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748/916 Question

1856 Views 7 Replies 5 Participants Last post by  Gforce
Will 916 pistons and jugs fit on a 748 block with 748 heads.
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Yes, it will make it a 853.

NOTE-make sure the 916 pistons/rings etc are the same weight as the 748 pistons/rings, otherwise you'll get vibrations, which will damage your engine. Another way is to rebalance your crank to the 916 piston combo.
piston mods

There's a 6mm bore difference and a 2.5mm stroke difference between the specs for both engines. Any suggestions on how (where) to lighten the 916 pistons to match the 748's (somewhere around the boss)?
I don't think you can use the 916 pistons for the 853 conversion because the deck height is not the same. There are plenty of "Drop-In" kits available from suppliers (like us).

To reap the full benefit from the kit, you should unshroud your valves so that the heads can take advantage of the extra area and flow better.

You can bore and replate the the 748 cylinders.
NCRick said:
To reap the full benefit from the kit, you should unshroud your valves so that the heads can take advantage of the extra area and flow better.
i've heard about this several times but still don't understand what it means. i know what it does. is it "kind of like" boring the head, that widens the area on the bottom of the head to match the new cyclinders. If so can i do it, i mean take it to a shop that machines stuff for cars, or does the Nakasyl (or however you spell it) need replating?

on the subject, what kind of numbers are returned by unshrouding. I gather an 851-3 kicks the 748 to about 104hp. Is that with unshrouding or w/out? Unshrouding +/- 2 hp or so?

on the subject and you knowing about the engines, what can be expected, on a 748, for 32mm intake and 30mm exhaust valves? and at what size valves does, if not always, does the engine have to running harder (higher) to get at the gains from valve size increase?

lastly for those of us like me, who opt for the thrifty route, how much you expect a 748 bore and plating costs? and for plating, is that something easily done or do i need to find the best shop in town to do it?
853 kits are readily available. They come with pistons that weigh the same as 748 pistons, so you don't have to rebalance the crank. You'd do well to weigh both and drill shallow dimples in the bottom of the piston crown to make both pistons weigh exactly the same. Be sure to ask your kit maker if that is the best place, but that is common.

Mine is at the DucShop getting just such a kit. It shold be ready late this week. Look up ducshop.com and give them a call. They should have a kit or 2 on the shelf. Mark is going to start having his own custom pistons made.
The 748 heads have the valves in tight to the small chambers, this limits the flow. By opening the area around the valves (on the outer edges and between the valves) the flow much better. I think the 853 with out this work will have quite high compression but will be rpm limited by the valves. The larger valve sizes gets you into some quite high end modifications involving ports and this may be more than what you are looking for. If not, I do have a set of used heads that were ported by BCM and unshrouded for an AMA Pro-Thunder bike. The valves are 1mm over and made the bike competitive in that class with 748 SPS cams and bottom end work. I would want $500 for those heads. Mark at the Duc shop is good people and knows his stuff. You can spend a bunch to go fast!

We get about $175 per Cylinder for bore and replate.
I don't have a kit maker. I was gonna try and make a set of 916 pistons and cylinders work on my 748 for the street. I can weigh the pistons and with a dremel lighten them up from underneath, open up the combustion chamber around the valves the same way (or have it done at a friends engine machine shop), but maybe it's not worth all of the work for maybe 3-4 HP. I'd love to get a kit and drop a couple of grand on my second bike, but I am a cheap do-it-yourself yankee.

You guys have been great with all of your help and advice. This has turned out to be a pretty good thread.
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