748SP engine in bits - any recommendations?
 

My 748SP racebike engine (ex Skidlids, ex Old Yella) is presently in bits for a bit of investigation work. It wasn't very well after the last outing at Brands and it seems that it needs some new pistons and crankshafts and a few other internal organs. Now, excuse my (very) limited lack of knowledge on subjects such as this but while it's in bits, is this a good time to 'upgrade' some parts.

For example, what goes into making the 748 into an 853(??). Very happy with the handling but wouldn't mind a little more grunt at the lower end of the rev range. Above 6-7000, it's wonderful but get the gearing wrong and nothing much really happens. Not looking to spend mega bucks but more of a case 'may as well while it's in parts' scenario. Not really sure what it needs at present but I want to make good use of it this season so a good going over is in order.

Suggestions and costs greatly appreciated.

Thanks all

simon

Have a word with Jeff at http://www.gtecperformance.co.uk/

I have a set of Barrells & pistons (one broken ring) and a crankshaft with rods for a 748 going cheapish if required

U2U me with prices and exactly what you have. May help cushion the blow on the rebuild.

BTW, what exactly is the engine in this bike? Was it an SP engine when you had it or was that an Old Yella 'mod'? Any more info on what exactly it is that I have?

Thanks

simon

Your engine was a 748SP engine which I got off a guy in Sommerset, the bits I have are from a 748BP, but don't think there is much difference.
Main thing you notice between a SP and a BP is the oil cooler the SP has.
Valve sizes and cams may be different but I've never really looked into it,

You can have both barrells & pistons with all but one ring (as it broke) and the crankshaft with rods for £75, I also have a few other engine internals if you need them,

748sp
 

Kev wont the 748 SP engine have ti rods or is that just the SPS??

not sure Chris, I remember hearing that people thoght the 748SP had Ti rods, but then again I also heard of a lot of people blowing up SPs by over reving.
Nelly will know

Let me know how you want paying and I'll U2U my address details. The deadline is Silverstone on the 12th so these will help greatly.

Thanks again and see you at the track

simon

Simon where are you based,
May have some time available to meet up or I could maybe find the time to run the bits over to Aylesbury

Sent you a U2U.

BTW, if you are interested in what it looks like now:

see here

and here

laters

simon

748 SP has steel rods but the race type (basically lots stronger than standard and about the same weight)

748SPS has Ti rods but not the race style, so lighter than standard and a little stronger (tho not as tough as the SP ones) a homologation thing really.

Corse rods are lighter and stronger than both but are outrageously expensive!

So, if your replacing rods with used ones i prefer the SP steel ones as they are bulletproof. also, the ti bolts on the ti rods can only be tightened 3 times and should be checked for stretch. (they are about £100 each! Ouch)

On the 748 the valves are the same size from BP to SP to SPS as are the heads (apart from cam profiles) But the pistons are high compression on the sp/sps not bp which is what makes them feel sharper than bp's. so if you're buying second had pistons be carefull you dont put the flat top BP ones in there or you'll have a low compression motor with SP cams and it wont be very happy.

if its apart, and you are buying pistons and barrels, get the 853 cc kit. its mega! but thats a different story!

when you have your pistons, rings, rods and crank on the bench, send them off to be balanced. daft not too when its apart, should cost about £100 all in and will make a big difference.

Good luck.

Is the compression ratio of a SP altered that much when fitted with BP pistons, I only have the Haynes manual for the 748 and it quotes 11.6 to 1 for the SP and 11.5 to 1 for the BP.
If the heads and valves are the same I wouldn't have thought dropping 0.1 on the compression ratio would make much difference.

Simon I should be able to get them over to Dave's no later than Wenesday afternoon although it may be as early as tomorrow if work doesn't phone needing my services and your still happy going with BP ones.
As I said I also have a few other internals so was there anything else required in the bottom end.

Dave's not been in today but I'll check with him what is needed and call / mail you tomorrow. I don't want to mess you about and make any unwanted trips so I'll confirm asap but depending on finances, it seems that we may go the 853 route anyway. Unfortunately, I don't have enough tech knowledge to know whether your parts will fit but again I'll try to confirm with Dave tomorrow.

Laters

simon