KR2 N891JF Brake Refurb

KR2 N891JF Enginetics Brake Refurb

created January 25, 2013 and updated February 2015 with Matco brake upgrade (see bottom half).

When I bought N891JF, I flew commercial out to Omaha to pick it up. An AP/IA had looked it over and declared it airworthy, but that didn't necessarily mean that everything on the plane was perfect. I was cautioned that the brakes were fairly primitive go-cart brakes, and would likely need attention after I got it home. Sure enough, full brakes were ineffective against anything over about 1500 RPMs, so a full-throttle runup was out of the question. I should have brought along a tie-down strap so I could so some serious full throttle testing, but I was being "smart" and traveling light. In retrospect, I'd have had a much safer takeoff if I'd done some wide open throttle ground testing with the POSA carb to learn its idosyncratic mixture/throttle relationship, but I figured I could get that experience with some taxi testing. That turned out to be an incorrect assumption, mainly because of the extremely poor runway surface at Millard. Most of my efforts were focused on keeping the plane under control, and not letting it throw me out of the cockpit!

Back to brakes though...while taxiing at Millard, I learned that I had more brakes on one side than the other, but not much on either side. Still, it was enough to taxi around safely and land with, so I put that on my list of things to address after getting it home. After arriving at home, I attacked the side that was almost useless, and removed all the air by pumping brake fluid in at the bleeder valve until the bubbles were all gone through the reservoir. I used a common pump-type oil can filled with brake fluid connected to the bleeder with a plastic urethane tube. It didn't take long to determine that most of the fittings leaked, and the caliper appeared to be leaking as well.

It was time to disassemble everything, determine the cause of the leakage, and fix it. I started with the "good" side. Disassembly revealed the simplicity of this system, and the name "ENGINETICS" on the caliper. There appears to be only two real moving parts, with one being the "cup seal" in the caliper housing that pushes against the pad assembly to energize the brake. I located the "Enginetics Aerospace" website, where they mentioned that although they were founded on go-kart brakes, but were now concentrating on building jet engines and other aircraft parts. It was clear they were no longer in that business. I was able to find a parts breakdown on the Rieken's Racing website that appeared to match my setup, although what was on the plane appeared to be about a third of what constitutes one side of a go-kart's front brake system! Another potential source was MCP Brakes, which bought the ENGINETICS brake line in 2004. Their site listed a few "Enginetic Internal Front Brake" parts, but the part numbers didn't match up with the breakdown, and there were no photos. It was the weekend though, and I'd have to wait until Monday to find out for sure.

I was anxious to disassemble the caliper, but didn't want to risk damaging the cup seal until I knew that I could find a replacement. One would assume that a brake designer would incorporate common standard hydraulic parts, however. I emailed Steve Bennett about the origin of the brakes, and he said he might have a couple of odd parts around that he'd send me. It turned out to be a new caliper and a new pad, so I was fearless in my desire to remove the cup seal at this point. It turned out to be a cup-shaped o-ring, and with about 50 psi of air applied to the backside, it blew right out.

More replacement brake pads might be another story though. Disassembly of the "bad side" brakes revealed pads that were completely soaked with brake fluid, not to mention broken in three pieces! I called the folks at Rieken's Racing, and they said they'd just sold their last pads earlier in the week! They recommended that I upgrade to something a little newer, and reminded me that these things were awfully old, and I should seriously consider upgrading my kart! They did say that MCP Brake was their source, and that I could check with them to see if there were anymore in the works. It turns out MCP had a new batch of pads just in from what appears to be a source in the Netherlands.

After removing the cup seal, the caliper and other metallic parts were soaked in kerosene overnight, brushed clean, and blown out with compressed air. I also removed some burrs on the caliper that were left after manufacturing. I also cleaned up the Asuza wheels and replaced all the bearings, since one was rough and noisy, and the others were probably due after 22 years.

After posting a request for info to KRnet, John Shaffer contacted me offline and told me he had sold those brake systems to KR builders. He also gave me a few clues regarding setting up the pad to caliper spacing using shims, along with the advice to minimize the gap to prevent blowing the seal out.

Monday rolled around, and I called Rieken's Racing, which yielded the dreaded "I sold the last set of those pads last week, and I don't think they're available anymore", but then I called MCP, their source, where I was told that almost all parts for those brakes are still available, except the calipers themselves. No problem there...all I needed was pads and cup seals. All I had to do was order 500 of each! But Mark at MCP kindly said he'd drop ship a pair to me, but I'd have to buy them through Rieken's, which was fine with me. $100 later I had a pair of both pads and seals headed to the house. It turns out the pad backings are now made of steel instead of aluminum, so each pad gained 3.1 ounces. The airplane just gained 6.2 ounces just by replacing the pads! But after looking at the wear patterns on the disks, I realized why they went to steel. The aluminum was flexing and allowing the concentrated load of the moveable cup seal to be the primary braking area, while the rest of the pad was flexing out of play. Steel stiffens the pad and allows the majority of the pad to work, so the brakes should be much more effective. I'll take the 6.2 ounce hit for that, any day. It's vital that a pilot be able to do a full power runup before takeoff to gain assurance that the engine is capable of a full-power takeoff. Is there any other kind?

The new cup seals and pads arrived.

All was reassembled, shims set up correctly, and brakes bled.

But even after a complete rebuild, they still would not hold over about 1700 RPM from the 2180cc Great Plains engine, which was running up at about 3200 RPM while connected to a tree with a tie-down strap. Still, they worked well enough to considerably slow me down at my 2600' long runway, but I were still capable of nosing the plane over as slow speeds.

UPDATE - November 2014...These brakes simply never worked well, despite all new parts and meticulous adjustment to make them as effective as possible, including caliper spacing (critical) and no air in the brake system. The pedals were fairly stiff, but it was not apparent to the pilot that there was more than about 1/3 of the braking power that I had in N56ML's Clevelands.

After putting about 60 hours and maybe 120 landings on the plane, I was flying into Barnwell SC for the 2014 Corvair College, and discovered I had no brakes. Good thing it's a 5000' runway there. If I'd been at home, I'd have ended up in either a swamp or a corn field, neither of which is particularly appealing. Of course I should have tested the brakes before I left, but given that they won't hold the plane at full throttle (not even close), I've developed the habit of pulling off the asphalt into the grass, and doing a quick runup in the grass to check engine operation. Apparently I never actually used the brakes to realize that they didn't exist before takeoff. Energy conservation, I guess. And yes, the checklist needs updating.

Anyway, even after bleeding from the bleeder up to the reservoir using an oil can from the bleeder end (thanks, P.F. Beck!), I still had no brake pressure on the right side, so further investigation revealed brake fluid puking out of the innards when the pedal was pushed. Probably a blown cup seal, which wasn't going to get fixed until I got home. But how to get into my 2600' long strip? I didn't even entertain the thought...I landed at nearby MDQ instead, with 6500' of runway. No problem, right? As luck would have it, given that it was the coldest morning of the year so far, I had topped the tires off before I left on Friday morning with 50 psi of air. Of course I didn't think of this until I was flying back home, or I'd have bled some out. I used well over 5000' of MDQ's runway to get stopped, and I finally switched the engine off to make the last turnout before the end. I found out this morning the runway has an uphill and a downhill, and I landed downhill. Duh. Mental note to attention to such things in the future.

I came back that night with new cup seals and pads, and rebuilt the brakes by flashlight, and this time the bleed job held. The photo above shows the pulverized brake pad and a cup seal that looks more like a fried egg than a brake fluid seal. Looks like neither belongs on an airplane! Apparently I'd overheated the brakes on a previous landing and the cup seal that is normally housed in the aluminum caliper melted itself to the stainless steel caliper cover, which put an end to the hydraulic system closure, allowing the fluid to leak out everywhere.
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Matco brakes Created February 2015

My brake failure at the Corvair College at Barnwell made me realize that the time to upgrade to "real" aircraft brakes had arrived so I set about finding something else. I was lucky enough to have Riley Collins email with an offer I couldn't refuse, for a set of old but unused Matco brakes, wheels, and tires. He bought them at a garage sale, apparently from a project that never flew, and passed them on to me. I am now installing a set of W50L Matco brakes, which look functionally similar to the Clevelands on N56ML, including piston diameter, o-ring, same brake pad, and even the same load rating. They were so old the guys at Matco barely recognized them!

It turns out they were indeed unused, never having grease in the bearings or brake fluid in the calipers. They just needed some cleaning up.

But one issue was that the 5 x 5.00 McCreary aircraft tires used tubes with much longer brass filler valve stem than the Cheng Shins have, which meant the hole for the valve stem had to "move".

I did a little research on Matco's site and discovered these were made of 535 aluminum, which requires 3356 welding rod, so I welded up the old holes.

I machined the weld beads flat and measured off the faces (exit points) of the two valve stems to determine the best place to drill the new hole, and then moved the hole outward a little more so the valve stem would protrude a little further outside the wheel center. Filling these things on the Cleveland wheels is tough if the tire is flat, because there's not much to hold when you inflate the tube!

I intentionally drilled the 5/8" hole to encompass the wheel half joint, so the valve stem wouldn't have to be inserted and make the hard turn to the exit on the outside of the wheel. This way, the valve stem is more "clamped" in place, and there's no fishing through the hole required. Don't ask how I know this! This also gave me plenty of thread sticking out of the wheel...far more than I had with the Clevelands on N56ML, a wonderful thing! The moral of this story though is to pay attention to the wheel description...I started with wheels for "aircraft tubes", and turned converted them to "industrial tube" wheels, in Matco terminology.

Just like the Clevelands, you have to be careful not to pinch the tube when you assemble the wheel halves into the tire. Lot's of "tire talc" (or baby powder) is applied to help the tube to find the right inflated position within the tire, and also to smooth the way for the two wheel halves to prevent pinching. I put just enough air in the tube to resist pinching, while still allowing the wheel half to be slid into the tire. I use 3 quarter-inch threaded rods to slowly tighten the wheel halves together, with about 5 checks of the crack to make sure the inner tube isn't being pinched. So far, I've never pinched one, and this is pair number six for me. There are other ways to ensure you don't pinch the tube, but this method works for me.

I know a guy (who will remain un-named) who pinched four tubes during assembly in two days. I know, because I was there for three of them, and he told me about the first one early on the first day. Obviously a slow learner, and suffice it to say that I wouldn't let him design or maintain anything I'd fly!

It turns out that the Matcos are about 1.5 pounds or 25% lighter (each) than the Clevelands, and are about .625" narrower from outer wheel edge to caliper on the inside, thanks to the Matco's somewhat "internal caliper". Here's a three pound weight savings, and the smaller diameter tires (see below) will save another two ounces. The Matcos are considerably wider than the fully internal caliper Enginetics brakes they replace though, so Jim Faughn's "sexy" wheel pants will have to be modified or replaced, as much as I hate to.

The tires that came on the wheels were 5 x 5.00 McCreary "aircraft tires", which look like "tundra tires" on a KR, so I started looking for the smaller lighter Cheng Shin tires and tubes like I have on N56ML. Neither AS&S nor Wicks has the 4,6, and 8 ply Cheng Shin x 11.0 x 4.00 - 5 tires, and Mike Stirewalt had posted his bad experience with the AS&S replacements, so I took a chance and bought some other "CST" (short for Cheng Shin Tire) tires off ebay. The photos were a bit scary, as the tires were deformed, but I figured they would look a lot lot better with air in them, and the price was right, so I ordered a pair from

I may buy a set of pricier Aero Classic 11x4-5.0 tires next, which are twice the price, but have a higher 8=ply load rating, and would likely last longer. I didn't come across these while looking for Cheng Shins, but they are available from Matco (among other places) at

This shows the "before inflation" on the left (which is a little scary looking), the "as inflated to 40 psi" in the middle, and the old Cheng Shin from N56ML on the right. It looks like the new CSTs are about an inch less in diameter (bonus), weigh a little less, but are only 4 ply and are rated for a max of 46 psi, as opposed to 70 psi on the old Cheng Shin tires. There may be better options, but I'll give these a try for a while. The smaller tires will improve the aerodynamics, as well as save weight.

Here's the first fitup, and it looks like this is going to work out nicely. I'll have to redrill the four mounting holes for the caliper bracket to align with the axle holes, and will have to make some pretty substantial shims to compensate for the poorly angled lower gear brackets, but it won't be as bad as I thought. The wheel pants are a re-do, however, and likely won't be done in time for SNF this year. Jim's "sexy" wheel pants will either have to be redone or replaced. It'd be a shame to trash them, so I'll start there. The 5/8" steel axles that were on the plane had substantial bends in them, but the 1.25" aluminum ones that I'm putting on are unharmed, even after one of them withstood enough force that broke the gear leg in its previous incarnation. The aluminum ones only weigh two ounces more than the steel ones, 15.4 oz vs 13.4 oz, but are far stronger.

All of the brakes mentioned above, including the Clevelands, use the MIL spec brake fluid, the current "standard civil aviation" version being MIL-PRF-83282D, sold commercially as Royco 782 among others.

Below is an interesting post that I found on Van's Airforce list, discussing specific DOT brake fluids.

"Let me start by saying I am the Wagner Brake fluid factory Rep for north Texas. I sell a lot of it. The reason the automotive mfgs. use Dot 3 or 4 fluid it its ability to absorb moisture. This cuts down on rust and corrosion. If you use Dot 5 which is silicone based the moisture stays separate and causes problems. The difference between 3, 4, and 5.1 is the boiling point. Most later model cars have aluminum cyl's and we don't sell 1/4th the master cylinders that we did when they were cast iron. For several years some of the auto mfg's were really pushing flushing after about 30,000 miles. Some have backed off this. When the fluid gets dark change it. When the fluid gets moisture in it the boiling point goes down. With out looking it up, I think 3 boils around 350, 4 around 425 and 5.1 around 500. We sell mostly 3 for cars and 4 for medium duty and off road. Very little 5.1 is sold. Some of the high end imports use 5. (silicone). There is not a large market for 5 so we don't sell it. "

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