Topic: Big Bore motor found, I need advice.

[RyanLilly]

Hey, I haven't posted in a long time, but I finally decided to get the old 74 750K out of the garage and build her into a cafe. I need some advice on purchasing a used motor that has been bored a bit.

The story goes: the current owner bought a 76 750, which had a 78 motor bored to 883, plus came with the original motor. The new buyer wanted the bike with the original motor, so the seller swapped the motors, but had to use the tappet covers, points cover, and alternator cover from the 883 to put on the stock motor. The 883 then sat under a covered porch for the next 18 months without tappet covers(among others).

First I know that 883 sounds fishy, I'm guessing the current owner is mistaken and its a 836, 849, 895 etc. Anyway I have no reason to doubt that I does indeed a larger bore.

When people do a big bore, do they also usually put in a new cam, springs, etc? My thought was that this would likely be the case.

But how bad should I expect this motor to be? Best case; there is some surface rust in the top end, and everything cleans up nicely. But what is the worst case? The entire motor is rusted throughout, and I can maybe salvage the pistons?

I think Ill maybe pull the valve cover and check it out before pulling the trigger, I would hope that I could still clean up a fairly rusted cam, Id just have to measure it afterwords, but what about springs and valves? Pretty much toast?

Thanks for any advice.

Ryan

[bucky katt]

possibly an 890?

[Gordon]

Don't believe it until you see it with your own eyes. 

[754]

Get or make a cylinder light, pop the plugs, see watcha have..

 Shine it in the tappet holes , look for rust

 Check the covers for freeze damage, drop the pan, see if water sat in it..

 I bought a lower end on here, the shifter cover was split from water sitting in it..

[Jim F]

I bought a 75 drag bike that was advertised that It had a  (If I can remember) 940 Kit

I went a looked at it and look threw the plug holes and knew by the pistons

that they where aftermarket but when I got it home and took the top end apart it was an 836

and the guy didnt return my calls so you just never know unless you know the person that does the

engine building

Jim

[MCRider]

Hey, I haven't posted in a long time, but I finally decided to get the old 74 750K out of the garage and build her into a cafe. I need some advice on purchasing a used motor that has been bored a bit.

The story goes: the current owner bought a 76 750, which had a 78 motor bored to 883, plus came with the original motor. The new buyer wanted the bike with the original motor, so the seller swapped the motors, but had to use the tappet covers, points cover, and alternator cover from the 883 to put on the stock motor. The 883 then sat under a covered porch for the next 18 months without tappet covers(among others).

First I know that 883 sounds fishy, I'm guessing the current owner is mistaken and its a 836, 849, 895 etc. Anyway I have no reason to doubt that I does indeed a larger bore.

When people do a big bore, do they also usually put in a new cam, springs, etc? My thought was that this would likely be the case.

But how bad should I expect this motor to be? Best case; there is some surface rust in the top end, and everything cleans up nicely. But what is the worst case? The entire motor is rusted throughout, and I can maybe salvage the pistons?

I think Ill maybe pull the valve cover and check it out before pulling the trigger, I would hope that I could still clean up a fairly rusted cam, Id just have to measure it afterwords, but what about springs and valves? Pretty much toast?

Thanks for any advice.

Ryan

An 888cc is a valid number. 67mm  I have run this size before. You can use this calculator with a stroke of 63mm to see what's really available.  Many sizes advertised are actually rounding up of the actual size, for marketing purposes. At 66.75mm, An 883cc Honda is very unlikely.   An 883cc is a Harley Sportster and that's what he may be thinking of.

http://www.nightrider.com/biketech/calc_displacement.htm

I think and correct me if I'm wrong but most pistons these days are only available in increments of 1 mm, so the 895cc kit of old, at 67.25mm, is likely not available. Might be hard to get rings for,

But anything over an 836 (maybe an 850cc, 65.5mm) requires resleeving the block and that's more trouble than many will go to. An 836cc at 65mm, is by far the most popular size. Ken at CycleX is pushing the 850, I think to have some one-upmanship on the ubiquitous 836.

I think many, probably majority of non-stock engines are a big bore kit and a pipe, and that's all.  Quick and effective improvement.  It takes a lot more money and committment to do more. A cam and springs without headwork may even be a step backwards.

[754]

Big Jim, you too eh..

 Yeah, I bought a 900...

 turned out to be a 836 with 350 pistond.. pinging engine that was....
 yeah, I got stung...

[scondon]

I left an engine outside under a tarp for one winter, and the cylinder bores rusted solid. Turn the motor over with a wrench, at least, before buying. Spend as much as you would on a stock 750 engine that can't be proven to run.

[754]

Why turn it over, look with a light before the rings push the rust up..
 Motor out, take the valvecover off and look at the cam & springs.., looks bad, pay lower..

[RyanLilly]

thanks for the input. I'll bring a few tools, pull the valve cover, look at the cam, and stick a flashlight into the cylinders. If it doesn't look to bad and turns over I'll make an offer.

Also its a 78 motor(not sure if its an F or K), are there any differences that would affect parts compatibility with my 74 motor?
At the least I hope to be able to use the cylinders and pistons.

[RyanLilly]

Well I'm Back, and now the proud owner of what is apparently a 1978 750F motor, bored to 836! After talking with the guy for a bit we concluded that it was a 836 motor, we are pretty sure of it, but the guy told me that I could take a few days to tear it down, and if everything wasn't right, He would refund my money, even have it in writing. When I got to his house I found out that the Porch was more of a 3 season room, so the motor really wasn't exposed to the elements, There is no rust in this motor(just a few spider webs and eggs  ) Popped off the valve cover and there is no rust at all! everything has a coat of slightly solidified oil and It looks, at first inspection, to be in good working order. I turned the crank with a wrench and it spins well feels like it builds good compression. So Later this week I'm going to tear it down, find out what pistons it has, and measure all of the clearances.

To top it all off, the guy told me to give him a call sometime if I wanted some stock, CB750 parts(gas tank, turn signals, side covers etc.....For FREE.  I'll post some pics when I tear it down.

The guy is pretty sure it was from a super sport, I there a way to tell from the Case number?

EDIT:

I looked up the serial # and Apparently its a 1978 K model.

[mcpuffett]

Nice find on the engine  , now go and get the free parts he offered you if you don't want them someone else may need them & you might get some $ 's back, cheers Mick.