Honda Prelude Transmission Rebuild & Head Gasket Refresh

M6HA 4 Speed Automatic due for a rebuild? Notes below on my experience, not going into great detail of the process because the factory service manual is pretty good at covering all the bases. Read on for the download link to the FSM.

My Symptoms:

Flare from 1st to 2nd when cold, light to medium throttle. Once warmed up after highway driving, flare gone
After shifting to 4th, about one second later it would slip momentarily, 200 RPM jump and back to normal

Before you start, do a compression test and record your numbers. The lowest should be no more than 15% drop from the highest. Mine were all ~210 PSI but head gasket was still blown, more on that later.

While the transmission is out and the engine on a stand, now’s a good time to do a leak-down test and replace the head gasket if necessary, timing belt and balance shaft belt, intake/exhaust gaskets, rear main seal, valve adjustment, and have a machine shop clean, check for trueness (resurface if necessary) and replace the valve stem seals. Repaint the valve cover too! Pics below.

At the bottom of this post will be some tools I use and recommend.

Leak Down Test

I mentioned my compression test above, and the numbers were satisfactory, but a leak down test revealed I was losing roughly 15% to 19% in cylinders #2 and #3. 74 PSI going in, cylinder #2 retained 60 PSI, and 66 PSI on cylinder #3. This was later revealed when the head was removed, the head gasket was blown between these two cylinders. It wasn’t evident while the car was running, idle was fine, power was good, but occasionally there were some hard starts where it would take a few extra cranks before it’d run.

I used this OTC Leak Down Test Kit.

Factory Service Manual

The FSM is pretty straight forward with the removals and installation of the engine, cylinder head, transmission, etc…

You can download it here (not my site): https://drive.google.com/file/d/1CXlO3pom6gVJIXK-QOQSRhuhyVd4vRKr/view?pli=1

Valve Cover

Have a nasty peel-y valve cover? Now’s the time to re-paint it!

I used an orange-based stripper which worked pretty good, then scraped and blew off the remaining paint with water and compressed air.

You can get the wrinkle paint on Amazon, and an etching primer.

With a clean valve cover, mask off the areas that you don’t want paint to stick to – spark plug holes, bolt holes, etc…
Apply the etching primer, I waited about 45 minutes before applying the wrinkle paint.
Follow the directions, apply three coats, horizontal, vertical and diagonal with about 7 to 10 minutes in between coats. After the last coat, it’ll look glossy – wait for it to flash off after about 10 minutes then using a heat gun make sweeping motions on the whole thing and you’ll see it start to wrinkle up. Scrape the paint off the letters with a flat razor blade. it’s ok if you don’t get it all, just the bulk of it. Then let it sit undisturbed for 24 hours. Lots of YouTube videos are available to see how this gets applied.

After it’s cured, mask off the letters to protect the valve cover, then use 80 grit sand paper with a flat sanding block and sand horizontally from side to size to give it that brushed aluminum look. Use the 80 grit to get the paint off, then switch to 120 grit for the final finish. You could go to 220 if you like but 120 was adequate for me.

SEM Self Etching Primer

VHT Wrinkle Paint

Scrape the letters with a flat blade while the paint is semi wet, it’s okay if you don’t get it all.

Once the paint is dry, use sand paper to make the letters look like brushed aluminum.

Mask off the letters so you don’t hit the valve cover. Use compressed air to blow off the dust.

Seals & Gaskets

With your engine on a stand, now’s the time to do all gaskets and seals while they’re easy to access! Don’t forget the balance shaft seal because it pukes oil when it goes out. The Fel-Pro head gasket set is good for most of the gaskets you’ll need but does not include the balance shaft seal, or the rear main seal. I ordered the Honda rear main out of personal preference.

Fel-Pro HS 26411 PT Head Gasket Set

Genuine Honda 91214-PLE-003 Oil Seal (80X100X10) (Rear Main Seal)

Torque Wrench

Follow the FSM for torque specifications when reinstalling everything. Gear Wrench makes pretty decent torque wrenches, I had to use my old click-style for some bolts requiring 8.7 ft/lb. But the 1/4″ drive goes below 10 foot pounds. To do the ~180ft/lbs on the crank pulley you’ll need the 1/2″ drive torque wrench.

GEARWRENCH 3/8 Drive Electronic Torque Wrench 7.4-99.6 FT LB – 85076

Transmission

Have a shop rebuild it. There’s several dozen o-rings, and you’ll need a bearing press during the rebuild. It was worth it to have a qualified shop do it for me. One thing I’ve found, if you use sport shift you will cause premature wear to the clutches and other components when downshifting because the logic in the transmission computer doesn’t handle it very effectively and will just dump the transmission right into gear whereas upshifting is smooth, downshifting is not. I think this is the main reason for these transmissions going out early, and not changing your transmission fluid when specified by the owners manual. Mine lasted 160k miles, so I don’t think that’s too bad. Use Honda DW-1 Transmission Fluid, nothing else!

If you’re near Sacramento, contact me for the shop I used, they’re VERY well priced for doing Honda/Acura transmissions. They noted my 4th gear clutches were completely gone and were metal-on-metal. Explains the occasional “jump” after shifting into 4th!

You will need to flush your transmission cooler lines (the ones that go into your radiator) to get the old nasty fluid out so you don’t compromise the new fluid you’re going to put in.

Lubegard 19001-UNV Kooler Kleen Transmission Cooler Flush with 2-in-1 Fitting

SeaFoam

I’m a big fan of Sea Foam Top End Cleaner. Honda engines are sooty, carbon-y little goblins and eventually you’ll get enough carbon buildup that will clog the EGR, cause misfires, foul the valves, etc…

SS14 Sea Foam Spray Quick Acting Top Engine Cleaner

Valve Adjustment

Get yourself the special socket with a screwdriver built-in, and some angled feeler gauges. 0.007″ on intake, 0.008″ on the exhaust. Follow the procedure in the FSM, you’ll adjust the valves on each cylinder in the firing order sequence, 1-3-4-2. Measure between the cam and rocker lobes, adjust until there is slight drag.

10MM Valve Adjustment Tool

Stainless Steel Feeler Gauge

Balance Shaft Belt Tension

Lots of debate on how to set the tension on this belt. It’s supposed to have a little slack in it, which is taken up by the timing belt tensioner under load, but the balance shaft belt has it’s own tensioner attached to the timing belt tensioner. They’re different, but the same…kinda… The FSM spells it out, but I have my own method.

If it’s too tight, it will make a loud whine when the engine is running. If the cover is off, don’t apply extra pressure to the tensioner thinking it has to be taut, it will be too tight.

Here’s how I do it, and you can do this with the engine in the car.