Spridget Tech Talk (MG Midget & Austin-Healey Sprite)
Hey guys
Forum information is befuddling my decisions. I'm rebuilding a 948 9c, new pistons, rockers, etc. Is it best to upgrade to a long stroke? What is different between long stroke 948 and 1098 chrank in 948?
One forum comment said a 1098 had less value than a 948, how's that?
It is a matter I am muddling over these days.
Where to start?
With a 948 block one can easily put in Mini Cooper 998cc flat top pistons. However, one would need 1098cc connecting rods (same length as 948, but the wrist pins are different). Using the 998 with 0.060" overbore, one gets a 1050cc engine. with good carburetion and valves (either a 1098 or 1275 head) one gets an engine with the same HP as a 1275, but without the torque. I add that this 998 set up (essentially a Mini Cooper Engine) is essentially square (bore equals the stroke) and so is highly efficient. It likes to rev high, and so is a real racing machine.
Short rods mean lots of HP at high revs and low torque. Conversely, long stroke means low end torque and lower revs.
The 948 was around for a lot longer than the 1098, and was used in lots of makes and models, so it has more applications. As it is under 1000cc it qualifies for racing in another class. It is also a very sweet and stalwart little engine. The 1098, a stepping stone to the 1275, has weaker bearings, poorer balance in my opinion.
948 and 998 cranks have a shorter stroke than a 1098. Late 1098 cranks have bigger bearings.
To put a 1098 crank into a 948 block you would have to grind out the block a lot since the conrods will hit the sides of the smaller (inside) block, lots of hassle. The small bearing (CG) crank does have a tendency to break but the big bearing one (CC) doesn't have that problem as much. Also the 1100 block is taller (to accept the extra stroke) so you would have to get very special pistons (custom made?) to fit the 948 block.
Alex- it is difficult to get a 1275 head to seal on a small block (and vice-versa), the water passages are not in the same locations and tend to give real problems.
BillM
OK, very interesting. I'll plan for turning the 948 into a 1050 bored 0.60 with 998cc pistons and 1098 rods.The square high rev engine sounds great to me.
A -206 or -295 head (from the 1100's)will help that engine rev much better than the stock 948 head (as Alex said).
BillM
i just bought a 2G295 head on ebay for $125! it comes with a warrantee and I can check it with a magna flux on Thursday. Now the fun is beginning. I'l remember that .120" overbore for the 948. my block appears unmodified in regards to shaving.
I am wondering now if the 1.34 lift roller rockers increase fuel efficiency under normal driving? Seems the answer would be no until i start hot rodding
Hi,
Just picking up on this discussion.
FYI, 948 and 1098 blocks have different width main bearing housings and consequently so do the cranks. The 1098 housing is quite a bit narrower as are the main bearing shells, a 1098 crank will therefore not fit a 948 block even though both blocks are the same height, the longer throw 1098 crank used shorter wrist pin to crown size pistons to get round this.
Paul N-M
Hello Paul,
Good to hear from you.
My plan is this: "With a 948 block one can easily put in Mini Cooper 998cc flat top pistons. However, one would need 1098cc connecting rods (same length as 948, but the wrist pins are different). Using the 998cc MiniCooper pistons with 0.060" overbore, one gets a 1050cc engine".
I am concerned that you may be saying that the rods from the 1098 will not work in the 948. Is that true in your opinion?
Should I instead settle for 948 pistons and 0.060" overbore?
Will the 948cc crankshaft rod bearings be a problem with 1098cc rods?
I assume the 998cc pistons come with such rods but I have not found a source of OEM rods for them yet.
I did find sources for the 998cc pistons and also for gapless rings for them.
I will be soon planning to purchase these as I have completed mag fluzxing and leveling a 1098cc cylinder head with 1275 intake valves, new valve seat rings, 3-angle valve seat grinding, new springs, special APT intake valves, and rockers with off-center bushings).
Hope to hear from you on checking these plans of mine. I have not yet worked on the block, just the head.
Cheers
Gerry
Hap, you are right about the Harland Sharp rockers. Nice looking, inexpensive units. But by the time I had the pedestals gound, special spacers made, elongated the pushrod holes, shimmed everything... I got them to wrork OK, but what an expensive pain in the a..!!
Gerry, you're welcome to call me, I guessing I'm the only one on this forum to actually have build a 1050cc 948 engine, and have a nice set of prepped, balanced 1098 rods with ARP rod bolts I'll sell you for deal. my contact info is on the below link on my post.
Hi Gerry,
I've been on holiday, just back and note your question.
I suggest you use A+ rods and pistons from an A+ 998. These rods and pistons are stronger even in standard form,7000rpm being possible. The rods can be lightened, balanced and peened or hardened (don't harden the caps though if you decide on this over peening). The big ends fit the 948 crank. Std 998 A+ pistons take you 948 block out to 998 obviously, oversize pistons can give you arround 1050 depending on size selected. You will probably find that on a dry build the pistons will be shy of the block deck by a few thou. Deck the block to bring flush. This is a good idea anyway to ensure decent gasket seal.
The std A+ pistons are of slipper design, very good for a std item. The rings are a bit on the wide side for very high revs, watch the ring to ring land fit. If you are going over 7500rpm on a regular basis use racing pistons.
The A+ pistons and rods use a diferent wrist pin design to that of the 1098 rods/pistons so have to be used in conjunction with each other. The wrist pin is more difficult to remove, a press tool being required to dismantle. Thats the only slightly difficult bit.
Good luck,
Paul
I came up with a deck height of .015" using 998 pistons in 948 block with virgin block deck. if you got a 948 block that has been decked then you got to start worring about piston compression height, factory deck heights can vary as much .010"-.020", so a block that had been decked could easily pop the piston out of the block, similar to the 1275s when using the taller compression height 9.7 to 1 pistons. I always tell folks to look at the block deck closely, the factory make stampings on them, diamonds with numbers in them, if you see that, the block deck is virgin and if the block deck then passes a straight edge check, then and only then can safely use the taller compression pistons.
Hap is correct in his observation. If the block deck does not have the original factory marks on it chances are its been planed at some time, problem is unless you were the first to do it you dont know by how much. A dry build will determine what you need to know. However I would be surprised if the block had been planed more than a 0.004/0.005'' to make it flat unless it had suffered from corrosion. This of course can happen to old 948 blocks round some of the water passage/gasket holes. If this is the case up to 0.025'' may need removed depending on how bad it is and if its your only block. All is not lost though as the A+ pistons can have up to 0.025 machined off their top faces so this set up should still be possible. It will just involve you in a bit of engineering at you local machine shop.
You don't say what c/r you are looking to build to. Decking the block reduces combustion chamber volume therfore increases c/r. The work done/proposed to your c/head could change its volume. Therefore both block and c/head modifications have to be factored in to your c/r calculations.
Regards,
Paul
I'm ready to try a dry build. The crank was 0.010 but otherwise the block does not look planed and the pistons are 5-ring OE. The Cam was stamped 3J9 and in good shape.
Does anyone know if 3J9 was OE? The lobes seem high.
i'm going to give the 998 pistons a try and should begin boring this Thursday or next. Sonic testing show very thick cylinder walls on all sides.
The Block had the original factury marks on it and it was surprisingly flat. I planned it only 0.003 in order to improve an area near the center with small depression. That meant I had to remove the aluminum plate with the engine code which I am determine to put back because it is a 9C, not a 9CG.
I am amazed at how small the standard 998cc pistons are compared to the 948 standards. They are no bigger in diameter than a 948 + 0.060. No wonder the big bore is 998cc + 0.060.
I am spending time looking over every point made on Graham Russell's Project: Big Bore.
948 bore standard bore is 2.48", a 998 standard bore is the same as the 1098 is 2.54".