Rebuild figures are a different animal from service settings. Before a block goes to the machine shop somebody has to decide which oversize, which regrind package and which shells, and get it right the first time. The tables below are the bore sizes, crank undersizes, oil clearances and wear limits for the XPAG and XPEG, taken from one consistent published source and attributed as such — with the sizes the factory itself declined to recommend marked as exactly that.
Where these figures come from
Every dimension on this page is drawn from W.K.F. Wood’s The XPAG Engine: Data, Service, Supertuning, first printed in 1968 and reprinted with club permission, which remains the most completely tabulated public source of XPAG and XPEG rebuild data. Where Wood records that a size was not recommended by the MG Car Company, or was supplied only by independent manufacturers, that note is carried through here rather than quietly dropped.
Rebuild data is not the place for a confident guess. If a figure you need is missing below, it is missing because it could not be verified from this source — take it from the Workshop Manual for your engine, or ask the technical registrar of the MG Car Club T Register. The companion settings and torque figures page covers running adjustments — tappets, ignition, torque, capacities — and deliberately leaves the machining data to this page.
None of this applies to the TA’s MPJG engine, which is a different unit. Identify what you actually have first: the chassis and engine number guide covers the prefixes, and the round-block change during TD2 production matters when ordering.
Standard dimensions
| Dimension | XPAG (TB–TF 1250) | XPEG (TF 1500) |
|---|---|---|
| Bore (standard) | 66.5 mm (2.6181 in) | 72.23 mm (2.8346 in) |
| Stroke | 90 mm (3.5433 in) | 90 mm (3.5433 in) |
| Swept volume | 1,250 cc | 1,466 cc |
| Bore/stroke ratio | 1 : 1.353 | 1 : 1.25 |
| Compression ratio | 7.25:1 (TD factory 8:1) | 8.33:1 (factory 8:1) |
| Cranking compression, throttle open | 90–100 psi | 145 psi |
| Head depth (new) | 76.75 mm (3.022 in) | 3.022 in |
| Combustion chamber, per cylinder | 45.5 cc | 44.75 cc |
| Gasket thickness, compressed | 0.045 in | 0.045 in |
| Firing order | 1-3-4-2 | 1-3-4-2 |
Piston oversizes and what they do to capacity
Boring is quoted on the diameter, and the piston carries its own running clearance — so the block is bored to the exact oversize, not to the oversize minus a clearance. The capacity and compression figures below are Wood’s tabulated values.
| XPAG bore size | Capacity | Compression ratio | Note |
|---|---|---|---|
| Standard | 1,250 cc | 7.25 : 1 | Factory piston size |
| +0.010 in | 1,260 cc | 7.30 : 1 | Factory piston size |
| +0.020 in | 1,270 cc | 7.35 : 1 | Factory piston size |
| +0.030 in | 1,280 cc | 7.40 : 1 | Factory piston size |
| +0.040 in | 1,290 cc | 7.45 : 1 | Factory piston size |
| +0.050 in | 1,299 cc | 7.50 : 1 | Factory piston size |
| +0.060 in | 1,309 cc | 7.54 : 1 | Factory piston size |
| +0.080 in | 1,328 cc | 7.64 : 1 | Not recommended by MG; independent pistons only |
| +0.100 in | 1,348 cc | 7.74 : 1 | Not recommended by MG; independent pistons only |
| +0.120 in | 1,368 cc | 7.84 : 1 | Not recommended by MG; independent pistons only |
| XPEG bore size | Capacity | Compression ratio |
|---|---|---|
| Standard | 1,466 cc | 8.33 : 1 |
| +0.010 in | 1,476 cc | 8.38 : 1 |
| +0.020 in | 1,486 cc | 8.43 : 1 |
| +0.030 in | 1,496 cc | 8.48 : 1 |
| +0.040 in | 1,506 cc | 8.53 : 1 |
The XPEG table stops at +0.040in because that is where the published sizes stop: the 1,466 cc engine is already a bored-out 1,250 block, so there is materially less metal left in the walls. Wall thickness is measured, not assumed — a good shop will check before committing to a size.
Crankshaft main journal sizes and regrind packages
| Package | Undersize | Journal diameter | Note |
|---|---|---|---|
| Standard | — | 52.00 mm / 2.0472 in | Factory regrind package |
| — | 0.002 in | 51.95 mm / 2.0452 in | Independent suppliers, not a factory package |
| R.1 | 0.012 in (0.30 mm) | 51.70 mm / 2.0354 in | Factory regrind package |
| R.2 | 0.020 in (0.50 mm) | 51.50 mm / 2.0276 in | Factory regrind package |
| R.3 | 0.030 in (0.75 mm) | 51.25 mm / 2.0187 in | Factory regrind package |
| R.4 | 0.040 in (1.00 mm) | 51.00 mm / 2.0079 in | Factory regrind package |
| R.5 | 0.050 in (1.25 mm) | 50.75 mm / 1.9980 in | Factory regrind package |
| (R.6) | 0.060 in (1.50 mm) | 50.50 mm / 1.9882 in | Not recommended by the factory; independent shells |
The bearing shells are supplied to match the regrind package, so the shaft and the shells must be ordered as a decision, not separately. Note also Wood’s observation that the recommended oil clearance is tight enough that rebuilders often take a further 0.0008–0.0014in off the diameter to achieve it in practice; that is a judgement for you and your grinder, not a figure to apply blind.
Crankpin sizes and regrind packages
| Package | Undersize | Crankpin diameter | Note |
|---|---|---|---|
| Standard | — | 45.00 mm / 1.7717 in | Factory regrind package |
| — | 0.002 in | 44.95 mm / 1.7697 in | Independent suppliers, not a factory package |
| R.1 | 0.012 in (0.30 mm) | 44.70 mm / 1.7598 in | Factory regrind package |
| R.2 | 0.020 in (0.50 mm) | 44.50 mm / 1.7515 in | Factory regrind package |
| R.3 | 0.030 in (0.75 mm) | 44.25 mm / 1.7432 in | Factory regrind package |
| R.4 | 0.040 in (1.00 mm) | 44.00 mm / 1.7323 in | Factory regrind package |
| R.5 | 0.050 in (1.25 mm) | 43.75 mm / 1.7225 in | Factory regrind package |
| (R.6) | 0.060 in (1.50 mm) | 43.50 mm / 1.7126 in | Beyond the factory range |
Assembly clearances
| Clearance | Figure |
|---|---|
| Main bearing oil clearance | 0.0008–0.002 in |
| Crankshaft end float (thrust integral with centre main) | 0.0014–0.0037 in |
| Big-end (crankpin) oil clearance | 0.0005–0.001 in |
| Connecting rod end float on the crankpin | 0.004–0.006 in |
| Piston clearance at the thrust face | 0.0021–0.0029 in |
| Piston ring groove side clearance | 0.001–0.002 in |
| Piston ring gap in the bore | 0.006–0.010 in |
| Compression ring thickness, new / minimum | 0.109 in / 0.101 in |
| Oil control ring thickness, new / minimum | 0.105 in / 0.097 in |
| Gudgeon pin diameter and fit | 18 mm (0.7087 in), two-thumb push at room temperature |
| Camshaft journal oil clearance, front | 0.0016–0.0040 in |
| Camshaft journal oil clearance, centre and rear | 0.0018–0.0037 in |
| Camshaft end float | 0.005–0.013 in |
| Tappet working clearance in its block bore | 0.0015 in |
These are XPAG figures, and the XPEG shares them: the bottom end, camshaft and gudgeon-pin dimensions are common between the two engines, with the XPEG rod reinforced for the larger capacity. Where a figure below is quoted as a range, the middle of the range is the target and the extremes are the limits, not the aim.
Machining and wear limits
| Item | Limit or practice |
|---|---|
| Block top face flatness | Flat within 0.001 in, or resurface |
| Cylinder out-of-round, taper or oversize | Must not exceed 0.003 in — beyond that, rebore |
| Boring practice | Bore to the exact oversize; piston clearance is built into the piston |
| Bore finish | Cross-hatch honed to a 25-microinch finish |
| Crankshaft scoring or out-of-round | Greater than 0.001 in calls for regrinding |
| Regrind accuracy | Grinding tolerance +0.0000 / −0.0001 in on the quoted sizes |
| Block already at maximum oversize | Sleeve back to standard, or take a banana block to 72 mm for 1,466 cc |
| Salvaging a badly undersized shaft | Arc-weld build-up is an expedient, not a solution; metal spraying is rejected outright on the XPAG |
One further piece of context worth carrying into a rebuild: original crankshafts had a reputation for cracking at the web between the front main and the first throw, and replacement shafts carrying part number 168828 on the third web, usually marked EN-16, were made in more suitable steel. If a shaft comes out of the engine cracked, that is the history behind it.
Reference dimensions for the machine shop
| Dimension | XPAG | XPEG |
|---|---|---|
| Main bearing bore in block | 56.54 mm (2.2181 in) | as XPAG |
| Main journal width, front and centre | 38 mm (1.496 in) | as XPAG |
| Main journal width, rear | 40 mm (1.575 in) | as XPAG |
| Journal fillet radius | 2.5 mm (0.100 in) | as XPAG |
| Crankpin journal width | 28 mm (1.102 in) | as XPAG |
| Connecting rod centres | 178 mm (7.0078 in) | 178 mm (7.0078 in) |
| Big-end bore diameter | 48.67 mm (1.9159 in) | 48.67 mm (1.9159 in), rod reinforced |
| Small-end bore | 18 mm (0.7087 in) | 18 mm (0.7087 in) |
| Camshaft bearing bore in block | 43.5 mm (1.7126 in) | as XPAG |
| Tappet bore in block | 23 mm (0.906 in) | as XPAG |
| Piston compression height | 45 mm (1.7717 in) | 45 mm (1.7717 in) |
What this page does not give you
No verified figure could be established here for: valve guide and valve stem clearances; valve seat cutting dimensions beyond the seat angle and widths; minimum head thickness after skimming; small-end bush finished size; flywheel run-out limit; or main bearing cap and big-end bolt torques, which belong to the settings and torque page and are flagged there where the factory sources disagree. Nothing at all is published here for the MPJG.
If you hold a manual page or club bulletin that settles any of those, please write in and it will be added with the source named and credited.
Common questions
- What is the standard bore and stroke of the XPAG engine?
- The XPAG is 66.5 mm bore by 90 mm stroke, giving 1,250 cc. The XPEG fitted to the TF 1500 shares the 90 mm stroke but is bored to 72.23 mm for 1,466 cc — it is a bored-out XPAG rather than a new engine.
- How far can an XPAG block safely be rebored?
- MG's own piston sizes run from standard to +0.060in on the XPAG. Sizes of +0.080in, +0.100in and +0.120in exist from independent piston makers but are explicitly not recommended by the MG Car Company, and any oversize should be checked against the actual remaining bore-wall thickness by the machine shop rather than assumed from the table. Once the block is beyond safe reboring it has to be sleeved back to standard.
- What does reboring do to capacity and compression ratio?
- On the XPAG, +0.020in takes 1,250 cc to 1,270 cc and the compression ratio from 7.25:1 to about 7.35:1; +0.060in gives 1,309 cc and about 7.54:1. On the XPEG, +0.040in takes 1,466 cc to 1,506 cc and about 8.53:1. The full tables for both engines are on this page.
- What are the crankshaft regrind sizes for an XPAG?
- Main journals are 52.00 mm (2.0472in) standard, with factory regrind packages R.1 to R.5 at 0.012, 0.020, 0.030, 0.040 and 0.050in undersize; an R.6 at 0.060in exists but is outside the factory recommendation. Crankpins are 45.00 mm (1.7717in) standard with the same undersize steps. Shells must be ordered to match the grind package.
- When does an XPAG crankshaft need regrinding?
- Any scoring, or out-of-roundness greater than 0.001in, calls for a regrind. The grinding tolerance on the published sizes is +0.0000 / −0.0001in, so the choice of grinder matters as much as the size chosen — poor grinding accuracy is the usual reason a freshly rebuilt bottom end does not last.
- What are the oil clearances for XPAG main and big-end bearings?
- Main bearing oil clearance is 0.0008–0.002in with crankshaft end float of 0.0014–0.0037in on the integral centre-main thrust. Big-end oil clearance is 0.0005–0.001in with 0.004–0.006in rod end float. Check with Plastigage on assembly rather than trusting shell markings alone.
- When does an XPAG cylinder block need reboring rather than honing?
- Cylinder out-of-roundness, taper or oversize must not exceed 0.003in; beyond that the block should be rebored. When boring, go to the exact oversize because the running clearance is built into the piston, then cross-hatch hone to a 25-microinch finish.
- Do these figures apply to the TA's engine?
- No. The TA uses the earlier MPJG engine, which is a different unit with different dimensions entirely. Nothing on this page applies to it, and this site does not publish MPJG rebuild data because no transcribed manual source could be verified.
Sources
W.K.F. Wood, The XPAG Engine: Data, Service, Supertuning (1968; club reprint, MG TABC library)
Primary source for every figure on this page
MG Car Club Y-Type Register — the XPAG files and rebuild diaries
Corroborating club material on rebuild practice
Wood, The XPAG Engine (1968 reprint) is a club reprint of a 1968 publication: authoritative on the engine as rebuilt in period, but a transcription nonetheless. The paper Workshop Manual for your model, and your machinist’s own measurements of the parts in front of him, are the last word.
Reference data only, transcribed with sources named. Confirm every figure against the Workshop Manual for your engine and against the actual measured condition of your parts before machining.