Topic: Stock Piston options

[Jenya79]

Hi all,

I am doing a top end on my 1976 CB750k and am looking for a set of stock pistons. I have seen a set for as cheap as $109 from the parts dealers like 4into1, for $209 from IMD (imdpistons.com/products/honda-cb750-sohc-300-piston-kits-4), and ones in the $500 and above range. Are the cheap ones a "run the other way" kinda thing? Is the IMD set sufficient in quality? Not aiming to change the bore or compression.


P.S. Before anyone asks, I had intended to only do the head gasket and the whole top end came off when I started removing the head. Furthermore the original pistons have a whole bunch of carbon built up so I thought I might as well do that job as well.


Cheers and thanks in advance

[Tracksnblades1]

I’d measure everything first…Honda is known for using the best…

Old carburetor dunk/soak type like Tyme will make short work of carbon…can make old pistons look new..
It wont fix worn ones, it’ll just clean up…

It’s priceless to know what you have so you don’t replace it with poorer quality..

I would use an OEM ring that’s a 61mm if your stock bore measure near new.
The original manufacturers’ used some of the best…
They warrantied them you know…Doubt if you’ll find any better quality…

[denward17]

Have you taken measurements and know that stock pistons will work?

A lot of this depends on what your plans are for the bike.  If a long term relationship for a rider, I would probably jump up either .25 or .5 mm and rebore with an aftermarket piston set.

I've used the cruizinimage (CI) sets in a 550 and 750, and both bikes run great, and the piston sets I received from CI appeared pretty good quality and all pistons were right on in measurements.

I've read here on the forum that someone thought that IMD pistons were the same as CI pistons, but I really don't know if that is true.  I think a few UK guys may run them.

[PeWe]

+5 for Cruzinimage pistons.
(0.50 mm need rebore cylinders.)Tight fitting 0.02mm in total clearance or maybe less.
Use the pistons for K7/8-F1 which will give higher compression and a more fun bike.
https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

My K2 got them and work fine. Take no oil, no refill during 5500km. Maybe 5-10 mm lower level in oil tank.

[newday777]

Stay away from 4into1.com for anything.  If there is a problem with a part you get from them, they have very bad customer service after the sale. It is buyer beware with them.

[newday777]

Hi all,

I am doing a top end on my 1976 CB750k and am looking for a set of stock pistons. I have seen a set for as cheap as $109 from the parts dealers like 4into1, for $209 from IMD (imdpistons.com/products/honda-cb750-sohc-300-piston-kits-4), and ones in the $500 and above range. Are the cheap ones a "run the other way" kinda thing? Is the IMD set sufficient in quality? Not aiming to change the bore or compression.


P.S. Before anyone asks, I had intended to only do the head gasket and the whole top end came off when I started removing the head. Furthermore the original pistons have a whole bunch of carbon built up so I thought I might as well do that job as well.


Cheers and thanks in advance

Carbon can be cleaned off the pistons, but the things you need to look at before deciding to stay with the stock bore is are the sides of the pistons scuffed and the cylinder bores scratched, and are the cylinder bores within spec(need to be mic'd with an internal mic to check for egg shape, wear beyond specs so you don't get smoking with new stock pistons/rings)
Most likely you will need to have the cylinders bored oversized to a minimum .050mm pistons and rings to clean them up. Stock pistons and rings usually won't work as honing will oversize and not correct egg shape cylinders.

[willbird]

I have thought that having the bores properly honed to be round, straight, and with a modern surface finish and then having Line2Line coat the stock pistons would be interesting. The coating for 4 pistons is $152 currently. Doing this is a good form of recycling IMHO and leaves the cyl to be potentially refinished again decades from now if need be. I have seen some vids of the coatings after thousands of miles of use in a 200+ HP drag and drive race engine and they looked GREAT. They can do bearings too.

[Nurse Julie]

Cruzinimage piston and ring kits are excellent. But, as others have mentioned, you need to have a good measure up to see what size you need. Even if everything measures up OK for a standard size set of pistons, if there are any signs of wear, scratches, water damege or rust, you'll need to rebore to oversize anyway.

[HondaMan]

Replacing pistons and rings in an existing bore won't work out very well. If you plan on using the engine for significant riding use, you need to at least step-bore it with new pistons and rings, otherwise it will never make full power and will use oil.

PM me your address for a couple of O-rings to put into your new head gasket: they must be thicker than the OEM versions now because the modern head gaskets are also thicker. These O-rings do not come in any gasket sets. If you don't use the thicker ones the new head gasket will leak oil, usually within 50-100 miles.

[Tracksnblades1]

I have thought that having the bores properly honed to be round, straight, and with a modern surface finish and then having Line2Line coat the stock pistons would be interesting. The coating for 4 pistons is $152 currently. Doing this is a good form of recycling IMHO and leaves the cyl to be potentially refinished again decades from now if need be. I have seen some vids of the coatings after thousands of miles of use in a 200+ HP drag and drive race engine and they looked GREAT. They can do bearings too.

That’s interesting.

Millennium Technologies is advertising nikaseal services for cast iron bores… not sure the cost but you can buy a new single Honda cylinder jug for about the same price and you don’t need a good core…I was thinking of having a few coated but just decide to go Honda New OEM…maybe I’ll sell them my good core and realize my savings..

Probably not, just keep them incase I want a big bore core…😁

[HondaMan]

LA Sleeve sells new sleeves, too...

[RAFster122s]

Part of the issue is the metal used for the cylinder's cooling body for the sleeves in initial heat cycles allowed the cylinder's to drift because of the poor quality metal in the casting. So, an align bore .25 or .5 will re-establish the alignment restoring the roughly 5% performance hit HondaMan had estimated this drift caused.
The egg shaped wear that is common with our motors also is corrected by the small overbore.
CI piston and ring kits are an economical path and I have not heard of any issues if the tolerances are correct and you don't break a ring on reassembly. Some have reported getting blocked from ordering the replacement rings when they contact CI about breaking a ring on assemble, it is a communication problem and puzzling behavior on CI's part.

[PeWe]

Order CI stuff via their homepage I added earlier. Work better than eBay.