Sebring parts

[Fossilrider]

Hey guys, Steve here. The '67(?) 350 Sebring I just picked up has been sitting for years and has had the top end removed. I am guessing the engine was torn down due to the piston being stuck in a rusty bore, since I honed the cylinder and found a pitted ring around the bore about halfway down the bore. This is speculation, but, since the bike is only showing 1222 miles, I figure it was parked for a period and water/ condensation caused the rust and someone removed the top end to fix it and never finished.? Anyway, I will check the bottom end, as best I can without tearing it down further, and rebuild the top end and see if I can get the old gal going again. I was looking for gaskets and wondered if this engine will be fine with only one paper base gasket instead of the setup that was on the cylinder (a thick aluminum gasket with a paper gasket on each side of it). I have seen a reference to the Sebring "low compression gaskets". I realize this will increase the compression, but will it cause any problems otherwise? Is the compression increase enough to make any real difference power-wise? One other question, how many over-size pistons are available for this engine and in what sizes? With the pitting in my cylinder, I'm pretty sure it would not clear with the first couple over-sizes. Thanks for any help. Steve

[Jordan]

Have you researched the freely available parts lists?

[Fossilrider]

Hey Jordan, thanks for the heads-up. I just checked the classifieds but most are ads looking for parts. I have located pistons and gaskets on ebay but as yet I'm not sure which over-size piston I will need. And, as expensive as they are, I want to be sure I get the right one. Regards, Steve

[Jordan]

Hi Steve,

See here:
https://www.ducatimeccanica.com/D250_M2 ... Manual.pdf

Pages 28 thru 31.

Of course, you'll be limited to current availability, but it's a start.

[double diamond]

Steve,
Of course, it could be that your Sebring only needs a rebore to be revived. But the initial problem that took it out of commission could be anything, which led it to being neglected, resulting in the rusted cylinder bore you have identified. Piston rusted in cylinder is common for engines that have been left to sit for many years but the neglect is usually the result of the engine failing for some other reason. I’ve seen some really strange things disassembling engines with unknown history…

The original narrow case Sebring piston is very low compression for a number of reasons. These pistons are hard to come by but do show up occasionally. Unless you find an original piston, you’ll be considering a piston with higher compression which leads to a number of issues with installation and compatibility with other components. You can install the cylinder without the aluminum plate but check squish clearance to be sure you have sufficient clearance between the piston and head. The stock cam is very mild so probably no issue with valve to piston clearance without the plate. If you suspect the pitting in the bore is deeper than one oversize will clean up, you can have the cylinder bored until it cleans up then measure it and determine what oversize will be required. Ducati replacement pistons are available in oversizes of .4mm, .6mm and .8mm. The aftermarket pistons are usually offered in oversize steps of .25mm.

By the way, the parts catalog Jordan directed you to is for the 4 speed 250cc Diana/Monza. Most of the parts in this catalog do not apply to the 5 speed 350 Sebring.

Matt

[Fossilrider]

Many thanks guys. Yes, I realize the reason the bike was parked could run the gamut from bottom end to electrical to fuel system.... . Then, once the engine stuck, it needed to be pulled apart. I don't want to do a complete teardown unless necessary so, since it is obvious that it must be bored, I thought I'd do the top end and try to get it going and see what it runs/sounds like. I always hate to start putting a bike back together that someone else has started to repair, because of the very reason you mentioned, but I'm just taking a leap, of faith here on this one. While I have your ears, what is the recommended piston/cylinder clearance on this engine? Thanks again, and I'll keep you posted. Regards, Steve

[Eldert]

Hi Steve

piston clearance depends on the type of aluminum the mfg used and how much cam and taper the piston skirt has
they should give you the correct clearance

the Sebring cilinderhead has a differand squish band angle as other Ducati singles so beware

Eldert

[double diamond]

You can find clearance for the original equipment pistons in the service manual. There's a pdf. version in the TECH section of this site.

Matt

[Fossilrider]

Thanks again guys. I'm not sure which of the pistons I've found (on ebay) I will end up with since the guy doing the boring hasn't let me know what it will clear up at yet. Hope to hear from him tomorrow. The pistons I found might be OEM or not(?) so I'm not sure if the sellers will know exactly what the set-up specs will be for the ones they are selling. I guess I can just give the guy doing the boring the specs from the manual for the OEM piston, if the seller can't give me the specific info for his piston. Steve

[double diamond]

Steve,

The aluminum alloy that the piston is made of has some bearing on the desired piston clearance, so if you end up with an aftermarket piston, this should be taken into consideration providing the alloy is identified. The other consideration is whether the piston is cast or forged (which also dictates what alloy is used). Cast pistons typically require a smaller piston/cylinder clearance than forged. So, for instance, Ducati part number 0606.47.700, which is a 3rd oversize narrowcase Sebring piston, would be fitted according to the piston clearance range described in the Ducati OHC service manual available in the tech section of this site. I’m pretty sure this is a cast piston based on the clearance but I would have to look at one to be sure. The 350 widecase piston is forged which is consistent with the clearance called out in the widecase service manual (0.145mm max allowable, 0.115mm min. allowable). If you decide to use a narrowcase Sebring piston (there are a few on ebay including the number identified above), use the clearance recommended in the narrowcase service manual. An aftermarket cast piston would probably be suited to this clearance as well in the absence of any instruction included with the piston. Forged piston, use the widecase specification unless specifically directed otherwise by instructions included with the piston. OEM Sebring pistons are identified as either Borgo or AE. The ASSO pistons you’ll find on ebay are cast (unless specifically identified as forged). The Omega pistons (also available from eurojamb.com) are forged. Both of these alternatives are much higher compression than the standard Borgo piston. You might want to have a look at Guzzino.com as well.

Matt