1275 Engine Rebuild Choices

Any topics pertaining to the BMC A-Series engine.

1275 Engine Rebuild Choices

Postby iceclimber » Tue Jan 10, 2012 11:18 am

Firstly, I joined today after finding the Spritespot site late last night. What a great ‘spot-on’ forum this is for Sprite & Midget enthusiasts.

I am very much a ‘newbie’ having bought a good condition Mk IV Sprite in the summer. I was very pleased with the performance except for the oil pressure (when up to temperature was under 20psi at tick over and under 30psi on a run at around 65mph @ 3000rpm) I wanted to have full confidence in the engine for 2012 and so decided to do a rebuild over the winter months.

Stripped down everything & all was looking OK (crank at -20 – Bore at +30) for a straight rebuild with all new bearings, & balancing of crank, pistons and flywheel, new oil seals, timing chain, starter, clutch etc. etc… etc ….. BUT then discovered deep scores in cylinders 2 & 3. These have been diagnosed as being due to gudgeon pins not fitted correctly using heat treatment and subsequently moving.

So I now need a re-bore at +60 and so will need new pistons …… & so it begins …… :roll:

Am now thinking, (& encouraged by good friend (who has a 100/6 and frogeye) to do more than just a simple rebuild. So, was thinking about HiComp 21253 pistons, converting head to 11 stud, going to 2 x 1 ½ SU’s (or 1 x HF4), but keeping standard cam & valve gear etc. my exhaust manifold is apparently already a ‘good-un’.
BUT have now been reading about cams, high lift rockers, gas flowed heads, uprated valve gear etc…etc… and am thinking what is the best return for the modest outlay I am prepared to spend.
i.e. Would 21251 pistons and a kent 266 (or MG metro cam) with gas flowed head (still 9 stud) with the standard valves & rockers, be a better than 21253 pistons & no change of cam ? It appears that there are 3 cams that would suit what I would be happy with 1.) the MG metro cam, the Kent 266 & the Morespeed ph2

SORRY post is so long ……… :?

O'h .... One last question ...... should I get the block & head 'crack tested' before I do anything ? Is this difficult or expensive ?
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Re: 1275 Engine Rebuild Choices

Postby pg_1966Sprite » Wed Jan 11, 2012 9:42 pm

Welcome to the group. There are some amazing people on these forums, I joined a few months ago, recently got my rig on the road after a long sabbatical, it has a 1275 engine.

As far as these performance mods, I would caution you to read the posts; I notice the more modifications you make the more issues occur. I know enough to be dangerous and know my limitations. : ) Would do careful research before committing to those changes. Every action has a reaction...

As far as the head, would get it evaluated/tested. It is the right thing to do especially if you are going to to rebuild.

Personally, I had a '71 MGB back in the day, a Weber downdraft and race headers only to that car and it SCREAMED with just those changes.

I blew a head gasket this week and pulled the head last night and sent it to get evaluated. I anticipate a full re-do, it looked kinda crusty.

Good luck, email me offline to recommend the best of the best posts if it helps. Cheers!
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Re: 1275 Engine Rebuild Choices

Postby Charlie Tolman » Thu Jan 12, 2012 1:40 am

If the cylinder head has a standard combustion chamber volume, then pistons 21251 will generate a compression ratio of 8.8:1, pistons 21250, 9.75:1, and pistons 21253, 10.0:1. Premium gasoline might be needed for 10:1, depending on the camshaft.

The MG metro cam has timing and duration values of 16-56 (252 deg.) and 59-29 (268 deg.),
and the Kent 266 cam has 24-56 (260 deg.) and 61-29 (270 deg.)
Corresponding values for the Morspeed ph2 cam were not consistent on the Internet, with the duration ranging from 270 to 280 to 276/286 degrees. If it indeed is near 280 degrees, it would not be a good choice for a daily driver.

Compression ratios and cam duration values should be "matched", or be complementary.
A low compression ratio with a large duration cam will generate a low compression PRESSURE, which is not desirable, and the engine will have low horsepower throughout its rpm range.
A high compression ratio with a short duration cam will generate a very high compression PRESSURE, which can lead to engine problems.

The combination of engine components employed in a rebuild should depend on the intended application of the engine: daily driver, cruising down the highway, weekend gymkhana or auto-cross, or competition road racing ?

Building on a budget ? Eliminate roller rockers and other fancy valve train components. With low to medium compression ratios, the engine does not need a conversion from 9 to 11 studs. For normal and/or infrequent spirited driving, the head does not need to be flow tested and receive flow enhancement.

Have a good machinist carefully measure the diameter and roundness of all journals on the crankshaft.

Charlie T.
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Re: 1275 Engine Rebuild Choices

Postby iceclimber » Fri Jan 13, 2012 6:30 am

Thanks for the replies ......

Charlie; thanks for the advise & I admit I should have read some of the other posts before posting my question. But I now understand the principle, that if I go for a higher comp piston, then I will need a cam with a larger duration. I have also been reading some of the information in the David Vizard book and see how you can select a cam to suit your intended driving requirements.
I was wondering if you could advise based on the following: I do not intend EVER taking the engine above 6000rpm, in fact I can only imagine very short periods above 5000rpm. My driving will be on country roads (i live in the Cotswolds / Worcs / Gloucs / Hereford area). What I would value most, is the ability when travelling along at say 45-50mph at say around 2500rpm, to be able to accelerate rapidly in 4th (will keep a 4 speed box) up to a max of 70-75mph. So, I presume this would indicate I am more interested in torque rather than top end power ?
best regards
Steve
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Re: 1275 Engine Rebuild Choices

Postby Mark T Boldry » Fri Jan 13, 2012 5:42 pm

My advice would be NOT to over tune your engine.....

The A series engine is held back by head and cam, so those are areas to consider firstly.

If it were mine, I would be looking at MG Metro, Swiftune SW5-07 and the new Minispares Phase 1 camshaft options.

Once you have chosen your camshaft, establish what compression ratio best suits that cam. I know for the SW5-07 it likes around 10.3:1, others may be different.

Head modifications should NOT be radical, in fact if you can find an MG Metro head, (non un-leaded has larger inlet valves), just machine to clean up and fine tune C/R. Avoid doing anything with the exhaust valve or porting.

You'll find that the SU HIF44 works VERY well and whilst it doesn't look right (a single carb on an MG.. :-( ) it'll give you great torque and fuel economy.

Finally (and most often missed) get your ignition distributor set up with the best advance curve to suit your engine spec. There are some good people in the UK who can do this. Distributor Doctor and H&H Ignition solutions are just two I can think of.

It goes without saying that you'll balance all the rotating and reciprocating parts and IF the rods are still the same ones that were used when the bores were scored, you'll either change them or convert them to bushes and use circlip retained gudgeon pins.

I hope this helps..

Mark.
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Re: 1275 Engine Rebuild Choices

Postby Charlie Tolman » Sun Jan 15, 2012 1:41 pm

Specification for several Sprite camshafts:
Valve timing: Intake (duration), Exhaust (duration), max lift angles, lobe center, and overlap.

A. MG Metro: 16-56 (252) , 59-29 (268); 110, 105, 107.5 L/C; 45 deg. O-Lap
B. Kent 266: 24-56 (260) , 61-29 (270); 106, 106, 106; 53 deg. O-L

C. Kent 246: 18-54 (252) , 54-18 (252); 108, 108, 108; 36 deg O-L
D. Kent 256: 21-53 (254) , 53-21 (254); 106, 106, 106; 42 deg O-L 1000-6500 rpm

E. Swiftune 5: 16-48 (244) , 52-12 (244); 106, 110, 108; 28 deg O-L
F. Swiftune 5-7: 10-52 (242) , 47-15 (242); 111, 106, 108.5; 25 deg O-L

G. Stock 1275 ?: 5-45 (230) , 51-21 (252); 110, 105, 107.5; 26 O-L

I have not been able to obtain consistent specifications for the Morspeed ph2 cam.
__________________________________________________________________________________

Comments based on your "low" rpm application:

A. Int closes a little late. Ex opens a little early. Lobe timing backwards. Smog cam? Adv. 4 deg? CR=10.8 max.
B. Int closes a little late (56 deg ABDC). Ex opens early (61 deg BBDC) . CR=10.8 max.

C. Exhaust duration a little small, but okay. CR=10.4 max.
D. Some say too tame for a 1275 ? **1000-6500 rpm. CR=10.2 max.

E. Duration too small (244) for a 1275. Not much better than stock. Okay for 948 and 998 engines. CR=9.2 max.
F. Same comments. Lobe timing backwards. 948 and 998 ? Smog cam? Advance by 4 deg.? CR=10.0 max.

G. Not sure if this is the stock 1275 cam. Intake duration very small. CR=8.8 max.

David Vizard, in his big book, states that a compression ratio of 10.5:1 should be the maximum for a cam with an intake closing angle of 52 degrees ABDC. Since his book was written in 1986, when available gasoline had larger octane ratings than does today's gas, I have reduced this CR value to 10.0:1. Other intake closing angles have scaled CR values, listed above.

If it was my car, and it would be driven according to your low rpm description, I would choose the Kent 256 cam.

If premium gasoline, octane 92+, is used, then the MAXimum compression ratio might be okay with an extra half-of-a-point ( 9.8:1 increased to 10.3:1, for example).

Charlie Tolman
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Re: 1275 Engine Rebuild Choices

Postby Mark T Boldry » Sun Jan 15, 2012 4:03 pm

Interesting comments on the cam specs Charlie, are these based on theory or personal experience..?

I have used A, B, D, E, F and G in similar 1275 units and from my experiences, my preference is the SW5-07 (the SW5 now superseded by the 07) followed by the MG Metro or Kent 266. When racing a Midget in the UK 3 years ago I ran a 649 'plus' and my hottest rival was running the MG Metro cam... I had the legs on him on the fast bits, but out of bends he was quicker.. :-( The engines were VERY close in specification with the only 'free' modifications being camshaft and C/R.

The C/R figure of 10.3:1 I mentioned for the SW5-07 is recommended by the company who developed the cam (Swiftune) and have done 100s of hours on the dyno with it.... How do you come by 9.2:1.. ?

I enjoy your knowledge and many comments on this forum Charlie and have great respect for them, Sorry to question you on this but am keen to know on what you base your comments.

Regards,
Mark.
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Re: 1275 Engine Rebuild Choices

Postby Talonaer » Sun Jan 15, 2012 4:36 pm

Great info here, thanks guys, the time and effort is really appreciated. For someone like me whose only just starting to learn about performance engine tuning it is very helpful!

Theres a good discussion on some A-series cams at: http://www.minifinity.co.uk/forum/viewt ... 22&t=10708
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Re: 1275 Engine Rebuild Choices

Postby Charlie Tolman » Sun Jan 15, 2012 9:13 pm

Mark,

There are two versions of the SW cams, the SW5 (early version) and SW5-7 (later version).
My recommendation for the SW5-7 was a maximum compression ratio of 10.0:1, not 9.2:1 as you stated.

I have used various cams in 17 years of SCCA road racing, with 948 and 1098 engines, with durations from 280 to 315 degrees, including scatter-pattern cams, and with compression ratios up to 15:1, in addition to various rocker arm ratios. I also have helped friends and acquaintances with engines in their street cars, in terms of cam selection, including a SW5 cam in a street driven 998 Mini.

My comments on the cams were based on a low rpm application, as stated by the person who started this thread on the forum. The cam should fit the application. For road racing applications, my comments and recommendation would be different.

Charlie T.
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Re: 1275 Engine Rebuild Choices

Postby iceclimber » Mon Jan 16, 2012 4:25 am

Charlie, / Mark, many thanks for the comments and advise.
I was going to give swiftune a call today as from cross-referencing to other forums etc. the SW5-7 does sound interesting. I appreciate that it does not fit exactly with the driving characteristics I described, but I have read a lot to suggest that it can pull well from around 2500rpm but really comes on song from 3500rpm.
I would like to use with the 21253 piston which will provide 10.0:1 comp. I will also of course fit the new uprated followers and get everything balanced. Do you think I should fit uprated valve springs ?
However, the SW5-7 appears to be quite low in your recommendations ? ( if I am interpreting the comments you made for the SW's correctly ? )
In retrospect, I suppose I should have better stated my driving characteristics, as they may have come across a bit tame ! I am a biker (or was) having just sold my CBF1000F, before that I had BMW's and Suzy's ....... SO although I will be doing only road stuff (no motorways ever !) I will be pushing it at times, as it is my natural instinct to do so, but as mentioned before the car would only have rare excursions past 5000rpm. (I will at some point also update suspension & brakes)
If however you still think that the 256 will meet my better clarified expectations then this would appear to be cam i should use, with the 21253 pistons.
thanks & regards, Steve
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Re: 1275 Engine Rebuild Choices

Postby Mark T Boldry » Mon Jan 16, 2012 5:07 am

Steve,

Im my experience, the Kent 256 will be a little mild for what I believe you are looking for. The SW5-07 works very well in my 1380cc Cooper S between 3200 through to 5500 rpm and through the same rev range in the 998cc engined Mk1 Sprite.

Remember it's not just the camshaft that will give you the performance, but a well chosen cocktail of parts, all complimenting each other.

Mark.
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Re: 1275 Engine Rebuild Choices

Postby iceclimber » Mon Jan 16, 2012 2:47 pm

Mark T Boldry wrote:Steve,
Im my experience, the Kent 256 will be a little mild for what I believe you are looking for. The SW5-07 works very well in my 1380cc Cooper S between 3200 through to 5500 rpm and through the same rev range in the 998cc engined Mk1 Sprite.

Remember it's not just the camshaft that will give you the performance, but a well chosen cocktail of parts, all complimenting each other.
Mark.


Thanks Mark, re: your comment above. This is why I am being a bit neanderthal in my questions, as I have started from a zero knowledge base :( 1.) My budget for the engine rebuild is only £500 ish :wink: 2.) I think I know the limits of the other cocktail of parts, 3.) I know what sort of driving performance I would like ........ However, I need this forum to help ensure I won't be dissappointed by the camshaft choice.
So, the cocktail parts are ......... a +60 rebore; new pistons (for the small extra costs I'd like to use the 21253 10.0:1), the exhaust & manifold are apparently a good after market & should be OK. I will use a single HF44 (friend has spare one) with suitable manifold, I also accept I will need to get distrubutor with best advance curve (matched to cam choice & CR). Head will have a very small amount of 'flow work' done. (can do better headwork in the future) Obviously all will be balanced, & finally am very happy to run the car on 95 or even 97 octane. Other than new followers, possibly uprated valve springs (if necc) better oil pump and new timing chain & sprockets (+ the obvious bearings etc.... etc...) thats about all I want to spend. (no way thats gonna be contained within £500 !)
So, the above are the cocktail parts that the camshaft should match to provide something close to the diving experience I hope to acheive.

I promise I will stop soon :cry:
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Re: 1275 Engine Rebuild Choices

Postby Mark T Boldry » Mon Jan 16, 2012 4:24 pm

Steve,

Most cams can be bought as a kit with followers and oil pump.

With regard to valve springs, use the lightest weight possible, you don't want to knock the lobes off you new cam.. most are good for 6400 rpm without concern.

I use standard A Plus springs but to do this, you may want to use the later triple groove collets, caps and valves...

Mark.
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Re: 1275 Engine Rebuild Choices

Postby iceclimber » Thu Mar 08, 2012 8:46 pm

Hi, well spring is just around the corner here in the UK and so i need to get my finger out and get the Sprite ready for mid April :? ..... have finally got the engine and all balanced parts back from the machine shop (after nearly 2 months !) it's now a 1380 block which I wasn't actually planning on - but as the T shirt says .... Size Matters !
Anyway, I have now bought all the parts I need for the rebuild and after weeks & weeks of deliberations I have settled on the Kent 266 cam :roll: have read that with duplex chain (new chain & spockets) I need to time the cam at approx 2 degrees advance, so this will be 104' - are there any other views on this ? As I am more interested in torque than top end power & will very seldom (if ever) go past 5000rpm, would it make sense to set even more advance ? .... say 4' ?
cheers
Steve
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