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MG MG Y Type - XPAG Pistons

Hello, I've just had my block & crank back from the machine shop. The pistons that came with it are marked JP 0200 060 and are 3 ring, solid skirt.

On checking via the internet, found out that they are made by JP pistons of Australia and are correct for the +0.060" overbore XPAG that the block is now. Several of the threads that I read about JP pistons are not particularly complementary, citing seizure due to uneven expansion, has any other user had experience of these pistons?

Secondly, the one piston, that is currently not on a con-rod, seemed a lot heavier than some other pistons that I have. When weighed, the JP piston was 403gm and the old piston, also +0.060 is 365 gm. Is this weight difference going to be a problem ? If so, is lightening the JP pistons an option, or should I purchase a different piston?

D P Jones

Hi David,

Can you tell me what the inside of the bottom of the skirt looks like? The pistons might be heavier because they have an extra thick section designed for a fourth (oil) ring groove to be machined below the gudgeon if desired. I went to the JP factory about 20 years ago and saw pistons as I described above. They were part number 510R. I remember because they were exactly the same build and part number as the +.120" "Weslite" pistons which have been running in my road-going TC since 1971 ... and are still going strong. (JP took over "Wise Engineering", who made the "Weslite" pistons, about 1980.) The 510R pistons were Racing alloy.

Lighter pistons are better for racing but it is not as important for a car which won't be run at 6000 rpm. If the pistons have the extra metal I described, they could be lightened by machining some of it off. It is important for the pistons to match each other in weight.

I looked in the archives but will try looking again. I have not heard of the seizure issues but perhaps they were caused by running too little clearance in the search for quietness. Solid skirt pistons MUST have more clearance and are noisier at start-up.

I have heard of JP issues with piston rings. JP told my brother it was due to poor quality from their supplier. We had cast iron oil rings (necessary in chrome bores) which were not round when compressed. That was about six years ago.

Bob Schapel
R L Schapel

Hi Bob,
Thanks for your information. The pistons I have do not have the extra material for the 4th ring at the skirt. The wall thickness is approximately the same for both old and new at 4mm. When measured in the bore at the skirt, the pistons are -0.003" at right angles to the wrist (gudgeon) pin and -0.006" in line with the gudgeon pin. The height of both pstons is the same.
The pistons have the number 200 cast into the inside of the crown.
Hopefully, this will not cause a problem, next job is to weigh them and equalise the weights.
Best Regards
D P Jones

Hi David,
The .003" clearance sounds about right to me but you should check that with a good engine builder. The other dimension is smaller because pistons are "cam-ground". I think pistons which are used for racing usually have a more emphasised cam-grind. I.e. they have a bigger difference between the two dimensions. Good luck with the rebuild.
R L Schapel

Hi David,
Sorry for a tardy response. I am not at all surprised to read a positive reply from Bob Schapel. I too have had excellent success with JP pistons as have many of my MG friends here in OZ. They are a significant improvement on the 4 ring Repco pistons previously available. I have used 3 ring JP pistons in the original XPAG in my TD and a B series engine in my YT. Both engines have done many high speed h/way and competition miles as well as suburban cruising without any problems. Some years ago now I did a partial, bottom end rebuild on the XPAG engine because of failing oil pressure via the crank main bearings. Apart from obviously replacing the bearing caps I fitted new rings to the JPs that were still in perfect condition, honed the bores and hey presto new engine!!!! Unfortunately I have no info on the part numbers or weights of the JP pistons I used but an email to JP might get you some answers to any questions you have.
R A Prior

Thank you all for your info. I've been onto JP and the 3 thou and 6 thou clearance is correct. I have balanced the pistons and rods so that I now have a piston/rod combination that all 4 sets are now within 1 gram. The crank/flywheel was also balanced when the crank was reground, so hopefully the engine will run quite smoothly
Thanks again
D P Jones

This thread was discussed between 12/07/2016 and 02/08/2016

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