270 Offy

[mayhugh1]

Terry,
Could you not use radiators like in you Rolls Royce V12 Merlin?(computer coolant rads), Just a question. I have been following along of another fantastic build
Cheers
Andrew

Ghosty,
Yes, I could. At first I thought that using a couple of them would be the solution to the starter shaft problem as well. The problem with pc radiators is that their inlet and outlet are on the same end of the core making them awkward to use in a model engine. My earlier Offy coolant pump testing showed I needed the radiator inlet up high and its outlet down low to get all the air out of the engine. I could have put a pair of them inside an enclosure and hid the plumbing to correct the I/O this like I did on the Merlin, but then I realized I was wasting a lot of real estate on non-cooling stuff. Remember, though, the Merlin also had a 25 oz coolant reservoir in addition to the two radiators and electric fans, and it got really warm while the engine was running. This time I thought I'd just go with a bigger reservoir and see what happens. - Terry

 

[Ken I]

Terry, been following with interest but this is my first comment.
As per usual another mind bogglingly impressive build - I'm in awe.
Re your comments on scalability of radiators etc.
The "fins" on radiators are wrinkled to induce turbulent flow - the difference in heat abstraction ability of the airflow through the radiator is much reduced if it does so in laminar flow mode. Ditto the flow of water inside to a lesser extent.
Flow velocity is also a major influence in turning laminar flow into turbulent flow.
As you said, radiators don't scale well - if you scale down an automotive radiator you will end up with a fragile gossamer thing but that surface area and thin sections densely packed into a small volume is what gives automotive radiators their ability. If you consider your surface area per unit volume of your scaled radiator I think you will find it to be a fraction of the original and laminar flow might halve that again.
I'm not sure how to translate that into helping your model - perhaps a lip on the fins, pierced and flared holes, stippling etc.
A larger reservoir will buy you more run time - but then perhaps that is all you need (or want).
FYI - Regards, Ken

 

[mayhugh1]

The three parts making up the radiator were machined from 6061 aluminum. Both tanks and the core's exterior were machined on my Tormach, but I did the core's interior manually on my Enco Bridgeport clone. Hogging out the interior required a 3-1/2" cutter stick-out, and so a 3/4" diameter end mill was used to limit the deafening chatter-induced squeal. I felt more comfortable working manually inside the deep narrow cavity where it was difficult to prevent re-cutting some of the huge chips. Without high volume flood cooling, the chips occasionally welded themselves to the cutter which, on the Tormach, would have resulted in a nasty crash. The front and rear faces of the simulated core were grooved using a 60 degree vee cutter.

All three parts were bead-blasted in preparation for paint. After sanity checking the hole locations with a mock-up of the Offy, the parts were bonded together by filling with JB Weld the seams that would have received solder in a full-size radiator. - Terry

 

[Skipper]

I built Ron‘s Offy engine. Couldn’t get the head bolts tight enough. Kept blowing head gaskets.
Skipper

 

[Peter Twissell]

Nice work, as ever.
When milling an item right through like your core block, I find it useful to start by drilling one or more large-ish holes through.
The holes provide a route fo clear chips, even if it means stopping the cut occasionally to brush chips towards the hole.
A drilled hole also avoids the need to plunge the cutter at each depth of cut.

 

[kvom]

There's something to be said for plunge milling in a chain for the entire length of the slot followed by a finishing pass.

 

[mayhugh1]

The radiator was painted with matte black Gun Kote which requires a 300F oven bake-out to cure. In the past I've had trouble with JB Weld out-gassing underneath this paint and creating bubbles in the finish. So, after the epoxy had been allowed to cure for a full day, the radiator received a one hour 300F pre-bake which seemed to solve the problem.

The radiator cap was machined from 303 stainless and then polished. Although it's vented to the atmosphere through a tiny central hole, the cap is sealed to the radiator with an o-ring.

The radiator's inlet and outlet were also fabricated from stainless. The lower outlet tube required a one inch "S-bend" to accommodate an offset between the radiator and the water pump inlet. It took a full day with jury-rigged bending jigs and lots of scrapped tubing to finally get a threaded tube to fit and look acceptable. The top inlet was simply machined from rod stock.

I've never come across a commercial miniature hose clamp that looked at home with the silicone coolant hoses I use on my engines, and so I decided to design my own. Two batches of clamps were machined to handle the hose o.d.'s (3/8" and 7/16" ) that I'm using on the Offy. The clamps' important dimensions are the i.d.'s which I experimentally determined to be optimum at .010" less than the hose o.d.'s. The slot width was .040", and 2-56 SHCS's are used to tighten them around the hose.

Finally, a pair of brackets were fabricated to attach the radiator to the display stand in front of the engine.

Before wrapping up work on the radiator, I'm considering machining a shroud for a pair of electric fans on the rear surface of the radiator's faux core. Although they'd likely be more cosmetic than functional, I found that mounting them a quarter inch off the radiator's grooved surface churns up a lot of turbulent air that might provide a bit of additional cooling. In any event, I have a couple six volt fans that are currently looking for a good home. - Terry

 

[dsage]

Beautiful work as usual.
Good call on the pre-heat. I hadn't considered it for JB weld. Thanks for that tip.
I do powder coating and it's necessary to pre-heat cast iron or cast aluminum to over the cure temperature of the paint to force the gas out of the casting. Then you let it cool a bit before applying the paint. Then back up to curing temperature. Otherwise, as you have found, you get what looks like rice crispies finish on the surface. A disaster in powder coating because it's near impossible to remove.
Thanks Terry.

 

[kuhncw]

Terry, Great work as always. Thanks for the hose clamp idea. Is the spring clamp in the photo for damping or something else?

Chuck

 

[mayhugh1]

Terry, Great work as always. Thanks for the hose clamp idea. Is the spring clamp in the photo for damping or something else?

Chuck

Chuck,
Yes, the clamp dampened the tall skinny workpiece during cutting. - Terry

 

[mayhugh1]

A shroud for a pair of 60 mm fans was machined from a block of aluminum and painted to match the radiator. When mounted, the fans stand 3/8" off the rear of the radiator, and the air they draw in around the sides of the shroud is blown over the top of the engine. The 6V fans are powered by the ECM, and their combined 36 cfm results in a surprising amount of felt cooling air flowing over the head.

One last coolant related task - leak testing the block - was something that should have been done long ago. The Offy's block, being separate from the crankcase and head, contains the cylinder liners and a number of oil return tubes. It will be closed up with a pair of gasket'd side covers that are secured by eighty 0-80 SHCS's. The cylinder liners which were close sliding fits inside their bores were sealed with Loctite 620. When filled with hot circulating coolant there'll plenty of opportunities for leaks to the outside of the engine or, even worse, into its oil.

For the leak check, the side covers were temporarily installed and the block filled with isopropyl alcohol. Sure enough, a leak showed up at the bottom of the rear liner. Although a block-to-crankcase gasket will be part of the final assembly, I didn't want to risk a major teardown later on.

Loctite 290 is a wicking grade thread locker that's sometimes used to solve metal porosity problems. A single drop deposited in the bottom corner of each liner quickly wicked around its raised circumference forming a meniscus. A few hours later, the meniscus around the rear liner had been entirely drawn into the gap causing the leak, leaving Loctite on the liner inside the block. After an overnight room temperature cure, the block was moved inside my (130F) welding rod oven for several more hours. The uncured drip inside the block was wiped away and a second drop of Loctite added the rear liner. This time its meniscus was still in place a day later indicating the gap had likely been closed. After several more hours in the oven the uncured surface Loctite was wiped away.

Thread locker was also dropper'd onto the topside of the block around all four liners even though there had been no indication of leaks on the top ends of any of the liners. After an overnight cure at 130F the block was retested, and it passed my alcohol leak test. - Terry

 

[dsage]

More great tips and tricks to somehow remember.
Thanks Terry

 

[mayhugh1]

I use George Trimble's method to make my piston rings. The only change I've made to his process is to use a normalization temperature of 975F rather than his originally published 1475F. In addition, I use a 200 lumen flashlight to check the fit of each finished ring inside a cylinder before it's installed on a piston. A ring that has any light leaking between it and its cylinder wall, other than through its running gap, is discarded.

My yields are typically limited by circularity issues that often show up during or after the blank's final machining. I use class 40 gray cast iron from a number of sources that's been laying around in my shop for years. The circularity errors have been as high as eight tenths over portions of the finished blanks. Typically, less than half of any blank passes my acceptance criteria of two tenths, and occasionally an entire blank is scrapped. Once a ring has been removed from a blank, it's nearly impossible to evaluate until it's fully finished and light tested. After slicing candidate rings from the well-behaved portions of the blanks, my yields are typically 80%-90%.

When starting a large batch of rings, I usually prepare several blanks from different sources. Even though only eight rings plus a few spares were needed for the Offy, I prepared three 3" long blanks. The Offy's rings require a finished diameter of 1.002", and so I started with one inch diameter raw material which actually measured 1.080".

All three blanks were turned down to 1.063" so they could be gripped in a standard collet during their final machining. The blanks were then drilled/bored to the rings' final i.d. for a depth of two inches leaving an inch spigot at one end for the collet during final machining. In order to relieve some internal casting stresses, the pre-finished blanks in my last few builds next have next received a 700F heat soak. This heat soak seems to have reduced issues with circularity errors which sometimes show up even days after the blanks' final machining. Rather than my usual five hour soak, an oven programming error soaked these three blanks for some 12 hours.

A few days later, the o.d. of the first blank was finished and polished to its final diameter with 600 grit paper. Quadrature measurements of the blank's diameter recorded continually along every half inch of its length during machining showed (surprisingly) negligible errors over the blank's two inch length. Using a .019" parting tool, I was able to get 25 candidate rings from that single blank, and so I didn't finish the remaining two blanks.

The inside corners of the parted rings were broken using a 1/4" diameter ceramic file. Using 600 grit grinding grease on a glass plate and a simple holding fixture, both flat sides were lapped to obtain a .001" clearance in the piston grooves. During combustion, the rings will seal against the lower walls of the pistons' groove to provide an important component of the combustion chamber's overall seal.

The Trimble article recommends a straight radial break in each ring for proper contact with the spreader dowel in the normalization fixture. Although 'good enough' results might be obtained by simply snapping the rings, I constructed a cleaver several years ago. After lapping and just before heating, each ring was cleaved and the running gap set to .004" with a diamond file. Each gap was verified inside a cylinder using a feeler gage.

The Trimble articles describe the construction of the fixture required to support the rings during their heat treatment. Equations were provided for the dimensions of the mandrel and spreader dowel which are its key components. This fixture isn't difficult to make, but its dimensions are specific to a particular ring diameter. I finally got to reuse one of my earlier made fixtures.

In the past I've sealed the fixture'd rings in an argon-filled stainless bag for protection during heat treatment, but the contents invariably wound up covered in a mysterious brown deposit. Although it wasn't difficult to remove, it was an annoying extra step that I began suspecting is related to the sulphur and lead that are alloyed into the free-machining steels used for the fixtures. This time I didn't cover the fixture, and only minor burnishing with a white Scotch Brite pad cleaned the o.d.'s up nicely.

My light tests yielded a dozen 'perfect' rings and only five that I labeled as rejects. The remainder were a bit off from being perfect, but in all likelihood would be 'good enough'. - Terry

 

[G54AUST]

Bumper issue Terry. I've printed this one off for Ron; later ron. This is now a reference source, going straight to the pool room library. (please refer to Aussie film "The Castle")


Kind Regards,

Trevor,
Melbourne, AU

 

[Charles Lamont]

I forget, does the Trimble method include a final turning stage, after heat-treatment?

 

[tornitore45]

Hi Terry, miss our get together...
That black ring holder, how does it work?
My question is how you direct the intense light to the ring but blank off the light from the center to blind you.
I suppose is all in keeping the ring holder centered inside the ring ID without blocking light where it needs to go.

Can you sketch the ring holder?

 

[mayhugh1]

Trevor,
It's probably because of the continent I grew up on, but I didn't understand your comment....

Charles,
The Trimble method doesn't include any machining after the heat treatment. The results of his mathematical analysis are that the correct size spreader dowel during heat treatment will result in a circular ring when it's inserted into the cylinder afterward.

Mauro,
I miss our monthly meets as well.
I've included a sketch that should make my light test procedure a little clearer. The ring is actually held inside the bore by its own friction with the cylinder. The pusher block referred to in my previous photo is a close fit to the cylinder bore and insures the ring is properly aligned in the bore for the test. The light source I use provides a ring of light around the ring's perimeter.

Terry

 

[doug tyler]

Have you ever checked the glass plate to see if it has stayed flat?

 

[mayhugh1]

Have you ever checked the glass plate to see if it has stayed flat?

Doug,
I usually discard them after after an hour or so use on each side or when the surfaces become too frosty looking to see through. I've never actually been able to measure any flatness errors across them. Even the ones I discard seem to be better than my surface plate. I bought a stack of 5"x7" glass panes some ten years ago from a local hardware store that was going out of business. The cost was just a few dollars and it looks like they'll last my lifetime. - Terty

 

[mayhugh1]

A number of the Offy's subassemblies including their associated gaskets were built-up and tested earlier as their constituent parts were machined over the past year or so. These included the water and oil pumps, carburetors, gear tower, magneto, head, and camshaft assemblies. The ECM, radiator, and the oil and fuel tanks were also completed and installed on a display/running stand. With all this prep work completed, final assembly should be pretty straightforward.

Before closing up the crankcase, though, I decided to make a last minute change to the upper half. The bearing supports which were integral to Ron's original single piece crankcase were machined away. They serve no useful purpose in the split crankcase and after being removed allow the pistons to be installed or removed with the block attached to the crankcase. After completing this operation, the block was secured to the upper crankcase using fourteen 3-48 SHCS's and a .005" thick Teflon gasket between them.

Oracal 651 adhesive backed sheet vinyl was applied to the upper half crankcase on the surfaces that will mate to those on the lower half. I used this 2.5 mil thick material in a similar gasket'ing application during the Knucklehead build. In this case it will be used to seal the crankcase halves together with a nearly invisible seam. Strips of .0005" shim stock were used to verify the seam will tightly close and not seep oil later.

The crankshaft, rods, and pistons (sans rings) were temporarily installed sometime earlier in order to check for any tight spots before the friction of the rings and rear oil seal hide them. With the rings now installed, the piston assemblies could be installed and the crankcase finally closed up. The crankcase halves, precisely aligned by a pair of dowel pins, were then bolted together with ten SHCS's. A tiny but important -002 o-ring seals an oil passage crossing the seam that will eventually deliver a portion of the oil from the scavenger pump to the engine's top-end. The rod caps and bolts were easily inserted and tightened through the access ports on the sides of the crankcase.

The rear oil seal was pressed into the rear housing which was then attached to the engine and sealed with an o-ring. The crank was spun with an electric drill while oil was injected into the engine's oil inlet with a syringe. Oil eventually showed up on the floor of the sump indicating that the pressure pump seemed to be working. - Terry