Engine and Transmission Tech

I’ve been scraping the internet, looking for cost-effective options to overbore a Rover 4.6 while having a cracked block repaired with top-hat liners (see my other thread “4.6 triage” for the reasons why).

I found the pistons from the Toyota T100 and 4Runner 3RZ-FE engine from 1995-2003, and I think it may be a real option for a cheap, but durable 4.8L build:

The piston is flat-topped, available in 95-96mm/3.74-3.78in bores, has a 24mm/.9445in pin that matches the OEM Rover piston, has a 35.5mm/1.398 comp height that’s only .015 less than the OEM piston. I’m not certain of the piston weights, but I think it might be about 100g less than the OEM Rover slug.

An aftermarket set of four anodized, Teflon-coated, hypereutectic pistons sells for 189 bucks.

Many Toyota guys add superchargers and turbos to the stock engine internals, and the engines have a rep for being bulletproof.

With a set of Buick 300 heads, I think I can make compression ratios around 9.5:1.
Anyone else out there tried these?

I think the OEM rover pistons were about .030 below deck, and the 300 heads have no quench pad, so there’s not much going on in that department.



Edited 1 time(s). Last edit at 09/19/2018 09:27PM by RDMG.

I have not heard of anyone using that piston. Sounds interesting, though.

Who makes that piston? How much dish?

Using these in a Rover 4.6, the piston will be 0.051" shy of the deck.



Edited 3 time(s). Last edit at 09/20/2018 11:34AM by MGBV8.

I don't think that combo will get tnear 9.5:1 compression ratio without shaving the head or decking the block.

Hi Carl,

These are the pistons I found:

[www.lceperformance.com]

Confirming the dimensions has been difficult, the details I gave above are a mix of sources.

There’s a forged piston for the same engine in the wiseco sport compact catalog that lists most of the specs I referenced above, but has a 10cc dish. I called their tech support line, and learned that their piston has the stock comp height but has an increased dish to lower compression for supercharging. I found some used OEM pistons on eBay, which have a basically flat top.

I entered a 3.78 bore, 3.2283 stroke, .04 gasket, 0 dish, and .05 piston depth below deck into this calculator to get close to 9.5:1:

[www.csgnetwork.com]

I can’t make my OEM 4.6 heads with a 28cc combustion chamber (skimmed once already) work under any combo, but a Buick 300 head with a 54cc chamber and thinner gasket seems to work. Since the heads need a lot of work already, skimming a bit to reduce volume isn’t a big deal. The plan also assumes top-hat liners, so decking the block will also be a marginal expense.

With a stock wrist pin, the Toyota piston seems to be an easy install. At the largest oversize, it give a 4.75L displacement. The larger bore also reduces shrouding of the valves in the Buick head.

A replacement set of RV8 4.6 pistons at Rimmer Bros is about US$1200, while the Toyota pistons are $380. Machining costs for the liners are the same either way.

If I sell my RV8 heads, buy Toyota pistons, and spend $1200-ish on Buick head porting instead of stock pistons, I think I come out way ahead?

Is there anything I lose by having the piston at TDC be .051 below deck? The 300 head doesn’t have much of a quench pad anyway?



Edited 1 time(s). Last edit at 09/20/2018 05:39PM by RDMG.

I do have a new set of 4.0 litre pistons, same as 4.6 but smaller dish at 13 cc. If these are of any help they are for sale. And way cheaper than $1200 !

One issue with the Toyota pistons (if the pic in Iceperformance link shows the actual items) is that the piston pins are floating. It means that you should have bushed con rods.

Dave I think you are onto something. The 95mm/3.74 Toyota piston looks very good as a replacement for a larger bore 4.0/4.6 piston. Compression height for the 4.0/4.6 is 1.413. The Toyota is 1.465. That is .052 higher. Have you seen Fred's thread on the cheap Chevy liners? Here is an even better deal on Ebay:

[www.ebay.com]



Edited 4 time(s). Last edit at 09/21/2018 04:59PM by mgb260.

Well now I’m really interested!

My compression height figure above came from the specs for the low-compression forged piston in the Wiseco catalog, so I think your reference is probably the OEM dimension.

With that higher deck height, I think the piston is about .020” above the deck:
5.90” OEM rod + 1.465” Toyota comp height +1.6145” half the 4.6 RV8 stroke = 8.9795”, and deck height is 8.960”.

Any guess what the volume of the dish on the Toyota piston might be? Now I’m concerned that 300 heads and a thick (.070”?) composite gasket may not be enough.

Also, what to do about the floating pins? There’s some discussion boards out there that say you can press fit floating pins into a press-fit rod? Alternatively, have a 24mm floating bush installed .020” lower than stock to get a zero-deck piston?

I was turned off by the obviously Chinese factory-direct pistons on eBay. You think they might be a safe bet? The price is ridiculously low.

I'm thinking just right for the 4.2's stroke with 6" Chevy rods. For the 4.6 you would have to mill the top ridge of the piston. Just leave out the piston lock rings, the pin will work fine pressed in. They are the same diameter as stock and will have the same interference fit. The pistons will be fine, Chris used the cheap Speed Pro (offshore) pistons with no issues. Have you checked Fred's liner solution?



Edited 4 time(s). Last edit at 09/21/2018 08:23PM by mgb260.

What I've been seeing on the forged pistons is that pricing isn't that good and by the time you buy 8 of them it can easily be over $1200, Volvo and Japanese. (for 97mm pistons to fit the 300, 3.818" std). You can have custom pistons made for less than that.

Jim

Hi Jim,

Totally agree on the forged pistons being not cost-effective. That's why the OEM-spec Toyota pistons are so interesting. I found a hypereutectic set of 4 for $189 (link above), and Jim found a cheaper set (not sure what the alloy is) for $80!

Coupled with Fred Key's low-cost top-hat liner for the Chevy 305, a 96mm (3.78") overbore of the Rover 4.2 or 4.6 starts to look like a reasonable approach.

Less than $400 all-in for the liners and the lowest-cost pistons. No need to modify the rods. Still have the $1,200 machining costs though. Then the $1,500-ish to massage the Buick 300 heads.

Add a few hundred to grind my crankshaft .010 under, and a few more hundred for new bearings and gaskets, and I could reasonably have a rebuilt, free-breathing, free-revving 4.75L Rover V8 for about $4,000.

Dave



Edited 2 time(s). Last edit at 09/22/2018 01:35PM by RDMG.

Maybe you could. Hope that all works out. If you keep your redline below 6K anyway. If you want it to rev freely a light flywheel helps.

I like to do all I can to keep the reciprocating weight down though. Forged rods and pistons and thinwall piston pins can let you remove as much as 12 ounces (roughly 300 grams) from each rod/piston assembly. That's up to 6 pounds for all 8. Makes a huge difference. Often you can find a suitable set of rods cheap on ebay for around 2-300 leaving a chunk of the budget for the slugs. Since cast pistons mostly run $400+ anyway, that extra 4-500 buys a lot. Light weight, and exactly what you want.

Those inexpensive off the shelf pistons often limit you and/or require additional machining operations to work right, and you usually don't consider those costs when buying them. Don't get me wrong, I still hope it can work out but there is a lot to consider and most sellers don't give full specifications on their listing. The usual problem is that you find a cheap piston and then you're stuck with either cast or the more expensive rods and then the crown of the piston has to be cut and you have to bush or ream the pin holes and the squish distance is no good and your stuck with 200 gram wrist pins and no piston offset. It doesn't take that much of that sort of thing before the custom pistons begin to look attractive, especially when considering that it extends your potential rev range by 1000-2000 rpm. And if you consider those few hundred dollars to be amortized over the life of the engine, it's peanuts.

If you are going to spend $1500 on the heads I do have to ask one question. You do realize that you can buy bare TA heads for $1600 right? They will outflow anything you can do to the 300 heads, possibly right out of the box. Need valves and springs of course. And rockers. Just thought I'd mention it. I have Mike thinking about what he can do to enlarge the chambers too. We'll see on that one. Again, here the custom pistons let you configure the crown to take advantage of the squish pad while keeping the CR within reason.

As far as spending $1200 on sleeves, I suppose that's quite a deal from what I've seen, and I'm sure you know my opinion on that. But if 60 lbs or so is that important I guess you are stuck with it. BTW, do you have a bare block with caps you could set on the bathroom scale? We don't have a weight for the 4.6 block, accurate or otherwise. We know it is heavier than the 215 but how much really? 20 lbs? More? Less? We just don't know.

Just another viewpoint.

Jim

Many thanks for the reality check Jim,

I tend to get overly optimistic about how things might work out.

My $1500 figure for the 300 heads might be way off. I was thinking that putting in Buick V6 valves would be the logical level of investment in that head, but I have no idea what the budget for that might be. I think those valves are about $20 each, so figure $400 for the valves and seats, another few hundred for the z28 springs and retainers. Likely $800 labor? Maybe that’s too low? If so, going to the TA Performance head might be a no-brainer. If so, that does take the cost estimate way up. The stock head combustion chamber is too small to support an overbore and a near-zero-deck piston, so the cost of either the 300 head or the TA head will have to be a factor.

I’m basically looking for the cheapest way to rebuild the engine, hoping there’s a durable overbore option that costs less than all OEM spec parts. I’ll then balance that against rebuilding a Buick 300.

I disassembled my RV8 bottom end this eve. Every con rod bearing was worn through to copper, and there are flecks of debris in everything. Main bearings weren’t worn to copper, but were pretty beat up. Some scoring on all the crank journals, but is uniformly distributed and only enough to barely catch a fingernail.

I really think the engine was more of a steam cleaning parts washer for the last few minutes of its past (maybe only) life. Bizzarely clean everywhere.

According to my digital luggage scale, the weight of a 2004 Land Rover Discovery 4.6L block, main bearing caps and bolts, cam bearings, and cam thrust plate is 76lbs.



Edited 2 time(s). Last edit at 09/23/2018 07:07AM by RDMG.

Dave, Many years ago I did the 300 heads with 2.3 Ford four valves and Z28 springs. You need new seats though. 1.74 IN and 1.5 EX. Valves are cheap on Ebay and check Alex's parts for seats, valve seals and Z28 springs. Do your own mild porting/cleanup.

Thank you Dave. So we now know that the 4.6 block weighs 16 lbs more than the Buick or Olds 215. We also know that the 300 weighs 64 lbs more than the 4.6 and about 50 lbs more than the 1800. Good information to have. I have found only minor weight differences in the rotating assemblies (a 340 crank weighs the same as a 215 crank for instance). So the weight differences in the blocks are the only difference worth considering.

There is no doubt that the TA heads are a higher cost option. How much higher is the real question. Or, is it worth it? We put TA heads on the 455 on a shoestring budget. Bought the bare heads, used the stock valves and springs from our iron heads, and a set of good stock rocker shafts we already had. In that case it was very much worth it to us, primarily because it made the exhaust possible, which is not a concern here. However flow bench testing by Dan Jones has demonstrated that the flow capacity is much greater.

As I understand it, here is how the Rover/SBB heads stack up from lesser to greater.
215 Olds
3.5L Rover
215 Buick
Other Rovers (later heads)
300 Buick alloy
Aftermarket Rover
300 Buick Iron
350 Buick Iron (not usable on BOPR engines in any practical sense)
TA
TA- 350 (near end of pre-production, not usable on BOPR)
with ported heads following the same order. One or two could be out of sequence and in some cases the differences are small, others are larger. Not a point to argue right now, I will defer to properly established flow numbers, and of course the quality of the porting job can make a difference.

As you can see we are in about the middle of the scale in the overall picture, which means that with the 350 cid engines the heads are the limitation. Less so with the 300 but still not nearly as good as the stock GM iron heads. The Buick mantra is, "Put your money in the Heads!" and these guys are the ones running the iron heads. What that should tell us then is that we have a lot of room for improvement. My approach to that was forced induction. Also not cheap or simple, but I can attest to its effectiveness. There is also a weight penalty of course.

Our current discussion about the head chamber clearly shows that the TA heads were designed as a bolt-on replacement for the stock Rover heads, and with a 300 based engine the only practical way to utilize the quench pad in the heads is with a deeply dished piston with quench pads. The dish volume depends on displacement and compression ratio but can easily be 30-50cc. That's a lot. So the first step in building such an engine is to call your piston maker and see if they will do it. And what the piston will weigh. It's pretty clear you won't find an off the shelf cast piston for that. So again, the custom piston widens the scope of possible builds. Jim N. posted a photo of such a piston, maybe I can attach it. The dish would need to be deeper but the design is good.

[www.v8buick.com]

Then there is the situation with the rockers which is still just a bit unclear. We know that the Chevy style pedestal mount roller rockers will work, but exactly which ones needs to be clarified. As will the expensive TA shaft rockers. I have not looked at the Chevy parts but presumably they are less expensive, maybe downright cheap compared to BOPR rocker shafts, so that could be a plus.

So then, the least expensive build is going to be the basically stock 300 with the 300 iron heads which will add another 50lbs or so to the weight and performance will be quite good. I have both heads and have weighed them but I can do it again to confirm that weight difference if you like. Your engine would weigh 100 lbs more than the 1800. That is acceptable for the MGB. It may not be acceptable to the individual psyche. That would be my starting point and it is a good one.

With the stock 300 alloy heads the weight will be 50 lbs less and the performance will not be as good. I suspect the car might be a little slower. You may or may not be able to feel the difference in the corners, I suspect not. But the psyche will be more accepting of it. Humans are weird, what can I say.

Once you start porting the 300 heads and adding big valves things can change. Last I heard (5 years ago?) a porting job was around $800. Could be over $1K today. That makes the TA heads a $600 premium since at that point the rest of the hardware is a wash. Is the better breathing worth the premium? That really depends on the pistons then, doesn't it? If you were building the engine for a 7K+ redline anyway, then the piston cost doesn't add any more than it takes to get the double D shaped dish. Otherwise it just depends on what you can find. In this case that $600 premium buys you an increased breathing capacity that has been documented By Dan Jones' flow bench results, posted in another thread on this forum so you can make that decision based on hard numbers.

Or you can just build the cheap engine. It really is quite good and absolutely the most bang for the buck. There will be few cars that will outrun it and that may never happen.

Jim

After a week or two of research, I’ve learned a few more details.

There’s two OEM versions of these pistons:

The 3RZ engine (2.7L Tacoma, T100, 4Runner) had pistons with a 1.476” (37.5mm) compression height

The 2RZ engine (2.4L Tacoma) had pistons with a 1.398” (35.5mm) compression height

Many aftermarket rebuild 3RZ pistons have a 1.465 compression height, to maintain stock compression after skimming the block and/or head.

The dish in each piston is about 12.5cc. (I originally thought flat top, but was mistaken.)

The NPR of America online catalog lists all the specs for the pistons, except for the dish ccs. I got that number from a Toyota guy who works with the RZ engines a lot.

I’m thinking the 2RZ piston with the 1.398” comp height could be a real winner for those who are putting top hat liners into a 4.6 engine, because the lower comp height give you a chance to mill the block deck to suit the new 95 or 96mm piston. In an otherwise stock rebuild, you can use the 96mm oversized pistons to get a 4.8L engine with likely lighter rotating mass.

I’m not yet sure what the best head combo would be, but I’ll be working options through a comp ratio calculator for a while. I was all set to run with a set of rebuilt Buick 300 heads with the taller 3RZ pistons and a thick gasket, but I might be better off building a 2RZ setup for the stock heads I have, and save for a set of TA heads down the road.

The 300 heads are an open chamber design with about a 55cc volume whereas the Rover heads are closed chamber with about 28cc. This makes a profound difference in piston choice and design just in terms of CR. Rumors have it that the 4.6L Rover heads flow nearly as well as the 300 heads.

Jim

> Rumors have it that the 4.6L Rover heads flow nearly as well as the 300 heads.

The 4.0L/4.6L heads are the best flowing of the production Rover heads, flowing 135 CFM intake and 106 CFM exhaust through 1.575" diameter and 1.350" diameter valves. They did not increase in flow above 0.350" lift. An unported set of Buick 300 aluminum heads (1.625" intake and 1.312" exhaust) flowed 154 CFM intake and 116 CFM exhaust. Depending upon your displacement and maximum RPM, the smaller port minimum cross-sectional area of the Rover heads can become an power limiting issue.

Dan Jones

So there you have it. Did you see that Carl?

Jim

(Thanks Dan)

I think the Buick 300 head will be perfect for a 96mm overbore using the 2RZ piston with a 1.400 comp height.

Using an online comp ratio calculator, I entered 53cc comb chamber, 3.78 bore, 3.23 stroke, .050 gasket thickness, zero deck, and -12.5cc dish to get a 10.3 compression ratio.

Those figures assumed .010 skimmed off the head for flatness and about .040 off the deck while finishing the top hat liners. Plenty of room to lower the comp ratio a bit by skimming less, but then the quench effect would disappear. Not sure how much quench you get from the 300 head anyway, maybe a little at the edges where the valves are not?

The 12.5cc dish figure isn’t confirmed, I think I’ll have to buy the pistons and measure myself.

Given the skimming and decking requirements for any top-hat rebuild, I think the piston is in the sweet spot. It may also be possible to find a combo that works for the TA heads, but then the piston strength may be an issue.



Edited 1 time(s). Last edit at 10/08/2018 07:46PM by RDMG.

Ok, so I’m going to take the plunge and buy a set of 2.4L pistons and 305 top hat liners, and set it all up to run with Buick 300 heads. I’ll start a new thread focused on how it comes together.

At the end of the day, if it all blows apart, I’ll have a nice set of aluminum heads to bolt onto a Buick 300!