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High Compression 3.2L Engine Build

I've bought and then sold SoS forged pistons over the course of this build. They are heavy and and fairly generic Wiseco slugs and more meant for boosted or stockish applications.

The tricky part is that piston skirts aren't perfectly round, so it would be very difficult if not impossible to get it back to the right shape and thats not really something I want to risk. I'm hoping Toda will be willing to sell me a single piston but they probably won't, in which case I'll just have to suck it up and buy another set.
Sorry it took me a long time to get back to you on your questions about dimpling.

Sorry about all the troubles too. After I ditched the first "machinist" I got lucky with the other ones doing my crank, heads, block, and rods. I had a great experience with Benson and even used to keep in touch with him after I got this back on the road.

My build 12 years ago now (wow) didn't get crazy with cams and valves as I wanted to be able to play around with both N/A and F/I, and I have. So, I optimized what the Honda engineers provided and focused on improving combustion efficiency. The pent roof combustion chamber is the standard for four valve heads, but not really a good design especially the higher CR you go. Over my almost 20 years here, I kept pics of NSX heads as they were removed, and noted the burn patterns and areas of high carbon. That's where I focused on dimpling. The pic below is the pattern I came up with near the intake valves. I re-welded the heads near the exhaust valves with some special features and I'm not going to show that part to the internet.

Same thing with the pistons. Before I embarked on this journey, I just asked SOS to send me their Wiseco pistons and ceramic coat the domes. As I got involved in the rebuild and getting into combustion efficiency, I ended up ditching those and having Wiseco do a special batch with less generous valve cutouts, etc. Then those were dimpled and ceramic coated (sorry, not showing those either). The dimple sizes and patterns I did on the head picture should give you an idea of what I did on the pistons too. Don't go crazy obviously as that can hurt you.

The before baseline dyno with my custom AEM tune was 255 RWHP. Afterwards with the same intake, exhaust, but this new engine with same valves, cams, and OEM 10.2:1 static CR, yielded 310 RWHP and that was with reduced timing as it was no longer necessary (I have another thread on here talking about that too).

Oh - While you have the heads off you may as well index a bunch of plugs to orient the ground strap where you want (that's a whole other discussion)....

You can spend a lot of time thinking about this stuff and obviously working on it. My $0.02 12 years later is don't get too deep into analysis paralysis. Good luck and hang in there. At least you can drive your car while doing this engine build. I had to stare at mine lifted in the air for about a year....

Thanks @Mac Attack! I tried not to get too specific in my questions about dimpling, the pics you showed of the head side corresponds to what I've seen from my own burn patterns and I have a pretty good idea of what I'm doing with my pistons I think. I've got a 3/16 ball head milling bit to do the dimpling. My plan is to print out paper templates to stick on the pistons to center punch where I want the dimples before drilling. Then once the dimples are drilled I'll blast the existing ceramic coating off of the piston tops after carefully masking everything else and reapply it myself. (it was not applied very well, some spots were too thick and ended up globby looking from the guy who did the coatings)

I've got a new piston on the way from Toda, they were very helpful and looked through their stock to provide one most closely matching the weight and size of the pistons I have already. $270 shipped so not bad at all, their support has been quite good despite the variety of dumb questions I've asked them about pistons and cams over the past year or two.

I've also decided not to bother with WPC treating the bearings. Instead I'm going to have a dry lubricant applied to them by Calico Coatings which will reduce clearance by about 0.0005" (0.00025 per side) but I'll still be just above the low end of factory specifications, which for coated bearings should be about perfect.

I'm not sure what to do about the block. Mountune is supposed to call me on Monday since their build shop manager has been out but I seriously doubt I'll here from them until I pester them more, but maybe I'll be pleasantly surprised. I'm not sure if I even want to send it back to them at all or just have a local shop hone it out a little bit more. If a shop can't do a good job on the first try, in my experience, they rarely do a good job in subsequent attempts and if anything just get sloppier. I really expected better from them.
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I'm a big fan of Calico. What main and rod clearances are you targeting?

I have no idea about Mountune, but I do know with a lot of shops that they won't put their best person on the job initially unless you're a special customer. If they supposedly have a good reputation, I'd tell the shop manager you are checking this and want their best person on it this time....
After talking to one of the guys at Calico about coated bearings I'm setting the uncoated rod bearing clearance to the top of the factory spec at 0.0023-0.0024", which after 0.00025"-0.0003" coating on each shell should put me right at 0.0018-0.0019". For the mains I'm doing the same thing and targeting 0.0019", which after coatings should put me right at 0.0014-0.0015". They say coated bearing can let you get away with tighter clearances but I don't see any reason to push it in that regard so I'm running ever so slightly tighter than the middle of factory specified clearances.

For measuring clearance I found that the bore gauge leaves scratches roughtly 0.0001" deep in each shell (measured thickness, scratched bearing with the bore gauge, then sanded with 2000 grit until the scratch was gone and measured again), so I got some 0.0030" precision shim stock and cut a strip of it to protect the bearing surfaces while measuring, and sure enough I get 0.0002" less every time. To remove measurement error from the shim stock I put it between the anvils of the mic before zeroing the bore gauge. I sanity check the measurements at the end without the shim to be safe but so far its worked great to prevent excessive scratching.

I found a great deal on Ebay for a M11x1.5 timesert kit so I'm trying to come up with a good way to perfectly center the drill fixture again. I'm planning on making some sort of extension piece on the lathe to stick out above the plate to make sure the drill bit can't end up a little crooked.

Yeah I've noticed a trend of shops not really caring about you unless you're famous. Mountune came highly recommended by several fairly well known NSX guys such as @stuntman, they did end up getting back to me and are sending me a label to send the block back for correction so that's good.
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I finally started on a tedious task I've been dreading: reshaping the combustion chambers for a 93mm bore size. I'm not really trying to change the shape so much as just open it up to fit the larger bore size and deshroud the valves as much as possible in the process. The 36mm valves are quite close to the sides of the chamber in 3.0 heads so in theory this could have some benefits in flow, and the removal of the sharp ridge around the edge could reduce potential hot spots.

These Endyn heads have already had some minor deshrouding work done but here's the changes I'm making for the larger bore size.


Screenshot 2024-03-01 at 9.41.48 PM.jpeg

The flat spot by the exhaust valves was a bit tricky. After talking about it with @Honcho I decided to just wing it with a milling bit on my drill press, which actually worked albeit its a crude solution.


The first chamber is getting there in this last pic but still needs a little more tweaking especially on the exhaust side. Then to do it 5 more times, measure CCs, and adjust as needed until they are all equal.

Slow and steady...

I fitted a set of green bearings and torqued up the mains, then checked clearances on each journal. The service manual specifies 0.0009-0.0019" clearance on the mains, so before coatings I'm targeting ~0.0017" clearance, and after coatings are applied I'll be right around 0.0012".

Its worth noting that Honda QC for bearings sucks, and color codes are largely meaningless, but I'll be measuring the actual thickness of each bearing once clearances are measured. And I'm using Toda bearing shells for final assembly instead of Honda since QC seems far better, they have more surface area, and they are a slightly harder material. The downside is pricing on Toda shells is 7750 JPY each, or $52 at current exchange rates, compared to $12-15 for OEM shells.

Main Tunnel Sizes (spec is 2.7165" - 2.7175")
1) 2.7168 - 2.7172 (0.0004" taper is pretty crappy for a fresh line bore)
2) 2.7168 - 2.7170
3) 2.7167 - 2.7169
4) 2.7167 - 2.7168

Clearance / upper thickness / lower thickness
1) 0.0015 / 0.0985 / 0.0985
2) 0.0013 / 0.0985 / 0.0982
3) 0.0012 / 0.0982 / 0.0985
4) 0.0017 / 0.0982 / 0.0986

From those numbers we can determine what thicknesses of bearings we need to theoretically hit my desired uncoated clearance of 0.0017.

1) 0.0984 G/Y (green is nominally 0.0985, yellow is nominally 0.0983)
2) 0.0982 Y/P (yellow is 0.0983 and pink is 0.0981)
3) 0.0981 P/P
4) 0.0983 G/Y or Y/Y

My estimates on 1 and 4 turned out to be spot on, but I don't have more yellow or pink bearings to test #2 and #3 so I ordered a couple more pink and yellow bearings to try it.

I'm also not thrilled with the taper on the #1 main, or the surface finish of the line bore job. It's a very rough finish so it won't securely hold the bearings or allow for effective heat transfer away from the bearings, so I ordered a 2.75" 800 grit flex hone to smooth the finish out. The bores are all pretty close to the middle of the spec so I can safely hone 0.0002ish out and still be safely within spec.

To correct the taper in #1, assuming mountune won't do anything about it, each main is split into 2 by an oil channel in the middle, so I'm going to put steel shim stock over the side thats larger, then do a quick pass with the hone and remeasure until the taper is reduced to 0.0002 or less, then remove the stock and do a few more swipes to smooth it out. Honing with 800 grit removes material extremely slowly so there's really no risk of taking away too much material or affecting bore geometry.