67mm Pistons ??

[grewth]

Today I got given this (allegedly) F1 cylinder barrel when I bought a pile of K2 bits.
I didn't look too closely because it was free.
But on getting it home it has 67mm bores, which would seem to be in oversized liners.
Liners are 70mm across bottom.
Only measured with a tape measure so far, but by my quick calculations that's 887cc or thereabouts.
Now I just wondering where I can get a set of pistons which will make this work.
Can anyone tell me if this was done from a kit, or engineered as a one off ?

 

[grewth]

Photos won't upload for some reason

 

[dirtdigger]

you need to get accurate measurements, there were and still are a lot of different overbore sizes. Cyclex and dynoman are the big places to get big bore pistons though. If they are 67mm for sure then it is a 890kit and cycle x has that size

 

[grewth]

you need to get accurate measurements, there were and still are a lot of different overbore sizes. Cyclex and dynoman are the big places to get big bore pistons though. If they are 67mm for sure then it is a 890kit and cycle x has that size

Thanks for that.
We have a selection of quality digital micrometers at work that I can access on Monday.
An inside micrometer would be better, not sure if we have one.
If the kit was from cycle x, it could mean a lot of expense for me, due to being located in the UK.
No NOS pistons or rings floating around on this side of the pond, if they're available at all I'm going to have to get them shipped to the UK.

 

[hinksy]

These guys in oz offer up to a 1240cc honda bigbore
https://www.rpengines.com.au/

 

[grewth]

These guys in oz offer up to a 1240cc honda bigbore
https://www.rpengines.com.au/

Thanks for that.
They are selling on ebay UK, at surprisingly low prices.
I'm glad that you posted that, because I until now I thought their rods were forged Aluminium alloy, something that I'd avoid like the plague.
Chrome Moly Steel rods for $1000 AU
That's less than £550 sterling.
Even with carriage and the dreaded VAT, that's still a bargain.
If they're any good that is !

 

[hinksy]

I have only heard good things about there work

 

[grewth]

I have only heard good things about there work

I was watching a clip of a dyno run of a 1032cc bike in the US.
A reliable 95 BHP at the rear wheel, with an externally standard looking bike.
OK, so you could get a lot more bang for your buck by buying some 20 year old sports bike and not modifying anything.
But I really like my old Honda's, and I reckon that I can put up with the early '70's handling and brakes, despite my advancing years.
Worth spending the money ?
Well yes if it's done properly, to me anyway.

 

[grewth]

Successfully uploaded at last !
But I'm not sure how I managed it

 

[hinksy]

Grewth , I`m sort of the same and I dont want the latest , fastest crotch rocket . I had honda fours and a suzuki gt triple when I was a lot younger . Then had a laverda 1200 for 24 yrs that had 80 hp at the rear wheel . After that I had a suzuki DRZ motard which was fully worked and only made 50 hp , then the trail version which only had 40 . So I have never had any massive horsepower beasts and like you , love the look of an older style bike . Not into doing high speeds anymore , just cruising on a nice day with me onda . Also have a look at their facebook page , there are lots of vids of their work

 

[grewth]

Well TBH the bores on that big bore barrel are not exactly brilliant.
No deep scoring, but plenty of light scuffing.
It's going to need a good hone anyway.
Then assuming that I can get some pistons/rings to suit, the $64 question is rods.
Apparently the 836 conversion is OK with the standard rods, maybe with F2 big end bolts added.
I'm thinking that 890 might just be pushing the standard rods too far.
And then with the cost of high strength rods factored in, a cheap project suddenly becomes expensive.
My thoughts are if you have to buy rods, why not go all the way out to 1030 or 1080 ?

 

[dirtdigger]

836 is fine with stock rods, thousands have run with no issues, especially just a road bike. If you want to race or something then upgrading rods is necessary regardless of the bore size. Just street riding you could probably get by with a 890 size if you used new bolts and had them shot peened. Rods are a weak spot in the 750, big bore and high rpms not a good combination. The Cycle x rods are an affordable alternative, I running them in a 970cc for many years but anything bigger gets Carillo rods. Rods are just the tip of the iceberg when going that big. I have well over $6000 in my 1030cc drag motor and I could have easily spent more. Almost everything needs work or upgraded when going that big if you want it to stay together.

 

[grewth]

836 is fine with stock rods, thousands have run with no issues, especially just a road bike. If you want to race or something then upgrading rods is necessary regardless of the bore size. Just street riding you could probably get by with a 890 size if you used new bolts and had them shot peened. Rods are a weak spot in the 750, big bore and high rpms not a good combination. The Cycle x rods are an affordable alternative, I running them in a 970cc for many years but anything bigger gets Carillo rods. Rods are just the tip of the iceberg when going that big. I have well over $6000 in my 1030cc drag motor and I could have easily spent more. Almost everything needs work or upgraded when going that big if you want it to stay together.

Thanks for that !
I'm old, and I appreciate a fat torque curve more than big horsepower figures at high rpm.
I appreciate that there are other classic bikes which lend themselves to tuning better (cheaper) than Honda's four.
Bikes with roller bearing crankshafts like Kawasaki's Z1 or Suzuki's GS series.
I just have a thing about the Honda
Putting $6000 into an engine is a bargain if it makes you happy.

 

[Old_Crow]

Putting $6000 into an engine is a bargain if it makes you happy.

Finally, someone who would understand why I spent a ridiculous amount of money to re-do the special factory paint job on my '79 Anniversary edition. Granted, I didn't spend $6k, but I could have painted it black or dark blue for a lot less...a whole lot less.

 

[hinksy]

It cost me $7500 ozzy dollaroos to rebuild my engine from the ground up , so I hear you . It hurt , but I just wasnt capable of doing some of the work involved and I wanted every bearing/seal/gasket replaced aswell as rubbers/chains and in the end pistons/valves/guides . Then It cost me transport fees/airfares and car rental , so at least another thou !

 

[grewth]

Getting bigger !
Now I've got 61mm, 67mm, and 72mm to choose from.
The biggest bore barrels are going to present a significant engineering challenge, forged rods definitely, other mods required also.
What I don't want to build is something that has a truckload of high frequency vibration.

 

[dirtdigger]

The 72 will run pretty warm. Carrillo rods are about a must with that big of bore. Upgrade to heavy duty primary chains and tensioner and timing chain are a must. You will need a good solid transmission and heavy duty clutch. That big bore will need a big head, cam and carbs to support it or it will be disappointing. You will need to have the top of the case bore for the sleeves if not done already. Many upgrades needed when going that big. Then managing the extra heat.

Did you take the 72mm engine apart or just have the cylinder?

 

[grewth]

Did you take the 72mm engine apart or just have the cylinder?

Unfortunately, I only bought the cylinder.
I appreciate that there's loads of research and planning (and spending) to be done before this thing will ever run again.
I'm looking at it as a long term project which might take a couple of years to come together.
Not sure about cylinder head work yet.
I've got a couple of F2 type heads, one seems to be quite a low milage example.
I'm not sure if using an F2 head would be a good starting point or not.
I'm on a learning curve here, and hopefully, if I don't make any rash decisions, I will end up with something worthwhile.
Or at least that's the plan

 

[dirtdigger]

yes F2 heads are a good starting point, you do have to do some work to convert them to use on K cylinders, There are pictures of almost everything you are looking at doing in my photo galleries on here. I have done all of this before.

 

[grewth]

yes F2 heads are a good starting point, you do have to do some work to convert them to use on K cylinders, There are pictures of almost everything you are looking at doing in my photo galleries on here. I have done all of this before.

Thank you !
I'll use those photos as a reference.
I've already noticed something about the 72mm barrel.
The "wells" around the cylinder stud holes which normally take the dowels and seals seem to have been welded up, and then the top face has been skimmed.