18V vs. 18G

I recently picked up a parts car with an o/d transmission. It's a 1970 with a 18G motor. My current '72GT has it's 18V motor.

Now that I've pulled the drivetrain out of the '70 to get the o/d out, I can't help but wonder about the motor as well.

If you had a choice, which would you rebuild? The 18G or the 18V. I'm not too well versed in engine spec changes over the years. I know some motors have bigger valves, but I'm not sure which ones. Any advantages/disadvantages to either model?

Thanks in advance for any information.

Robert,
Glad you asked that question.. I will be looking at a 1965 parts car next week and it has an 18G engine while my '72 has an 18V. I also have an 18G in my MGA. So I may have to go through the same decision process as you. Let's see what other member will advise..


Cheers,
Ahmed

I don't think there was much of a change in the block itself. The timing chain was a double row (better) on the pre 18v blocks. I think the biggest difference laid in the heads, which were set up for unleaded fuel and loads more emissions on the 18v. Your right I bet the valves were different also. Up to 70, 18GH, the heads were a little higher compression I think. Probably rebuilding the 18G motor is the better way, but theres alot more experience here (forum) than I got.

Be careful as to which 18G engines you are talking about. 18G and 18GA engines are 3-main bearing, 18GB up to 18V are all 5-main. The biggest change from the 18GB series to 18V was larger valves, and a valve relief "cut-out" in the block. Otherwise, the blocks are pretty much the same.

You can have an earlier block machined with the cut-outs to safely fit an 18V head.

As stated, the earlier engines also had a two-row timing chain, but the 18V can be changed just by installing 2-row sprockets, chain and tensioner.

Only other change that is important is the rear rocker pedestal has an offset oil feed on the later heads. However, the pedestal can easily be changed if you buy a rocker shaft assembly of the wrong kind.

There are some minor changes to the block. The 18V has the notches in the cylinder walls to accommodate the reduced valve clearance from the 18V head. Someone recently said that it wasn't to eliminate contact but rather to unshroud the valves.

Some of the 18V heads had larger intake valves. That might be a good reason to go with the 18V. All 18Vs had different combustion chamber volumes than 18G. The essential part is that you must match pistons and head or you won't get the right compression ratio. You can mix/match block and head (e.g., use the 18GH block and the 18V head) if you take this into account. If you have the big valves that might be a nice thing to do but you will have to notch the cylinder walls to match.

You should use the double row timing chain/pulley (18GH). You should use the short lifters and long pushrods (18V).

I don't know if your local emission laws would be a factor. I.e., would they require you to meet tougher emission specs if you use the later engine?

Mostly, I would look at the condition of the block and head. I would go with the one that needed less machining. I would also be strongly swayed to put the correct engine (or at least the block) in the intended car. That will make it a bit more authentic.

Although mixing and matching components is permissable (and may have advantages) it will also drive people crazy down the road if you ever sell the car.

I would build the G motor as it will most likely be higher compression (meaning higher performing) than the 18V. The G series had shallow dish pistons while the V had the deeper dish. Heads on the G motors had a larger volume (~42cc) as opposed to the Vs that had a smaller volume (~38cc). The valve size (and volume) will depend on the head casting number not the engine plate number so have a look at http://flowspeed.com/cylinder-ident.htm to figure out what you've got.

You can probably swap the V head onto the G block and pistons to raise compression but you'll have to add cutouts to the block (I think.)

In addition to the mentioned differences are the rods. The 18GJ has angular split rods which weigh 980 gms. The early 18V rods weighed 840 gms had a straight split and still had the floating wrist pin. The head on the 18V also had larger intake valves. I'd go for that one in a rebuild. The heads can be positively identified by taking the valve cover off and looking for the casting #. The 72 head should have a 12H2923 ID#. That's the big valve head. The earlier one probably has the 12H2389. In a rebuild the first thing to do is magnaflux both heads because they are notorious for cracking. The even earlier 12 H906 heads don't seem to do this as often.

I located and fitted an 18V head onto my 65 "B" (18GBUH engine no.) many years ago, after noticing in the road tests that the MK2's were a bit faster than the 5 bearing MK1's.Thats when I ascertained that the inlet valve size had been made larger.(for the Australian market cars anyway, dont know about yours)I also had the head "ported and polished" by a cylinder head shop who do a lot of racing engines. I did not modify the block however.( From what I've read today I obviously should have),I can say that by the admittedly unreliable "seat of the pants accelerometer", you definitely can notice the difference( A bit faster and seems to spin a bit more freely) But the difference practically is not huge. Interestingly, my "car nut" neighbour noticed a distinct change in the exhaust note after the change.

Ok, FWIW, here's my thoughts, the early 5 main crankshaft and angles rods will work fine in a motor, but if you asking me what I like to work with the best, it's not either of those items, I prefer the straight rods, they are two versions of those, I prefer the floating pistons, one less thing you need special machinery for, and the flat side crank IMHO is the best MGB crankshaft. As far as the stock compression ratios, all that can be re-done in a rebuild with decking heads and using the repalcement (higher compression pistons) so that can be easily worked out, and almost any combination of early or later parts can be worked out to a good 9.5 compression ratio. Only the pre smog heads came stock with the bigger valves, bigger valves can be fitted to any head.

The straight rods are easier to properly resize, and the flat side MGB cranks is for sure the Cadillac of the MGB cranks. Pulling your pan will tell you what you have with your G series motor.

Looks like with the two motors you can possibly get the best of what BL had to offer. How cool is that!

I believe there are also differences in transmission mounting (engine back plate) between 3-main and 5-main (????)

mac townsend Wrote:

Hap, I'm trying to pin down the smaller flywheel idea for a trans swap I'm doing. I'm told that the early B flywheel is smaller, but have yet to pin down whether that is for a three or five main motor.
I picked up a Magnette/MGA flywheel that is 12" diameter, but it has a different crank bolt pattern, and is heavier by about 10# to boot. What I need is a 12" flywheel that will bolt to the 18 GK crank, and hopefully is lighter than the 13" 19# wheel used on the later 1800s.
(The rest of the story is a 300ZX transmission and a TD/Sprite style starter, all screwed together with a new alloy rear plate to suit, all for a 50's style Special)

What flywheel do I want? (part number or model year or motor serial series, anything would help).
Thanks,
Dan Wilson
Colorado Springs
jbrwky59@hotmail.com

When I swapped a rebuilt 18V for my original GB engine, I used the flywheel, backing plate, starter from the GB and put them on the 18V. I was mating the engine to a T9 conversion and did not want to have to get a new bell housing. Point being the GB (5 main) engine also has the smaller flywheel.

In Burgess' book , he gives the 18V block a slight preferrence over the 5-main G series block for much the same reasons as Hap posts above. Burgess notes the irony in his preference for the 18 V because the earlier cranks were forged and usually one would prefer the block with the forged crank.

dannyw Wrote:

Ok, can I jump in?

Is there anywhere on the wonderful internet that shows specific instructions/dimensions on how and where to lighten a 5 main 18V flywheel?

Here is a reference to machining MGA flywheels:
http://www.mgaguru.com/mgtech/clutch/ft_202.htm

Now, has anyone got this machining data for the early three synchro B flywheel? From what Hap says, the 12" wheel is if anything, heavier than the 13". I may need that info.....dan

PS
Anyone else have any info on fitting the 12" flywheel to a later engine? "Need " *
the smaller diameter, also "want" less weight as the car it will go in should be under 1400#.
* the 13" flywheel is a very tight fit in the Nissan 300zx trans bellhousing, thought that going to the 12" early B flywheel would kill two birds.....am I dreaming as usual?

Guys on most flywheels you are limited on how much you can remove from a flywheel by how it is made, in the case of the MGB flywheels the ring gear is pushed on from the rear to the front, so while you can machine the front side for lightening outside of where the clutch and pressure plate attaches you must still leave a shelf for the ring gear to rest against, in the later flywheels I machined this down to about .0100" then on the back you can machine down until you begin to face off on the ring gear on the later flywheel, on the earlier unit, it's a little more tricky because it is already concaved in this area . I did a post on this with pictures last year, use the search feature, lots of good stuff benn discussed here in the past, just plug in lightening flywheels and use all date, you'll see many discussion about and even picture examples .