 37/64 and threaded it with a 3/8-NPT tap. I was going to just buy a solid brass plug and screw it in (and then cut off the top) but all the brass pipe plugs these days are hollow. So I made a plug from solid brass and screwed it in. This time I added JB Weld to the plug before screwing it in (suggested by another racer). Then hacksawed off most of it and finally flycut about 0.003" off the entire head to clean things up.


Here's two photos of an old head I haven't fixed yet. You can see the plug in this one receded about 1/8" (this came from a parts car engine that supposedly ran).

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Here's a photo (below) of my current 1275 head before repair. You can see the old pressed-in plug has moved in a little bit.

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Here's flycutting the head (below) on a Bridgeport to bring the new screwed-in plug and head to the same plane.

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Here's the finished head below (still need cleaning in the ports and valve seats). You can see the piece of the plug I sawed off before machining. One of the new Nural pistons can be seen too. These are dished and make about 9.75:1 compression on a stock head. My lightly cut head probably brings that closer to 10:1.

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Also, I rechecked the bores today. They are basically 2.820" dead-nuts top to bottom (this is +.040 in a 1275). One was like 2.8205 nearer the top but that's hardly a concern.

Top of piston is 2.8175 so I have .0025 clearance. Should be OK. The skirts measures 2.818 to 2.819 but to be honest the skirt usually just expands out to meet the cylinder anyway. Once it get's close, heat rejection tends to keep at a more or less constant diameter.
My guess to the previous scuffing is simply lack of lubrication. The car did have an oil leak and it was worse with the sump full. Later in the 2013 race season, I often started with 1/2 quart low (since this reduced leakage) and ended some sessions lower than the safe mark on the stick. I did see oil pressure drop from time to time during turns too. Pistons #2 and #3 were most scuffed but #1 had some extra skirt wear too.

My feeling about the oil leak out of the scroll is that most of it was caused by blow-by pressure in the sump. In my opinion, even decent venting can't overcome this. At least one of the rings had a 0.035" end gap (correct number should be around 0.010") and one ring may have been cracked too. What's interesting is that car would still wind up to 7000 RPM in 4th and turned basically same lap times this year as previous two years (when it wasn't leaking other than the usual, minor drip).
I really hate to say this but sometimes I miss the 1500 engine this car used to have. It never leaked a drop and finished several 4 hour enduros without consuming any oil at all. And on the 1500 engine, part of the rear seal on the pan was made from little, carved wooden blocks (really!).

This week I try to balance the pistons at least to a reasonable level (the Nural and County pistons are always pretty bad in this respect). Then a very light hone, clean the block out well and back together it goes.

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Fri Mar 21, 2014 11:03 am
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Nial -- I run .0055" piston-to-wall clearance (yes, five and a half thou). I know that sounds like a lot, but this is what my favorite British car race engine builder recommends (Venolia forged pistons). Venolia only recommends .0035" as I recall, but Bob Griffith says that's too tight, and I trust Bob. I assume the County pistons are cast, so I'm not sure how that translates.

You probably know this, but the right place to measure the piston O.D. is pin height, 90 degrees off the pin axis. Pistons are not round (they are barrel-ground -- actually oval) and the sides are not straight (tapered, smaller at top than at bottom) so that under load, under heat, they become cylindrical -- theoretically at least.

.0025" clearance, measured at the top of the piston, sounds pretty tight to me ... if I measured mine at the top, the clearance would probably be something like .007 or .008 !!!

Boy, that brass plug pressed in the head is really a dumb OEM design! Great idea to thread it. I wonder if a steel plug would be closer than brass to cast iron in thermal expansion coefficient? Although maybe the brass "gets tighter" when hot, which might be a good thing.

BTW, for anyone else reading this thread -- if you're just getting into building your own race engine, I very highly recommend getting a copy of the book "Engine Blueprinting" by Rick Voegelin. It's the best book in my library on race engine building -- answers a lot of questions!

Mark


Fri Mar 21, 2014 4:02 pm
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Yes, County pistons are cast and many are now "Made in China" and are supplied with Grant cast iron rings. I used a set by the owner's direction in a TR-6 engine I redid a year ago and they seemed to be pretty consistent weight wise and dimensionally. Probably as good as the AE Hepolite ones. Pin bores were spot on also.
Impressive repair Nials.
Back in my college days we did something similar but being Cornell it had to be exotic so we made a "plug" that would be an interference fit thread, then chilled the part in nitrogen and inserted it in a warmed head. It was then rough cut off with a mill and the head was then run on a Blanchard to trim and true it (.002" removed). I never heard that the fix failed. I only wish I still had access to all the toys in Riley Robb Hall. Sadly most of it was all sold off years ago.

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Sat Mar 22, 2014 8:15 pm
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Good stuff in here.

We're going to be building up a 12G940 head, and dry-decking it; came with a Mini we purchased, and thought we'd give it shot to see how it does compared to the stock cooling solution. The previous builder used steel plugs in the water passages, but had not needed to address the brass plug as the head was relatively new and gently used (before it was destroyed by a wayward valve). Nial's method sounds like the ticket.

Russ' sounds even cooler, since you get to use liquid nitrogen. Now if only we had a Blanchard around...I watch Craigslist, but they never seem to turn up... :(

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Sun Mar 23, 2014 8:33 pm
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Another option for the piston scuffing issue would be dry film coating

http://www.calicocoatings.com/campaigns/dry-film-lubricant/

http://swaintech.com/race-coatings/

http://www.techlinecoatings.com/articles/Coating_Pistons_Article.htm

http://www.jepistons.com/TechCorner/PistonCoatings.aspx

These use one type coating on the head of the piston for heat rejection and another on the skirt for lubrication. Obviously, from the last link, you can really get carried away with this stuff, but it is an option.

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Mon Mar 24, 2014 8:45 am
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Nial,
I am following your article with great interest. I too replaced all the brass plugs with threaded stainless steel plugs, though none where interfering with the head gasket.
As I swallowed a valve last time out I am rebuilding the engine including a new set of Venolias. You mentioned using a dingleberry hone on the cylinders. Can you tell me what grit you are using and how many passes you use? Flexhone seem to suggest only 6 or 7 passes but this seem rather light. Of course I don't want to remove too much material, I'm already at maximum.

John

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Mon Mar 24, 2014 8:54 am
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