Houston, we have a problem ...
The first photo shows the highest outdoor temperature I've experienced in my entire 75 years, and that includes three years spent in the Mojave desert. For you metric guys, that's 45.5 celsius. When this photo was taken, it was 99F inside my shop, and I was setting up to paint the block.
In preparation for paint, the block was prepped for bead blasting. The coolant passages and several machined surfaces inside the block needed shielding from the glass beads, and so a few days of throwaway work were spent making screw-on cover plates to protect those areas. Every threaded hole was filled with a screw. The lifter valley between the decks was also bead blasted even though it would remain unpainted. An easily removable top plate was machined to cover it. The lifter bores and oil drain-back holes in the valley were also plugged with rubber stoppers to keep the beads out of the cam bore and the bottom end of the engine. After bead blasting, the cover plates were removed and the entire block scrubbed with Simple Green followed by Dawn dish detergent and plenty of warm water. After a thorough drying with compressed air, the cover plates were reinstalled for painting. The block was air brushed with Gun Kote's 'Anno Blue' and then oven cured at 300F.
The liners which had been machined to protrude .003" above the unfinished decks were installed next. They, as well as their mating surfaces inside the block were pre-cleaned with acetone. Loctite's instructions for slip fits recommends installing circular parts with a twisting motion to insure full coverage of the adhesive during assembly, and I've learned through bitter experience to follow those recommendations. A rubber stopper shoved into the top end of the liner acted as a temporary aid to assembly them to the block using Loctite 620.
During assembly, Loctite was unavoidably smeared across the surfaces of the liners inside the coolant jackets. Left on those open surfaces it wouldn't cure and would eventually contaminate the coolant. After a 24 hour room temperature cure plus an extra 12 hours at 140F for good measure, the coolant jackets were flushed with acetone.
Acetone leaked out the bottoms of three of the liners in bank one carrying uncured Loctite with it. I've no explanation why the Loctite on those three liners didn't cure since as far as I knew I'd met all Loctite's requirements: 1) all machined surfaces were smooth and bright and had been cleaned with acetone, 2) their .002" radial gaps were well within 620's .015" max spec, and 3) steel met the requirement of having at least one active metal in the bonded pair. In fact, in order to avoid this very problem, a generous half inch wide sealing flange had been provided at the bottom of each cylinder at significant cost to coolant capacity.
A third cure cycle, this one at 190F for 1-1/2 hours didn't seem to solve the problem. After cool down, a second flush was performed but this time using Loctite's 7471 activator. This yellowish liquid is a volatile copper ion-carrying solvent intended as a surface primer when bonding together two 'inactive' metals such as aluminum. The hope was for the additional free ions to kick off a cure during a fourth heat cycle since the yellowish liquid seeping from the bottoms of the three liners was obviously in contact with the uncured 620.
In retrospect, I should have immediately started another heat cycle to see if the copper would have activated the 620. However, after some solvent flash time, Loctite 290 was injected into the coolant jacket feed port above the leaking sealing flange and allowed to also seep out the bottoms of the leaking liners. This thin wicking variation is intended to thread-lock existing assemblies. The plan was to allow capillary action to draw it into the leaking areas and cure. The block was then returned to the 140F oven.
Four hours later, a 25 psi coolant jacket air test showed the leaks to be sealed. Unfortunately, I'll never know if the activator had finally kicked off a cure of the 620 or if it was the 290 alone that plugged the leaks. In any event the block was returned to a 140F oven overnight. An acetone flush performed the next day confirmed the liners were still sealed.
Since I really don't want to revisit this nasty problem after the engine is running and the liners are exposed to oil and coolant, I decided to add some additional insurance. The bottoms of the liners protrude slightly through the surface of the block, and these were dropper'd with Loctite 290 which formed menisci. Very little adhesive was drawn in which was a good indication that the Loctite above it was cured (either 620 or 290). Just in case, though, the block was allowed to cure for another 8 hours at 140F before wiping away the menisci. For good measure I injected a second dose of 290 into both coolant jackets allowing it to puddle on the floors of the jackets and then put the block in the 140F oven overnight. The jackets were thoroughly flushed with acetone the next day with still no sign of leaks.
This is the second time I've used Loctite 290 to rescue steel liners sealed in an aluminum block with Loctite 620. (I had a similar experience with the Offy engine.) I also had a different but similar issue with 620 during the Merlin build.
Be aware ...
A Loctite applications engineer once told me that Loctite products have a very long shelf life and will continue to work so long as they're able to flow. The 620 I used was purchased five years ago. (Be aware there are a slew Chinese copies in nearly identical red bottles being sold on Amazon.) I've recently noticed that Henkel is now marking their bottles with the date of manufacture and a two year 'use by' date. And so, for a hobbyist, small bottles are probably recommended. In the future I'll likely start using activator on all slip fit inactive surfaces (aluminum, stainless, etc) regardless of what metal they're being bonded to. The issue with using activators and why I've tended to avoid them when theoretically possible is that they tend to decrease Loctite's ultimate bonding strength some 20%.
In any event, some 60 hours after installing the liners, work on the block was wrapped up when the angle plate and machining fixtures made last year were located and the deck surfaces finally finish machined. - Terry
