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MG TD TF 1500 - XPAG Piston height
|Hello all, I'm a newbie here so go easy on me.|
I've just acquired a TF. It is an abandoned restoration that was well along. The lower end of the engine is assembled with new crank, cam, pistons, etc. One thing that caught my eye is that the block has been decked. A quick shows the pistons are crowning slightly - worst case approximately .007". It this acceptable on this engine? Although experienced with other engines, the XPAG is new to me.
I have used a solid copper gasket on another engine that was severely decked to salvage the block. It worked but was somewhat difficult to get it to seal antifreeze. Those pistons were so high that they were threatening the head.I haven't seen a stock gasket yet to measure but I thought it would cost me nothing to ask those experienced with this engine.
Here's a little info that may help:
On the standard TF:-
Stock c/r = 8.1 to 1
Depth of stock cyl head = 75.16 mm
Combustion chamber capacity = 39.3 cc
Cyl head gasket = .045" - 1.143 mm
Capacity of head gasket = 4.5 cc
The above all from Bentley's WSM
If your pistons are standing proud of the block, you basically have two choices:- a) remove the pistons and mill the surface or b) use a thicker head gasket.
To test, install the head gasket of choice. Then I suggest you put two pieces of Plastigauge on the top of one piston, one under each valve. Replace the head (cold) and torque it to factory specs = abt. 45 lbs/ft. Remove the plugs and hand crank the engine several rotations.
Remove the head and carefully remove the Plastigauge strips and measure them. As a starting point, I wouldn't tolerate anything less than .075" but .1 would be better. When that engine gets hot, that dimension will reduce.
|Gordon A Clark|
|Your clearances should be fine with what you have unless you're planning on using domed pistons, then a quick check like Gord mentions above will calm your mind. I had to eyebrow the pistons when that happened.|
Solid copper head gaskets are available for the XPAG in various thicknesses from www.headgasket.com (also known as www.gasketworks.com). In order to lower the compression I stacked two solid copper gaskets in a supercharged engine that had domed pistons and have never had a problem with leakage (knock on wood).
|I don't remember how much you can get away with exactly, but my guess is that you should be fine. My pistons sit proud of the deck as well, and they are high compression pistons with very tall centers. See attached photo.
|Thanks guys, these are flat tops, .060 over. I think the head is stock thickness. I thought it was shaved quite a bit but the information I had was for a TD. I checked it just before I had to run out on a trip an have not made an accurate check. If it has been surfaced, I think only slightly.|
I have a couple of days layover in London this week. Is there anyplace related to MG that I should see while I'm there?
You might be interested in a visit to Brown & Gammons in Baldock. B&G is a big operation run by the Gammons family. They have for years, supported MG motorsport and are big suppliers to the T-Series crowd. Be sure to pick up one of their parts CDs. They are also the source of several T-Series “trick” parts including the light alloy flywheel, and the MGB trunions to mount MGB shocks on your T, to mention just a couple
B&G is easily reached by train - about 40 minutes from King’s Cross Station, and a short walk once you arrive in Baldock.
If you enjoy doing the tourist thing in London, two items of interest that often escape the average visitor to London, include the Silver Vaults and the nearby Churchill War Bunker.
I’ll be there in a couple of weeks (Sept 26th). Too bad we can’t work out something together. I will however, be at Stowe this coming week-end for the British Invasion.
|Gordon A Clark|
|Well, I've had a little more time to research. |
Gordon, is that .045 gasket thickness a compressed figure? If so, I think I may be OK. The research I've done suggests that a .035 is minimum practical IF, the RPM is low enough to prevent rod stretch, the pistons a a snug fit in their bores to prevent rock, the bearings are snug.
If the .045 is the compressed gasket figure, I should have a minimum of .038. The other conditions should be met as the crank, bearings and pistons are new and the XPAG is certainly not a high RPM engine.
Interestingly, my research also indicated that increasing the squish area beyond .050 or so rapidly negates its value in creating turbulence in the combustion chamber and, in fact, may increase the possibility of pinging. That seems counter intuitive since the compression would be lower but the loss of turbulence apparently is dramatic.
This is no race engine so I'm not inclined to run at the minimum as in a running engine it closes to almost nothing- perhaps desirable but leaves little room for carbon buildup.
My other thought is to remove the pistons and selectively machine for a zero deck height thus controlling with the gasket.
Comments or suggestions?
|Not quite sure how one would measure a gasket in the compressed mode, so I can only assume that the .045" is before installation of a new gasket.|
Maybe someone out there can measure a new one. I have an old one I can measure tomorrow but I don't believe that will be of much value.
As Gene has indicated, your best bet is a new solid copper head gasket from Gaskets Works. It looks like the other options, including my suggestion of milling the pistons, is getting expensive.
Best to use the K.I.S.S Principal!
|Gordon A Clark|
Since you quoted the cc capacity of the gasket from a workshop manual, I guessed it might be taken as the compressed height volume.
Been there before on the solid copper gasket and I may go that route. That engine quoted a deck height and I was on the other side of it to salvage the block. So far I haven't found a deck height number for the XPAG but I think I'd be happy at zero. First I want to do a survey of the all the piston deck heights. If I have them machined to a zero deck height naturally I'll have to have them balanced as well.
This work was done, as best as I can establish, over 20 years ago. The fact that the head machine work was done but it was never installed, leads me to the conclusion that the mechanic assembling the engine back then had some concerns. The quality of the other work done to the car is first rate.
I'd looked up that info for a friend earlier...here's what I came up with for gasket crush:
Bob Gp (MG vintage racer like Mike): .045"
Dirk D: 0.047 to 0.050"
John Taylor: 0.050"
Lee J sent the measurements of uncrushed (new) head gaskets: "I happen to have my micrometer handy , plus two new XPAG head gaskets , one for banana, one with round holes. The round hole version is .067" thick. The banana version is .056" thick. This is measuring across all folds on both ends.
So, all being said, it looks like 0.047 would be a safe figure to work from. Remember, a solid copper gasket will not compress!
Thanks! That's exactly what I was looking for. Stuck in a hotel room on a layover, I rely on what I can find on the internet from folks like you, Gordon and the rest who responded. With that information, I'll order up a gasket set and continue with the assembly. I intend to install a modern front seal and am inclined to chance it with the original type rear seal. I hope I don't regret that. That's not a set-in-stone decision and I'd like to hear opinions. The crank is brand new so I have no intention of messing with it but the Moss seal is an option. Incidentally, the price of the crank back in the '80s when this restoration was started was $850. !
Anyone have a preference for gasket suppliers? I have a one night layover in London this week and I may visit the Moss store there.
Contact me offlist about that front gasket seal...it's not as easy as it seems.
|I'm in the process of a full rebuild and the method used was to deck the block, then install the crank with rods and pistons. Then machine the pistons for a zero deck height. Then a full ballance of the rotating mass. I've yet to drive it as Len's cam is being installed at the moment, but I'm REALLY looking forward to the results.|
We'll actually land at the Luton airport using airport transfers tonbridge and will stay the day there. It turns out Baldock is only about 17 miles away, just a few miles from the town of Hitchin where we often stay at the Priory, an ancient hotel that was once indeed a Priory.
If possible I'll make the trip over. Their rear main seal kit looks interesting. Maybe if I can get it sans VAT I'll buy one.
|Gene and Steve,|
do you have any sources for the HC pistons for the XPAG?
|R.S. Ralph Siebenhaar|
|Hi Ralph, mine were not easy to find. In the end I had to go one size larger than planned because it was all I could find. Worst case, you could get a set of custom pistons made up.|
I did drop by Brown and Gammons; nice shop.
I picked up an XPAG rear seal kit. It seems to be exactly the same as the Moss kit. If I successfully recover the VAT, It will save me but a few dollars.
It looks cool though!
I have that same seal on my engine rebuild. I'm looking forward to trying it out. Though u won't see it after the trans is on, I sure enjoy the anodized red color. It looks fast!
|Just a thought on head gaskets, I had Hussey Copper in PA make a solid copper gasket for my banana head TD. They have the CNC stuff for the one made for my car and could make one to any thickness if one desires. They are good people and make gaskets for all kinds of street rods and racing cars.|
|Tom Maine (TD8105)|
This thread was discussed between 10/09/2011 and 15/09/2011
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