Fitting a 250 cam in a 200/175 head

[Rick]

The search for parts to build my 1957 175 replica is going a little slow, but I have found a few things. The cam I found is pretty roasted- probably beyond refacing. Comparing it to a 250 cam, there's not a lot of modification required- here's a picture, the 175 cam on the left, 250 on the right.

175vs250 cam.jpg

The features opposite the drive end are the same diameters, only 6mm shorter overall. There's a shop in town that should be able to grind the 250 cam down to match the 175 dimensions- but I'd like to know if anyone has done this before, and can point out anything I'm not considering.
The layout of the rocker arm shafts in relation to the cam axis measures the same- it's really hard to measure the location of the valve guides in relation to the cam, but it sure looks about the same.
So, I'm planning to have a 250 cam reground and use 250 rocker arms, unless someone points out where I'll have a problem.
A 250 cam bearing carrier fits in the 175/200 head, and the bevel drive box cover is the same- the tunnel through the head where the rocker arms ride on the cam is a little smaller on a 175/200 head, but I don't see a problem with opening it up if required.
Any advice would be appreciated.
Rick

[Rick]

I'll probably make a spacer to fit between the head and the cam bearing carrier to check things out before I spend the money to have the cam ground.
Rick

[JimF]

On a side note, there seems to be a 175 gasket set on eBay Italy right now. Item 150731739998

Maybe these are easy to come by, or mabye it's easy to make your own... I just thought I would point it out before the auction closes.

Jim

[Eldert]

Hi Rick

i shortened a couple of cams to fit a 175 / 200 head . i did it on the lathe .
if you go the spacer route in between head and cam bearing carrierblock dont forget to make a groove or something
like that for the oilfeed to the camshaft

Eldert

[Rick]

Thanks Jim- the gasket set is listed on the Lacey web page, so it can probably wait until I have at least 2 parts that have a gasket between them...
And thanks Eldert- always good to hear from someone who's actually done what I'm just planning. The head I found has been abused- will take a lot of work to make it functional, but good enough to serve as a mock-up. I didn't intend to run the head with the spacer- just wanted to dry fit to see if the 250 rocker arms were the right length and geometry. Did you use rocker arms and valves from a 250?
Rick

[DewCatTea-Bob]

only 6mm shorter overall. There's a shop in town that should be able to grind the 250 cam down to match the 175 dimensions- but I'd like to know if anyone has done this before, and can point out anything I'm not considering.
Any advice would be appreciated.

____ I-myself once installed a (wild/hotter) 200-cam into a 250-head but, never tried to use the longer 250-cam inside the smaller 175-head. _ However I was told by another Duke-fellow (back in the '70s), that he did so by simply fitting a thinner bearing (in place of the stock-size) into the bearing-support/cap. _ I think he told me that the particular bearing he had chosen for his mod.job was also a roller-type (instead of ball-type), and, I think he also modded his bearing-support a bit as well - (real foggy here, but I think he said he had to remove about 2mm from inside it somehow).
____ Anyhow, have you not considered installing a 160-cam, instead ? ...
While 160-cams are rather common, I've never directly-compared one of them with a 175/200-cam, and I don't now recall the exact differences (of their stock fitment dimensions), (real foggy here on this now but, I'm thinking I recall a difference of 8mm [with the 250-cam]). _ So the 160-cam may be the same length or perhaps only 2mm shorter than the 200-cam, which means you'd probably have an easier fitment situation to deal with (rather than with a 250-cam).
Although if the 160-cam is even shorter, then perhaps you could then possibly make-use of a 160's bearing-support/cap, as I recall that that support/cap has a longer nose-insert length.
__ The standard 160-cam, (there's a wilder 160-cam as well), would likely be somewhat down on high-RPM power (compared to your 250-cam), but also more likely in line with the (original)- intended function of your 175-head.
Perhaps someone of us has a 160-cam (which they can easily get their hands on) to measure-out for you ?
A 160-cam should be fairly-easy for you to acquire an example of, from somewhere !


Hopeful-Cheers,
-Bob

[Rick]

Bob-
I had a 250 cam and head, but no 160 parts, so I'll try to borrow one to see it it's a better prospect.
I did think about sinking the bearing deeper into the carrier- it would probably work. That 10mm x 10mm stub on the end of the cam is a bit of a puzzle to me- seems to be just a way to create a cavity for the oil to flow into on it's way down the middle of the cam. I could probably just cut the 10mm stub off and leave a small cavity open to the oil inlet, but I worry that the Ducati engineers put that stub on the cam for some greater purpose, so I'm reluctant- especially when oil flow to the rockers and cam is concerned.
I also wondered if a sealed bearing in the cam carrier would force more oil through the cam instead of lubricating the open bearing.
Rick

[DewCatTea-Bob]

" I had a 250 cam and head, but no 160 parts, so I'll try to borrow one to see it it's a better prospect. "

____ Do you have such a prospect-source fairly handy, (or just eBay) ? ...
(I do have a spare 160-head I could sell or trade you.)
__ I'm surprised that it seems no-one has yet given you any 160-cam dimensions by now ! _ I would've expected that at least Eldert would've.


" I could probably just cut the 10mm stub off and leave a small cavity open to the oil inlet, "

____While doing merely just that would of-course indeed decrease it's overall-length, the shaft's remaining left-end ODs would still need to be ground-down to fit.


" I also wondered if a sealed bearing in the cam carrier would force more oil through the cam instead of lubricating the open bearing. "

____ That certainly stands to reason of-course, as the oil-pressure would then have to become increased (for both the top & lower ends),, however, the purpose of the oil for such type bearings is not so much for lubercation as for carrying-away heat from the bearing !
It may be possible that a shielded-bearing could possibly continue opperating with the higher temperature of no such cooling-effect but, I-myself would not wish to do such experimentation, (as bearing-life would no-doubt be cut-short).


Hopeful-Cheers,
-Bob

[Rick]

Bob,
I know someone with a 160 cam, I'll measure it next time I go over there.
Cutting off the 10x10 stub would have gone along with sinking the bearing further into the carrier, but I think reworking the cam would be easier.
I was considering the sealed bearing since it looks like there will be no oil going to the cam lobes and rockers until the cavity in the carrier fills up with oil to create some pressure to push the oil down the cam- I've seen a few fried cams and rocker arms but the bearings looked OK, so I thought it might make sense to divert some of the oil to the cam/rocker surface by filling the cavity faster- definitely not a completely cooked plan- just a thought. If everything is working to spec, the stock setup will be OK.
Rick

[Bevel bob]

Hi Rick, I think the system is low pressure compared to a motor with plain or shell bearings, if you try to build pressure in the head you will just divert the flow to leak past the cover bush or send it all to the b/end. A popular mod is to fit a restrictor at the bevel end of the cam, a roll pin etc. Some motors have this from the factory ??.This sends more oil to the lobes.