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3.9 interim block -v- 3.9 normal block pistons
Posted: Sun Dec 23, 2012 10:19 pm
by task
I'm building an engine for a MGB GT.
I have a 3.9 block from a 1996 Discovery, it's an interim block but is missing a piston/rod.
I also have a 3.9 from a 1993 Range Rover that was apparently run with low oil pressure but has a complete set of pistons. I haven't stripped the engine yet so haven't seen what damage there is, if any.
Are the pistons the same?
I had planned to get the interim block honed and fit the pistons from the earlier engine with some new rings.
Open to other suggestions though
Posted: Mon Dec 24, 2012 1:54 pm
by POAH
pistons are the same AFAIK
Posted: Mon Dec 24, 2012 5:01 pm
by 3xpendable
Hi task,
I'm doing the same as you, and having recently bought an interim 3.9 i've ben doing some research. The only difference is the crank driven oil pump means the crank has a longer pulley keyway, and the minor differences in the block. The main bearing caps are slightly different in shape too but this doesn't affect the pistons
Any chance of some pics of your conversion?
Posted: Sat Dec 29, 2012 12:01 pm
by spend
For 3.9's there are 5 piston/block bore grades:
std = nom size to + .0075mm
A = .0075 - .0150
B = .0150 - .0225
C = .0225 - .0300
D = .0300 - .0375
(Grades should be shown on crown & block face)
I think this is why there is occasionally confusion over the different specs used for the RV8 bore - 3.7" (93.98mm) or 3.7015" (94.02mm), hence you frequently see the simple median of 94mm used.
Posted: Sat Jan 05, 2013 6:50 pm
by task
spend wrote:For 3.9's there are 5 piston/block bore grades:
std = nom size to + .0075mm
A = .0075 - .0150
B = .0150 - .0225
C = .0225 - .0300
D = .0300 - .0375
(Grades should be shown on crown & block face)
I think this is why there is occasionally confusion over the different specs used for the RV8 bore - 3.7" (93.98mm) or 3.7015" (94.02mm), hence you frequently see the simple median of 94mm used.
How are these identified on the pistons themselves?
I've stripped both engined down and found the following markings on the pistons themselves;
Early Engine;
AM413
758404
Hepolite
22059
AE HRC1828
The top of the piston has a "dimple" in it and is marked with an L
Late Engine;
413P
758414
AE HRC 1828
22059A
the top of the piston has the following marks 22059 Z07 M6 MY
I have 7 of the later pistons and 8 of the early ones.
Assuming the pistons are usable I would fit them to the later block. The crank that goes with these pistons looks good to so I would consider using this also, depending on which front cover I use. I'm limited to space with the MGB and I believe I can only use a SD1 or P6 front cover and pulleys, which seem hard to find.